sussawea®
t? Sees e
oe
SC gra ee tte wy 2 Ce
Radio Library Vol. IV
Radio Receivers and Servicing
RADIO RECEIVERS By K. M. MacILVAIN, E.E.
RADIO ENGINEER; MEMBER, INSTITUTE OF RADIO ENGINEERS
SERVICING OF RADIO RECEIVERS By L.Gssic STAFF:
Published by INTERNATIONAL TEXTBOOK COMPANY SCRANTON, PA.
Copyright, 1928, by INTERNATIONAL TEXTBOOK COMPANY
Copyright in Great Britain
All rights reserved
Printed in U.S. A.
INTERNATIONAL TEXTBOOK PRESS Seranton, Pa. 93898
PREFACE
The popular interest in radio is due chiefly to the reception of programs transmitted by radio broadcasting stations. An immense industry has been built around this branch of radio, giving employment to thousands who by their training and experience are qualified to serve it.
This volume was prepared especially to acquaint the reader with the fundamentals of radio reception and with methods of locating and overcoming the difficulties in radio reception. The instruction on Radio Receivers begins with the crystal detector and is followed logically by regenerative receivers, radio-frequency amplifiers, neutrodyne sets, reflex set, superheterodyne receivers, short-wave receivers, single-side-band receiver, power amplifiers, a.-c. receivers, and loud speakers.
The Section on Servicing of Radio Receivers contains practical instructions for locating and remedying troubles in radio receivers, loud speakers, power units, and acces- sories.
This instruction will be helpful to the men in the ‘industry, such as operators, set builders, dealers, sales- men, and service men. In fact, every set owner will profit by this instruction inasmuch as it will acquaint him with the possibilities and limitations of his own set and teach him how to obtain the utmost service from the equipment he may have.
INTERNATIONAL CORRESPONDENCE SCHOOLS
CONTENTS
RadiotRecervers 260 ie hes vee ss 60a 1S at A Pundamental [Theory of Operation): os ieee ee ease es Perey stall etecbOrs .. 40 cae hie RRC Um cae Urea aan Os
Macuitiinie 1 Ube. ELECUOIS «ec eee eee teeta rule 4
Diode detector; Bias detector; Detector using grid condenser and grid leak; Reception of undamped waves; Interception and detection.
Rerenerative Receivers. hee ls che eee
Single-circuit receivers; 300—-19,000-meter commercial receiver; Regeneration by tuned-plate method.
eaciosrequency Am plihétss2 saa eee eed ee ce
Untuned-transformer coupled radio-frequency receiver; One-stage tuned radio-frequency with feed-back; Two-stage tuned radio- frequency receiver; Neutrodyne receiver; Reflex receiver; Superheterodyne receiver; Short-wave receivers; Single-side- band receiver.
Pucio-eraciency: Aimplifiers., «rc cur. ceisler ait
Types of audio-frequency amplifiers; Transformer-coupled audio- frequency amplifier; Impedance-coupled audio-frequency amplifier; ([ransformer-resistance coupled audio-frequency amplifier. —
Power Amplifiers and Power Plate Supply..............
Advantages of power amplifiers; Power amplifier and power supply with full-wave rectification; Power amplifier with B and C eliminator; Receivers with a.-c. tubes.
RCMEELE TOC UCETS » scary (iiits 015 ter) arcu ete el sale! eden eR ass Ge Telephone receivers; Speakers.
penvaicine OF Radio HeCelVerss ¢ oca-aicegh ee ee ae
Teremernl INStrucllONs v5 cee cc) ou dls cledaee 2 UAT ae ety in
Classification of receiving sets; Precautions; Service-shop equip- ment; Portable tool kit; Serviceman’s conduct; Obtaining information from customer; Relation between length of service and failure. ,
rouse Dicsmourees.of LrOUDIO.. s« falc. -kiearee Ree
Trouble in accessories; Outside interference.
Seating OF RECEIVING ets vyacthe ae bd fe ee tee :
Weston a.-c. and d.-c. tester; Testing battomeoversred et Testing a.-c. operated sets.
Specific Troubles and Adjustments................ :
Simple testing equipment; Variable-condenser troubles; esting fixed condensers; ‘Adjustment of neutralizing condensers; Ser- vicing of power units; Servicing of radio speakers.
14— 18
192569
L018
(hee dae
96-103
104-141 104-110
111-117
118-129
130-141
a ‘ hi Be » ¢ « i" a?
RADIO RECEIVERS AND SERVICING
RADIO RECEIVERS
FUNDAMENTAL THEORY OF OPERATION
In the course of radio reception, the receiving antenna is subjected to the field of a traveling wave emanating from a radio transmitting station, and radio-frequency currents are induced in the antenna system. This radio- frequency energy is very feeble and some appreciation of this fact may be derived from the following discussion.
A non-directive radio transmitting station will be con- sidered. Since it is not directive,.energy is radiated from this transmitter with equal strength in all directions. Ata given distance from the transmitter the energy radiated is scattered over the entire surface of a sphere having a radius corresponding to the distance from the transmitter. Owing to the relative size of the receiving antenna, the latter can only cover an extremely small fraction of the sphere in question, hence the minuteness of the induced currents.
At the receiving station the antenna functions to inter- cept the traveling waves from the transmitting station and the action is manifested by the very feeble radio- frequency currents in the antenna circuit. The problem at this point is to establish a means of sensing the inter- ception of radio waves. In regard to the feasibility of visualizing the current in the antenna circuit, it should be
2 SR ADIO SRBC EDV ERs
considered that a radio-frequency milliammeter would not be sufficiently sensitive to indicate the value of the eurrent.' Even if it could, it would have too slow an action to follow the dots and dashes of the telegraph code at the speed with which they are transmitted in normal operation. In the case of radio telegraph communication, if a meter of sufficient sensitivity could be produced whose indicating element could follow the dots and dashes of the code, the signals could be read by the eye, but this is extremely impractical and it has been found that aural reception of radio telegraph signals must first be effected.
It might be stated at this point that in large commercial radio telegraph receiving stations these received audio- frequency signals are ampli- fied and then rectified, pro- ducing dots and dashes in Se ee the form of unidirectional
g e (j pulses. These pulses are applied to sensitive re- corders that record the dots
; and dashes in ink on a moy- ing tape. This makes it possible for the receiving operator to receive by either the eye or the ear or both.
In the case of radio telephone reception, visual reception would be unintelligible, so this necessitates aural reception. Thus in the case of radio telegraphy and radio telephony aural reception is necessary. A fundamental receiving circuit is shown in Fig. 1. The antenna a is connected to one end of the coupling coil 6, and the ground ¢ is connected to the other end. The radio-frequency currents induced in the antenna pass to ground through the coil b. The antenna circuit is not tuned to any particular wave- length, hence it is considered aperiodic.
= Fic. 1
AN DOGS@RVICIN Gr: 3
The two coils 6 and d constitute a radio-frequency step- -up transformer. ‘This is desirable, owing to the fact that the feeble energy in the antenna circuit consists of a rela- tively high current and low potential, and what is desired for application to the detector is a low current at a high potential. Thus, the potential available across the coil b is stepped up to a relatively high potential, which is available across the coil d. This potential is still further increased by shunting a condenser e across the coil d so that the combination may be tuned to the frequency of the incoming wave. This tuning operation not only produces maximum voltage cross the coil d and the condenser e at the desired wavelength, but it also selects the wavelength desired and tends to suppress the application of signal voltages on other wavelengths to the detector circuit.
The function of the detector f is to change the radio- frequency signal into an audio-frequency signal that can be applied to the phones g. The thought arises at this time, why not connect the phones directly in series with the antenna circuit. If there is enough energy in the output circuit of the receiver shown in Fig. 1 to actuate the dia- phragm of the telephone receivers, or phones, g there should be sufficient energy to accomplish this operation in the antenna circuit. <A consideration of Fig. 2 will help explain why the phones cannot be made to function in the antenna circuit.
A series of wave trains that are sent out from a damped- wave radio-telegraph transmitting station, such as a quenched-spark transmitter, are shown in Fig. 2 (a). Each time the transmitting key is pressed down, wave trains are sent out at an audio-frequency rate, possibly 1,000 per second. Each one of these wave trains is made up of radio-frequency oscillations that are of the order of 500,000 cycles per second, if the wavelength is 600 meters.
4 SRA D 10) RIEC. BA Ven Ris
Even if there were sufficient. current in the antenna circuit, caused by the incoming signal, to operate the receiver diaphragm, the diaphragm could not follow the radio-frequency changes in current because it has a period of its own and also possesses a certain inertia which pre- vents it from vibrating at such a high rate. Even if it
kadio -Frequencty (pur
Rectified Current
N x & &§ + G8 x © 9 Ow LES e > ge oO (© inde, &
could vibrate at such a high rate, the human ear would not be affected, since the highest frequencies audible to the human ear are between 16,000 and 20,000 cycles per second.
It might then be reasoned, why does not the telephone receiver connected in series with the antenna circuit follow the average change in current. The answer is, it does.
AND SERVICING 5
The average change in current is zero, as will be noted from a consideration of Fig. 2 (a). The radio-frequency variations in current go as far in the positive direction as in the negative direction and the average is zero. The function of a detector is to change the nature of the radio- frequency in such a manner that its average value will not be zero, and it does this by rectifying the radio-frequency input as shown in view (0b).
The detector is a device that has unilateral conductivity. It allows the passage of current in one direction only. The natureof the current in the output circuit of the detector is shown by view (6). The detector suppresses the negative waves, and the average of the positive waves results in a positive current that is applied to the phones. The telephone-diaphragm vibrations for each wave train are indicated by view (c). The pulses of current indicated in Fig. 2 occur at an audio-frequency rate, 1,000 per second, hence the diaphragm in the telephone receivers can respond to the current variations and vibrates in synchron- ism with the current pulses. This vibration sets up sound waves that are sensed by the ear. Thus, every time that the transmitting key is depressed a 1,000-cycle note is heard in the phones.
In the case of radio telephony, the radio-frequency oscillations vary in accordance with the audio-frequency signals which it is desired to transmit. The radio-fre- quency input to the receiver is in the nature of a radio- frequency current whose amplitude is varying at an audio- frequency rate. The negative halves of the radio-fre- quency oscillations are chopped off by the action of the detector, and the phones record the audio-frequency varia- tions in the current in the detector output circuit. The term detector is misleading. This device is virtually a rectifier and the phones detect the incoming signal.
6 SR ADIO SRECEIVERS
CRYSTAL DETECTORS
There are a number of crystals that have unilateral conductivity; that is, they offer a high resistance to the passage of a current In one direction and a low resistance to the passage of current in the opposite direction. The following is a list of some of the crystals that have this property: iron pyrite, galena, molybdenum, bornite, and carborundum. A characteristic curve for crystals of this type is shown in Fig. 3. It will be noted that the voltage applied to the detector is plotted horizontally and the resultant current through the detector is plotted vertically.
A consideration of the character- istic crystal curve will reveal that, if an incoming oscillation produces approximately equal and opposite potential variations across the detector, the output current dur- ing the negative half cycle is negligible as compared with the output current during the positive half of the cycle. This effects detector action, or rectification.
Current
Fic. 3
VACUUM-TUBE DETECTORS
DIODE DETECTOR
A three-electrode vacuum tube may be used as a detector or an amplifier or as a combination of both, according to the method of making connections. When it is used as a detector alone, it functions as a diode, or two-element tube, and not as a triode, or three-element tube, so it follows that a two-electrode vacuum tube can also be used as a detector.
The method of connecting a three-electrode vacuum tube to make it function as a detector in a receiving circuit, without using its amplifying propensities, is shown in
AND SERVICING a
Vig. 4. The vacuum tube with its plate and grid con- nected together is inserted in place of the crystal detectro as previously’ explained. The plate-grid connection forms one terminal of this type of detector and the negative filament connec- tion is the other detector terminal. In the course of broadcast reception, there is small chance of distor- tion occuring in the detector circuit when this scheme of connections is used.
¢—~
Fic. 4
BIAS DETECTOR
The scheme of connections for the bias detector are shown in Fig. 5, and the characteristic plate-current grid-voltage curve for a three-element tube is shown in Fig. 6. The grid of the tube ts held sufficiently negative by means of the bias, or C, battery, Fig. 5, to cause the incoming signal voltage to operate on the lower bend of the plate-current curve as shown in Fig. 6. The operat- ing point on the plate-current curve is such that the negative half of an incoming potential oscillation causes far less change in the plate current than the positive half of the oscillation. A train of oscillations applied to the grid of this type of detector tube causes an average change of plate current which is positive, hence detector action and amplification are both effected. One of the advantages of this type of detector
Fie. 5
8 SR ADIO CRECEIVERS
is experienced in the course of radio broadcast reception.
The output of this type of detector is quite free from dis-
tortion caused by overloading,
because the grid of the tube is
held negative. Detector ac-
Alate-current tion takes place by virtue of
the fact that the negative half
of the incoming potential cycle
operates from the lower bend
Vga Brae of the plate current curve
downwards, and the positive
half of the incoming oscilla-
Grid Voltage tion operates from the lower
ae bend upwards. Thus, it would
be necessary for the grid voltage to be such as to carry
the plate current to a value on the upper end of the characteristic curve to produce distortion.
Another advantage of this type of detector over the diode type is that in the diode the unilateral impedance characteristic of the tube is the only feature that is made use of, whereas, in this case, its ability to amplify is made use of and the incoming signal voltage is applied to the grid of the tube; thus, its effect is multiplied in the plate circuit by the amplifica- tion factor of the tube.
Plate Current
C-Battery Voltage
DETECTOR USING GRID CON- DENSER AND GRID LEAK
A schematic circuit arrangement for effect-
. -A +B ing detector action by -B
using a grid leak a and Fig. 7
grid condenser b is shown in Fig. 7. The action is shown eraphically in Fig. 8. The incoming signal voltage is
» a — =|
AND SERVICING 9
applied to the grid of the tube c, Fig. 7, and the grid is alternately positive and negative. When the grid goes positive, it not only causes an increase in current to the plate of the tube, but the grid itself accumulates some of the electrons that are flowing from the filament toward the plate. When the grid goes negative it does not lose
+ Time ——- QS 0 : *® ss | Q'S | $o_ | | - SO i 3 Q - S sh Time required for § Grid Charge to Leak off
+
Lp
Plate Current
Ss
Time ——»
Motion of Feceiver
Time ——>
Ide, ots
all of the electrons that it has accumulated, owing to the fact that it takes longer for any appreciable amount of electrons of leak off, than the time duration of the negative half of the cycle of the oscillatory signal input, this being a function of the value of grid leak used. Thus, before the electrons leak off from the grid, the latter goes positive,
IO CR ADIO CRB GE) Livan iRes
again and accumulates more electrons. It is in this man- ner that the grid gradually becomes more and more negative during the passage of a wave train. The sub- sequent effect is to cause an average change in the plate current that is less than normal and it is in this manner that detector action is effected in this type of detector. The function of the grid leak a is to allow the negative charge on the grid to leak off between wave trains and, to prevent the electrons from leaking off between oscilla- tions. RECEPTION OF UNDAMPED WAVES
The fundamental circuits discussed have all been for the reception of waves whose amplitude changes at an audio- frequency rate. For instance, in the case of the signals from a quenched-spark transmitter, the wave-train fre- quency is, say, 1,000 cycles per second; therefore, the amplitude of the radio-frequency waves reaches its maxi- mum value and its minimum value 1,000 times per second, and it is by virtue of this fact that the detector is able to produce the desired sound which is heard in the ear phones.
In the case of icw. (interrupted continuous wave), the continuous waves generated at the transmitter are cut in and out at an audio-frequency rate by means of a chopper. If the chopper turns the radio-frequency oscillations of continuous amplitude on and off 1,000 times per second, the amplitude of the transmitted wave will reach its maximum and minimum 1,000 times per second. It is by virtue of this fact that detector action at the receiver produces audio-frequency sounds through the medium of the ear phones.
In the case of radio-telephone transmission and recep- tion, the amplitude of the radio-frequency oscillations generated varies at an audio frequency rate according to
AND SERVICING II
the frequency of the speech or music that it is desired to transmit. It is by virtue of this fact that detector action changes the modulated radio-frequency input into audio- frequency currents.
The reception of undamped, or continuous, waves differs somewhat from the foregoing. An undamped wave is one
+
(a)
(d)
(e) Fic. 9
whose amplitude remains constant and it is necessary that the amplitude of the radio-frequency signal applied to the detector should vary at an audio-frequency rate in order that the detector can function. To change the input into the audio-frequency desired, in the case of ew. it is neces- sary to provide means at the receiver of changing the amplitude of the radio-frequency input at an audio- 4—2
12 SR ADIO SRECEIVERS
frequency rate so that its presence may be manifested by sounds in the ear phones.
In Fig. 9 (a) is shown the nature of the incoming ew. signal that is applied to the detector in the circuit shown in Fig. 10, which is a schematic wiring diagram of a cir- cuit for the reception of undamped-wave signals. The traveling wave is intercepted by the antenna. The induced currents in the antenna system pass through the antenna coil a, setting up a magnetic field around this coil. This magnetic field threads through the coil 5b, inducing currents therein of the same frequency as the induced currents in the antenna circuit. Owing to the
fact that coil 6 and con-
denser ¢ are tuned to the
same frequency as that a
of the incoming wave, there will be maximum voltage across the coil b and the condenser c. There is a local genera- tor d of radio-frequency oscillations having a frequency 1,000 cycles greater (or less) than that of the incoming signal. Energy at this frequency is induced into the detector circuit by means of the coupling coils e and f. The current generated by the local oascillator is represented in view (6b), Fig. 9. Thus these two frequencies are superimposed one upon the other and there is a resultant beat frequency, shown in view c, which is equal to the difference between the two radio frequencies, or 1,000 cycles. The amplitudes of views (a) and (b) are simply added for each interval of time, and the result, as shown in view (c), is an alternat- ing current of periodically increasing and decreasing amplitude, the alternating current being at radio fre-
isis, 1G
AND SER VIECING 13
quency, and the rate of change of its amplitude, from maxi- mum to minimum, being at audio frequency. The nature of the current in the output circuit of the detector is shown in view (d). The negative half of each radio-frequency oscillation is chopped off; hence, the average change in the rectified current occurs at an audio-frequency rate, and it is this audio-frequency change in the rectified cur- rent, shown in view (e), that actuates the diaphragm of the telephone receivers.
Considering Fig. 10, a vacuum tube could be used at d to generate the radio-frequency oscillations and a crystal detector at g. Again, a single three-electrode tube could be used in a circuit to function as a detector, an amplifier, and an oscillator, as will be explained later.
INTERCEPTION AND DETECTION
From the foregoing discussion it is found that the recep- tion of radio signals is fundamentally a case of intercep- tion and detection. ‘There must be a means of intercept- ing the electro-magnetic waves travelling through the ether and a means of changing the radio-frequency cur- rents induced in the antenna system into audio-frequency currents so that they may in turn be changed into sounds _of audio frequency intelligible to the human ear. The crystal detector changes the radio-frequency current into an audio-frequency current and the telephone receivers effect the change from audio-frequency currents to audio- frequency sound waves.
Greater sensitivity, or reception from a greater dis- tance, is effected by increasing the amount of signal energy applied to the detector. This can be accomplished by increasing the efficiency of the antenna system or by amplifying the radio-frequency input before application to the detector.
14 | SRADIO “RECEIVERS
Greater output volume with a given amount of signal energy available in the detector output circuit is a func- tion of the amount of audio-frequency amplification effected.
The fundamental elements involved in radio reception have now been considered. The next consideration will be the different types of receivers, so that it may be learned how the fundamental elements are embodied in the various receivers designed for different wavelengths and duties.
REGENERATIVE RECEIVERS SINGLE-CIRCUIT RECEIVER
The schematic wiring diagram of a single-circuit receiver is shown in Fig. 11. The detector input circuit is tuned by means of the inductance coil a and the condenser 6. The antenna lead is connected directly to the grid input coil a, and the ground is connected directly to the low end of this coil. The grounded end is also connected to the rotor plates of the tuning condenser 6.
A grid condenser c and grid leak d are connected in series with the grid lead to effect detector action. The grid return , is connected to the nega- tive filament terminal. This is the scheme of connections when using a
= Ata +8 UX-200-A detector tube. > Fic. 11 8B :
With some other types of tubes slightly better results are obtained by bringing the erid return to the positive filament terminal.
The plate of the tube is connected to the positive B-battery terminal through the feed-back coil e and the
AND SERVICING 15
phones f. It is by means of the inductive relation between the feed-back coil e and the input coil a that regeneration is effected. Regeneration is the feeding back of the radio- frequency signal energy from the plate circuit to the grid circuit, thus allowing it to be reamplified, or boosted, again. This regenerative effect may be carried to the point of oscillation. ‘This point is where the tube starts to oscillate, thus generating oscillations of continuous amplitude and of a frequency that is determined by the constants of the tuned input circuit, consisting of the coil a and the condenser 0.
A circuit of this type can be used to receive damped or undamped radio telegraph signals or it can be used for radio telephone reception. With all the elaborate receiv- ing circuits that are in existence today, there are many that can not equal the performance of this little single- circuit receiver that was one of the first types of broadcast receivers to appear.
300- TO 19,000-METER COMMERCIAL RECEIVER
A schematic wiring diagram of a standard commercial _ receiver that is used on ships for the reception of telegraph and telephone signals on wavelengths between 300 and 19,000 meters is shown in Fig. 12. The receiver proper * provides adequate switching arrangement for covering all wavelengths between 300 and 8,000 meters, employing either a crystal detector or a vacuum-tube regenerative detector. There is a long-wave attachment for this set that allows for tuning in signals on wavelengths as high as 19,000 meters. There is also a two-step amplifier attach- ment for increasing the output volume of the received signals.
The antenna is connected to a contact arm that can be moved to different taps on the primary winding a. The
CR ADIO SRECEIVERS
16
ACN) RV. CLIN, G 17
low end of this winding is connected to the stator plates ofa .00045-microfarad variable tuning condenser b through two external terminals c. These two external terminals are jumpered together when the receiver is being used on the 300- to 8,000-meter band, but when it is desired to tune in stations between 8,000 and 19,000 meters a primary loading coil is inserted at this point in the circuit. The rotor plates of the primary tuning condenser b are connected to ground.
The secondary circuit is coupled to the primary circuit by means of the coupling coil d, which is inductively coupled to coila. Other than the coupling just mentioned, there is no coupling between the primary and secondary circuits. A shield e is inserted between the two circuits.
The .00032-microfarad secondary tuning condenser f is shunted across the three coils d, g, and h. Coil d is the coupling coil between the primary and secondary circuits; coil g is the coupling coil between the plate and grid of the detector tube (when such is used) to effect regeneration; and coil A is the tapped secondary tuning coil. Two external terminals 7 are connected in series with the secondary inductance coils to allow for the insertion of a secondary loading inductance for tuning above 8,000 meters.
The stator plates of the secondary tuning condenser f are connected to one of the poles of a four-pole double- throw switch, which is used to change from a crystal detector to a vacuum-tube detector. The two positions of this switch may be designated by T and C, T being the tube position and C the crystal position. When the switch is in the tube position, the high side (stator plates) of the condenser f is connected to the grid of the tube through the grid leak and grid condenser unit. When the switch is in the crystal position, the high side of the con- denser is connected to one terminal of the crystal detector.
18 CR ADIO SRECEIVERS
The plate of the vacuum tube is connected to the feed- back coils 7, the two external terminals k for the long-wave tickler, and the contacts of the change-over switch. These contacts connect the reactance I to the plate of the tube in the tube position, and to the second terminal of the crystal detector in the crystal position.
This reactance J is tuned by means of the condensers m and n to the frequency of the signal energy in the output of the detector (audio frequency). The other end of this reactance J is connected through the phones o to the positive B-battery terminal or to the rotor plates of the secondary tuning condenser f, according to whether the change-over switch is in the tube or the crystal position. One of the filament leads to the vacuum tube passes through the contacts of the change-over switch, so that the tube filament is not energized when the crystal detector is being used. A push button p is provided, which, when depressed, short-circuits the plate coil 7. This is known as the oscillation test.
This type of receiver is to be found on the majority of ships at sea at the present time. ‘There may come a time within the next few years when the same receiver will be used with a stage or two of radio-frequency amplification ahead of it, but results obtained with this set are at pres- sent of such a high standard that it will be some time before it will be superseded by a later model.
REGENERATION BY TUNED-PLATE METHOD
In Fig. 13 is shown a method of effecting regeneration without establishing inductive coupling between the plate and the grid coils. The antenna circuit is aperiodic (untuned), and the grid input circuit is tuned to the incom- ing signal by means of the inductance coil a and the condenser b.
AN DOSER V UCINIG 19
Regeneration is effected by tuning the plate circuit to the incoming signal by means of the inductance coil ¢ and the condenser d. The feed-back from the plate to the grid circuit of the tube is effected by virtue of the capacity coupling between the grid and the plate that is inherent within the tube itself.
RADIO-FREQUENCY AMPLIFIERS
UNTUNED-TRANSFORMER COUPLED RADIO-FREQUENCY RECEIVER
A schematic wiring diagram of a receiver that has three stages of radio-frequency amplification ahead of the detector is shown in Fig. 14. This receiver employs
-A +A +B -B
reels
fl
untuned transformers between the stages of radio-fre- quency amplification. ‘The antenna is connected to one side of the primary winding of the first radio-frequency transformer a and the ground lead is connected to the other end of the same winding. One side of the secondary winding is connected to the grid terminal of the first radio- frequency amplifier tube b and the other end of the same winding is connected to the movable contact arm of the 400-ohm stabilizing potentiometer c. The extremities of this potentiometer are connected across the A-battery supply leads.
20
ms! () ()
SRADIO “RECEIVERS
Fic. 14
AND SERVICING 21
The two remaining radio-frequency transformers d and e have their primary windings connected in series with the plates of the two radio-frequency amplifier tubes 6 and f, respectively, and their secondary windings in series with the grid circuits of the two radio-frequency amplifier tubes f and g, respectively.
The low side of the secondary winding of each of the three radio-frequency transformers is connected to the movable contact arm of the stabilizing potentiometer c. The function of the stabilizing potentiometer is to afford a means of supplying a small positive potential to the grids of the three tubes in question, which tends to keep them from oscillating.
There is no frequency selection ahead of the detector tube in a receiving circuit of this type. All energy induced in the receiving antenna is passed on to the first radio- frequency amplifier tube and thence to the two succeeding stages, where all incoming radio signals are boosted in voltage for application to the detector tube. The function of the radio-frequency amplifier system in this circuit is to step up the voltage of all the radio-frequency signals that reach it through the medium of the antenna.
The input to the detector is tuned. It is here that a selection is made of the particular signal that it is desired to receive. ‘The tube g may be considered the output tube of the radio-frequency amplifier. The output circuit of this tube is coupled to the grid circuit of the detector tube h through the plate coil 7 and the grid coil 7. The detector input circuit is made selective by means of the tuned cir- cuit consisting of the coil 7 and the condenser k. The wavelength range depends on the values of the inductance and capacity of these devices.
Regeneration is effected in this circuit by means of the coil / in series with the detector plate circuit and coupled
24) ‘RADIO SRECEIVERS
to the detector input circuit by means of the inductive relation between the coils 7 and l. The condenser m is a radio-frequency by-pass condenser, which functions to by-pass the radio-frequency currents in the plate circuit of the detector tube around the phones n and the B bat- tery, as they offer a relatively high impedance to the pas- sage of currents at radio frequencies. The impedance of the path for radio-frequency currents in the plate circuit of a regenerative detector tube should be as low as possible so as to effect regeneration, if desirable, to a value just below the oscillating point.
ONE-STAGE TUNED RADIO-FREQUENCY WITH FEED-BACK
Circuit Connections.—It was previously pointed out that the old single-circuit receiver with regeneration was one that would offer good competition to many of the elaborate receivers that have been produced since the inception of radio broadcasting. A modification of this receiver is shown in Fig. 15. There is one stage of tuned radio-frequency amplification which boosts the signal voltage before application to the detector tube and also selects the frequencies desired. Regeneration is effected by a feed-back from the plate circuit of the radio-fre- quency amplifier tube to the antenna circuit by means of the coupler a.
The grid circuit of the radio-frequency stage is really a radio-frequency filter. It effects the greatest voltage for application to the grid of the radio-frequency amplifier tube at that frequency to which it is tuned. I¢ will also pass frequencies several thousand cycles greater and several thousand cycles less than that frequency to which it is tuned, but with less and less efficiency, depending on the number of cycles difference between the frequency in question and the fundamental frequency and on the sharp-
AND SERVICING 23
ness of tuning of the circuit containing the inductance coil b and the condenser c. This sharpness of tuning is a funetion of the amount of resistance in the tuned circuit; the less the resistance the sharper the tuning and the greater the resistance the broader the tuning.
In the course of radio broadcast reception the receiving antenna is subjected to the field of a traveling wave con- sisting of a carrier frequency with side bands usually up to 5,000 or 10,000 cycles on either side of the carrier. The carrier is the radio frequency that is generated by the
Bias
Fre. 15
apparatus in the transmitter and upon which the audio frequencies that are to be transmitted are superimposed. The audio frequencies that are usually transmitted in the course of a radio broadcast are those up to 5,000 cycles. The superimposition of this 5,000-cycle audio-frequency band on the carrier effects the transmission of what is termed the upper side band, with limits of the carrier frequency and the carrier frequency plus 5,000, and the transmission of what is termed the lower side band, with limits of the carrier frequency and the carrier frequency minus 5,000 cycles. ‘Thus it can be seen that it is necessary to pass a 10,000-cycle band through the tuning circuits.
24 FRADIO SRECEIVERS
It is very fine to have sharpness of tuning, or selectivity, which means the passage of a narrow band of frequencies, but it is not desirable to have too great a degree of selec- tivity, as this would mean that some of the frequencies in the side bands would be chopped off and distortion would ensue. In order to effect undistorted reception the loud speaker must reproduce all the audio frequencies that are trans- mitted by the broadcasting station, it being assumed that the broadcasting station isputting out anundistorted signal.
List and Description of Parts.—It is important in con- sidering the construction of radio broadcast receivers to obtain the best apparatus. If inferior apparatus is used,
Sy Lf
Gh
Fic. 16 Fic. 17
it may work well for a short time, but there is no assurance that satisfaction will be long-lived. In Fig. 15 the follow- ing apparatus is required:
a—Variocoupler. This device consists of two coils, a stator a, Fig. 16, and a rotor b. Coil a is wound with 30 turns No. 24 d.c.c. (double-cotton covered) wire on a 3-inch form. Coil b is wound with 30 turns No. 24 d.e.e. on a form that is free to turn within the 3-inch form of coil a.
b—Inductance coil, Fig. 15. Thisis a 44-turn spiderweb coil tapped at fourth turn for the antenna connection, and wound with No. 24 d.c.c. on a 2-inch form, as shown in Fig. 17.
AND |) SERVICING 25
ec and d—Variable condensers, Fig. 15, .00035 micro- farad.
e—Grid condenser, fixed, .00025 microfarad.
f—By-pass condenser, fixed, .1 microfarad.
g—By-pass condenser, fixed, .002 microfarad.
h—Radio-frequency transformer. ‘The secondary wind- ing s is a 44-turn spider-web coil, No. 24 d.c.c. on a 2-inch form. The primary winding 7: consists of 6 turns, No. 24 d.c.c. wound on the outside of the secondary coil.
.2 and j—10-ohm rheostats.
k—400-ohm potentiometer.
l—Grid leak, 3 megohms.
m—Vacuum tube, UX-201-A and socket.
n—Vacuum tube, UX-200-A and socket.
In addition to the foregoing, it will be necessary to have a panel; a base board; about 15 feet of bus wire; control knobs for the variable condensers, rheostats, potentio- meter, and variocoupler; the required A and B batteries, (A, 6 volts, B 90 volts, tapped at center for detector-plate connection) ; and a pair of telephone receivers (2,000 ohms). If a two-stage audio-frequency amplifier is used in con- junction with this receiver, it is permissible to have the radio- and audio-amplifier tube filament temperature controlled by the same rheostat.
TWO-STAGE TUNED RADIO-FREQUENCY RECEIVER
Circuit Diagram and List of Parts.—The one-step tuned radio-frequency receiver is the first step beyond the single- circuit tuner, and the two-step tuned radio-frequency receiver 1s the next step beyond the former. It is quite easy to construct a receiver having a single stage of radio- frequency amplification that will operate with satisfactory stability, but it is not so easy to effect stable operation with two stages of radio-frequency amplification, because
26 (RADIO SRECEIVERS
the inductive and capacitive feed-backs between the radio- frequency stages tend to cause the radio-frequency amplifier tubes to oscillate.
The inductive feed-back is caused mostly by the inter- linking of flux from the tuning coils in the radio-frequency amplifier stages, and the capacity feed-back is caused mostly by the inherent electrode capacity within the tubes themselves. In this receiver the inductive coupling between successive radio-frequency stages has been minimized by the use of closed field coils. These coils are termed D-coils, or figure-8 coils.
A schematic wiring diagram of the receiver under consideration is shown in Fig. 18. The actual apparatus used in the construction of this set is shown in Fig. 19. The following is a list of the material.
a—D-Coil, 3-inch diameter with 14-turn primary and 56-turn secondary.
b—D-Coil 3-inch diameter with single 56-turn winding (tapped at turn 14).
c—D-Coil 3-inch diameter with 14-turn primary and 56-turn secondary.
d—<Audio-frequency transformer (6 to 1).
e—Audio-frequency transformer (2 to 1).
f—Output transformer (1 to 1).
g—.0005-microfarad variable condenser.
h—.0005-microfarad variable condenser. .
i—.00025-microfarad variable condenser.
j—.0005-microfarad variable condenser. k—.00025-microfarad grid condenser.
I—.002-microfarad by-pass condenser.
m—Four .1-microfarad by-pass condensers.
n—200-ohm potentiometer.
o—6-ohm rheostat.
p—10-ohm rheostat.
27
ol SERVICING
Ob - INES ogee G29 +
O06 +
O'SE/ ON ie
= ail
SI “OIA lasts as eR) oe eee Jseee =)
tla | | 4
ib
Se
io)
TE Q0Q0)
WOOOD
~
Am
28
CR ADIO ‘RECEIVERS
Bos
L
AND SERVICING 29
g—5-megohm grid leak.
r—3 megohm grid leak.
s—Two double-circuit output jacks.
t—Single-circuit output jack.
u—Filament switch.
v and w—Two UX-201-A amplifier tubes and sockets.
x—U X-200-A detector tube and socket.
y—UX-201-A amplifier tube and socket.
_g—UX-171- power-amplifier tube and socket.
The above is a list of the material that was used by the writer in the construction of a receiver to aid in the description of the functioning of this particular type of set.
Construction of Radio-Frequency Transformers.—The main feature in the receiver shown in Figs. 18 and 19 is the type of radio-frequency transformer used to minimize interstage inductive coupling, and, although this type of coll has been used commercially for some time, the writer had the honor of being the first to present them to the broadeast public through the medium of radio magazines. When the first receivers of this type were constructed there were no coils on the market that were of the particular type embodied in this set, so it was necessary for the set builder to construct them himself. Therefore, the construction of a radio-frequency transformer of the D-coil or figure-8 type will be discussed and this discussion and subsequent theory should give one a good idea of their inherent characteristics.
The following is the description of the construction of the D-coil: Procure a piece of bakelite tubing 3 inches in diameter and 34 inches long. Cut a slit 4 inch wide through the side of the tube extending from one end to within 2 inch of the other end. Cut a similar slit directly opposite. The transformers a, b, c, Fig 19, show the physical characteristics of the D-coil. Four terminals,
hel
L----}-/fA ~~ t—--~--
= coimille All wi
AINED |S eR EVIL CO LNG 31
which may be labeled 1, 2, 3, and 4, are located around the end of the tube that is not slit. The reason for putting them at this end is because there is space that is free from the winding and this end is more solid, as there is no slit in it. Two of the four binding posts on each trans- former are shown in Fig. 19.
Use No. 24 d.c.c. copper wire. A half-pound spool will have enough wire for all three transformers. When preparing to wind either transformer a or c cut off 20 feet of the wire from the spool for the primary coil. Fasten one end of this primary wire to the inside of terminal 2. Fasten one end of the wire left on the spool to the inside of terminal 3. This wire will form the secondary coil. Wind both of these wires together through a slit and first around one half of the form, then through the opposite slit and around the other half of the form. This is continued until 14 complete turns have been wound on the form, whereupon the free end of the primary winding is brought to terminal 1 and connected thereto. The secondary wind- ing is continued until 56 turns have been wound. ‘The free end of the secondary winding is then connected to termi- nal4. The terminals /, 2, 3, and 4 of transformers a and c, Fig. 18, are connected in the circuit as indicated in the figure.
The coil 6, Fig. 19, is wound in a manner similar to coils a and c, except there is only one winding made up of 56 turns of No. 24 d.c.c. Coil bis tapped at the fourteenth turn and from there is connected to the plate of the tube v, Fig. 18.
Radio-Frequency Receivers With Figure-8 Coils.—A circuit digram of a receiving set with two stages of radio- frequency amplification, a detector, and three stages of transformer and choke-coil coupled audio-frequency amplification is shown in Fig. 20. The construction of this set is shown in Fig. 21. The following is a list of parts indicated in Fig. 20.
a2 “RADIO
Fic. 21
SRECEIVERS
a—Feed-back coupler.
b—Three figure-8 radio- frequency transformers.
c—A u dio - frequency transformer (6 to 1).
d—200-henry impedance.
é—Double - impedance coupler.
f—Speaker filter.
g—Three .00035-micro- farad straight - line fre- quency condensers.
h—.00025 - microfarad grid condenser.
a—Three .05-microfarad fixed condensers.
j—Two .1-microfarad by- pass condensers.
k—.002 - microfarad by - pass condenser.
l—Three .00025-micro- farad fixed condensers.
m—200-ohm potentio- meter.
n—'T wo 6-ohm rheostats.
o—10-ohm rheostat.
p—s-megohm grid leak.
g—500,000-ohm poten- tiometer.
r—100,000-ohm grid leak.
s—single-circuit output jack.
—Filament switch.
u—6-point switch.
AND SERVICING ae]
v—Two UX-201-A amplifier tubes and sockets.
w—UX-200-A detector tube and socket.
x—-Two UX-201-A amplifier tubes and sockets.
y—UX-171 power amplifier tube and socket.
Since the main feature of the receiver shown in Fig. 20 as well as of that shown in Fig. 18 is its ability to minimize inductive interstage coupling, it will be well to find the reason for this effect. The drawing shown in Fig. 22 will aid in the explanation of the theory of the closed-field coil in question. This theory applies to both the double-D and the figure-8 coils.
A current through the secondary winding of this transformer passes in one direction through the wind- ing on the side marked a and in the opposite direc- tion through the winding on the side marked b. The lines of force emanating from section a and those emanating from section 6 are in opposite directions. These lines are additive through the centers of section a and 6. The effects of the stray lines of force in opposite directions from sections a and 6, in the area surrounding the coil, have a tendency to neu- tralize, as may be observed from the arrowheads on the lines representing the lines of force in Fig. 22.
The flux density, caused by the current through the wind- ings of the sections a and ), is greatest through the centers of the two sections and is a minimum around the outside of the coil. This is the reason why this type of coil is termed a close-field coil. The field is closed through the center of the two sections. ‘The foregoing discussion also shows why the external field is a minimum.
j2iek ee
34. FRADIO SRECEIVERS
NEUTRODYNE RECEIVER
Neutrodyne-Receiver Theory.—If efficient and stable operation in a radio-frequency amplifier system is desired, it is necessary to eliminate both the inductive and capaci- tive feed-back. The D-coil or figure-8 coil type of receiver shows one method of eliminating, to a great extent, the inductive feed-back. In the neutrodyne receiver a method of neutralizing the capacity feed-back is put into practice.
Thus, with the inception of capacity neutralization for receiving sets it became possible to eliminate both the inductive and the capacitive feed-back. A combination figure-8 coil receiver with capacity neutralization con- stitutes a decidedly worthwhile receiver. In the majority of the standard neutrodyne receivers, which feature capacity neutralization, inductive coupling is minimized by setting the coils at a definite angle to each other. The position of the coils causes the lines of force emanating from one coil to pass through the other coils in a direction parallel to the wires that constitute the winding of the coil through which the flux lines are passing. As long as the lines of force from one coil remain parallel to the wires in the winding of a second coil, there will be no flux inter- linkage and the inductive coupling will be zero.
A schematic wiring diagram of a section of a radio-fre- quency amplifier is shown in Fig. 23, the radio-frequency amplifier tubes being shown at a and b. The input circuit of tube a is tuned to the frequency of the incoming signal by means of the inductance coil ¢ and the condenser d. The condenser e, shown by dotted lines, represents the internal plate-grid capacity of the tube a. The output circuit of tube a is tuned to the incoming signal by virtue of the close coupling between the coils f and g, the latter
AND SERVICING 35
coil in parallel with the condenser h being tuned to the same frequency as the combination of coil ¢ and con- denser d. The condenser 72 is a by-pass condenser for radio-frequency current from the positive B-battery terminal to the negative A-battery terminal.
The maximum signal voltage in the output circuit of tube a can be considered as existing across the inductance coil f in view of the fact that the lower end of the coil marked 2 is practically at ground potential, owing to the radio-frequency by-pass condenser 7, which is of sufficiently large value to offer very little impedance to the passage of radio-frequency currents.
'
' «.: e < Pid
‘
i}
1
This maximum signal voltage that exists across the coil f also exists across the internal plate-grid capacity of the tube, represented at e, and the effective resistance of the erid-filament circuit, represented at 7. The two quantities in question are connected in series from the upper and lower terminals of coil f, shown at / and 2. The reason why the grid to filament circuit can be considered an effec- tive resistance j is due to the fact that the voltage that is considered is the voltage that is applied across the coil f, and the frequency of this voltage is the frequency to which the combination cd is tuned. Therefore, for this frequency (the resonant frequency), the capacity reac-
36 FRADIO “RECEIVERS
tance and the inductive reactance of the circuit cd neu- tralize, and the resistance of the circuit is all that is left to impede the passage of current at its resonant frequency.
If the frequency of the voltage across the coil f were greater than the resonant frequency of the circuit cd, the latter would be an effective capacity, and if the frequency of the voltage across the coil f were less than the resonant frequency of the circuit cd, the latter would be an effec- tive inductance.
When the capacity e is quite small, its reactance is quite large, capacity reactance being expressed by the formula
r c
=——— ohms 2nfC in which X,.=capacity reactance, in ohms; a=constant 3.1416; f=frequency, in cycles per second; C'=capacity, in farads.
From the foregoing it can be seen that the potential at the grid terminal of tube a, due to the voltage across coil f, is above that of the ground by virtue of the cur- rent from filament to grid, but is nearer the lower end 2 of coil f, owing to the fact that the voltage drop across condenser e is much greater than the drop from grid to filament.
If at any instant the polarity at point / is positive, then the polarity at point 2 is negative, the two points being 180° out of phase. The grid, being nearer to point 2 than to point 7, will be negative when point / is positive, and this is the condition for regeneration, for here is a voltage on the grid of the tube a that is of the same frequency as the voltage in the plate circuit of the tube and, furthermore, this voltage on the grid is negative when the voltage at the
AND» SERVICING 27
plate is positive. It is to be noted that this also is the condition for self-oscillations: excitation voltage on the grid, 180° out of phase with the plate voltage.
It might be interesting to note why the tube has a greater tendency to oscillate on the lower wavelengths, during the course of patrolling the broadcast wave band, than on the higher waves. As the wavelength decreases the frequency increases. As the frequency increases the reactance of the internal plate-grid tube capacity decreases. Both of these facts can be substantiated by considering the formula for converting wavelength to frequency, in which
__ 800,000,000 ~ wavelength
and the formula for capacity reactance, in which
1 on 2nfC
As the reactance of the tube capacity decreases the voltage drop across it also decreases and the potential on the grid becomes higher, hence more grid excitation, greater regeneration, and, subsequently, greater tendency to oscil- late.
One method of applying a neutralizing condenser to a stage of radio-frequency amplification is shown at k. The neutralizing scheme is virtuaily a wheatstone bridge. The neutralizing condenser k is connected from the grid terminal of tube 6 to the grid terminal of tube a. The points 2 and 3 are at the same potential, so far as the high- frequency currents are concerned, owing to the fact that point 2 is a radio-frequency ground, through the medium of the radio-frequency by-pass condenser 7, and point 3 is metallically connected to the negative filament lead, which is at ground potential.
38 FRADIO SRECEIVERS
The signal voltage that is being fed back from the output circuit of tube a can be considered, in this case, as existing across the points / and 4 with the intermediate points 2 and 3 at ground potential. If the coil f is equal to coil g, the point 2 or 3 is midway between the extremities / and 4; hence if the grid is also made midway between / and 4, as far as the voltage across the coils f and g is concerned, the grid will be at ground potentials with respect to the feed- back voltage, because the points 2 and 3 are at ground potential. In this case, this can be accomplished by making the neutralizing condenser k& equal to the plate- grid capacity e.
In receiving tubes of the UX-201-A type the plate- grid capacity is of the order of 6 micro-microfarads, thus the neutralizing condenser k should have nearly the same capacity. However, in most radio receivers employing tuned radio-frequency amplification, there is more induc- tance in the coil g than there is in the coil f. This is due to the fact that there is a step-up ratio effected in the coupling transformer between the output circuit of the tube a and the input circuit of the tube b. The reason for this is to boost the signal voltage available in the output circuit of one tube, through the medium of the coupling transformer, for application to the grid of a succeeding tube.
If the inductance of the coil g is four times the inductance of the coil f, four-fifths of the voltage drop from 1 to 4 will occur across coil g, and in order that condenser k should function to maintain the grid at the same potential as the points 2 and 3, there should be the same voltage drop across it that there is across coil g. This can be effected by making the value of condenser k one-fourth that of condenser e, for the capacity reactance varies inversely as the value of the capacity, the equation for
AN DS ER VLOUNG 39
capacity reactance being,
aE: | +] +] #/ 7 +e h Me= oO nfC whereas the inductive reactance XS varies directly as the
value of inductance, the equation being, X, Soom LA
Therefore, if the value of the capacity of k is one-fourth that of e, the reactance of the former will be four times greater than that of the latter and the voltage drop across the former will subsequently be four times greater than the voltage drop across the latter. This is the con- dition that must exist to maintain the grid at the same potential as points 2 and 3, which means that it is at ground potential as far as the feed-back voltage is con- cerned. If condenser e has a capacity value of about 6 micro-micro- farads, then 1.5 micro- microfarads will be re- quired at k for complete neutralization.
Fic. 24
0000 ie
40 CRADIO °RECEIVERS
Standard Neutrodyne Receiver, 200 to 300 Meters.—A schematic wiring diagram of a standard neutrodyne receiver is shown in Fig. 24. The following is a list of the receiving-circuit constants as well as a list of the material needed in the construction of this set.
a—Three radio-frequency transformers, the primary of which consists of 13 turns No. 24 d.s.c. (double-silk covered) on 23-inch form and the secondary of 50 turns No. 24 d.s.c. on the same form.
b—Three .0005-microfarad variable condensers.
c—6-ohm rheostat.
d—12-ohm rheostat.
e—3 megohm grid-leak resistance.
f—.00025-microfarad grid condenser.
g—.002-microfarad radio-frequency by-pass condenser. h—.1-microfarad by-pass condenser
7—Two 1.5-micro-microfarad neutralizing condenser.
4—Two UX-201-A amplifier tubes and sockets.
k—U X-200-A detector tube and socket.
The antenna lead, Fig. 24, is connected to one end of fe primary coil of the first PriNeirectt toe transformer, the other end of which is connected to ground. The secondary winding of this transformer is tuned by means of the .0005- microfarad variable condenser 6. The ground lead is con- nected through to the negative filament lead. The rheo- stats are in the positive filament lead.
The primary winding of the second-radio-frequency transformer is in series with the plate circuit to the first radio-frequency amplifier tube. The secondary winding of this transformer is also tuned by means of a .0005-micro- farad variable condenser. |
The B-battery supply for the plates of the radio-fre- quency amplifier tubes 7 should be between 67.5 and 90 volts.
AN De {SERV LCUN-G 41
The neutralizing condensers 7 are connected from grid to grid. From a consideration of the preceding discussion of neutrodyne theory the value of these condensers should be of the order of 1.5 micro-microfarads, if the internal plate-grid capacity of the tubes 7 is of the order of 6 micro- microfarads. It is a good idea to use variable condensers at 7 of such a maximum value that it is possible to pass through the optimum point.
The solenoidal type of coils used in this receiver have a large stray field and it is necessary to minimize the effect to as great an extent as possible. The figure-8 type trans- formers do not have a large stray field, but when solenoidal transformers are used, the inductive effect between the trans- formers must be in some way controlled. One method of doing this is shown in Fig. 25.
The three transformers a,b, and care tilted. The BiG 49 line of force emanating from coil a pass through the winding of coil b in such a direction that they are approximately parallel to the wires that constitute the winding of coil b. If the lines of force from coil a do not cut through any of the winding of coil b, there will be no coupling effected between the two coils.
The lines of foree emanating from coil b pass through the winding of coil c approximately parallel to the wires in that winding and the lines of force from coil b also pass through coil c approximately parallel to the wires that constitute the latter coil. In this manner inductive coupling is minimized but it is not eliminated entirely. It is worth
42 SRADIO SRECEIVERS
while to have the coils mounted in such a manner that they can be tilted at any angle, from zero to 90° with the horizontal. If this is done it is possible to orient the coils to the point of minimum inductive coupling and this angle is somewhat critical.
Other Methods of Stabilizing Radio-Frequency Ampli- fiers.—Besides the potentiometer and neutrodyne methods of suppressing oscillations, there are other ways of obtain- ing similar results. In a large number of commercial receivers, grid resistors are used in the radio-frequency stages to minimize the tendency to self-oscillation. The grid resistors, as the name implies, are connected in the grid circuits of the radio-frequency amplifier tubes. The most common position is between the grid of the tube and the stator plates of the tuning condenser. A resistance of about 1,000 ohms in each of the radio-frequency stages will in most cases maintain the amplifier in a stable con- dition.
Another favorite method of stabilizing radio-frequency amplifiers is to change the overall efficiency of the set by controlling either the filament or the plate current of the radio-frequency tubes. In a large number of sets the rheostat controlling the filament current of the radio- frequency tubes acts also as a volume control and, inci- dentally, as an effective means of reducing the energy to the point where the set remains stable.
Then, there are the so-called “losser’’ methods. A shorted coil mounted near the tuning coil will absorb sufficient energy to effect stable operation. Similarly, by mounting the tuning coils near the variable condenser, sufficient energy will be dissipated in eddy currents to obtain the same results.
The proper use of, the shield-grid tube will eliminate any tendency to self-oscillation and, at the same time, obtain
AND SERVICING 43
extremely high amplification. The manufacturer’s instructions accompanying this tube should be followed in all cases.
There are numerous other ways of preventing radio- frequency amplifiers from oscillating and introducing distortion into the receiver. Most of these are based on the introduction of opposing voltages, on proper balancing, on losses, and on changing the phase relation between the Individual circuits.
Shielding.—Shielding is a means of confining the mag- netic fields of coils and conductors to a restricted area. To be effective, shielding must be designed with proper relation to the parts to be protected. When correctly applied, it increases the sensitivity and selectivity of a receiver because the shields exclude external disturbances an minimize internal interference.
Electromagnetic shielding, to be effective, must be complete. The smallest crack or opening is sufficient to spoil the whole receiver and it is imperative, therefore, that great pains be taken with the work. Shielding is not purely a mechanical operation, as it requires technical design as well, based on the action of the radio-frequency circuits in the set.
The following facts compiled by the Aluminum Com- pany of America apply to the art of shielding:
1. Within limits, the effectiveness of shielding increases with the frequency and with the conductivity and thick- ness of the metal sheet used.
2. At frequencies in the broadcast range, relatively thin sheets of aluminum or copper aresatisfactory. Number 20 B. & S. gauge and heavier is used.
3. Aluminum and copper of the same thickness are equally efficient, for practical purposes, in radio-frequency shielding.
4—4
44 RADIO. RRC E DYES
4. Complete metal shields of the can or box type, well grounded are the most effective.
5. Such shields should make good electrical contact at the joints, and the holes or outlets should be kept as few as possible.
REFLEX RECEIVER
In a reflex receiver, the amplifier tubes are made to function as radio-frequency amplifiers and as audio-fre- quency amplifiers also. The neutrodyne receiver lends itself best for reflexing, since the radio-frequency amplifier circuits are so well neutralized that 90 volts can be applied to the plates of the tubes without danger of unstability of operation. It is also to be noted that the normal voltage for an audio-frequency amplifier, using UX-201-A type tubes, is 90 volts. A radio-frequency receiver that is so unstable in operation that no more than 45 volts can be applied to the plates of the radio-frequency amplifier tubes without making them oscillate is not particularly adapted for use as a reflex receiver, since the maximum allowable plate voltage would be limited to 45 volts by the radio- frequency amplifier tubes. This would mean that the audio-frequency amplifier tubes would have to be operated at this potential, which would not be conducive to efficient reflex amplification, and a separate audio-frequency amplifier should be used.
The schematic wiring diagram, Fig. 26, shows a receiver employing three tubes a, b, and c, having two stages of radio-frequency amplification, a detector, and two stages of audio-frequency amplification. The constants that applied in the case of the neutrodyne receiver, apply here as well, with the addition of two-audio-frequency trans- formers and several fixed condensers. The transformer d has a 6 to 1 ratio of turns, while the transformer e has a 2 to 1 ratio of turns.
9¢ “OT
46 . “RADIO SRECEIVERS
The radio-frequency circuits have already been dis- cussed, hence the audio-frequency circuits which are reflexed, will now be considered.
The plate terminal of the detector tube c is connected to one end of the primary winding of the first audio frequency transformer d. There is a .002-microfarad radio-frequency by-pass condenser across this winding. If desirable, regeneration may be effected in the detector plate circuit by means of a variometer connected in series with the detector plate lead. This variometer should be capable of tuning the plate circuit in question to the various frequencies in the wave band for which the receiver is designed.
The secondary winding of the first audio-frequency transformer d is connected in series with the grid return lead from the tube a. There is a .002-microfarad radio- frequency by-pass condenser across the secondary winding of the audio transformer. The function of this condenser is to offer a low impedance path for the radio-frequency currents in this part of the circuit. In some types of audio- frequency transformers the secondary winding of itself has sufficient capacity to by-pass the radio-frequency currents without the application of the by-pass condenser.
In cases where a radio-frequency by-pass condenser is desired in circuits carrying audio-frequency currents, the value of the by-pass condenser must not be so large as to by-pass audio-frequency currents as well. The larger the value of a condenser the lower its impedance to the passage of alternating currents. The higher the frequency of the alternating currents the less the impedance of the condenser to the currents of that frequency. Thus, a con- denser could have such a value that it would offer a low impedance to the high-frequency current but would offer a fairly high impedance to the low-frequency current.
AND, SERVICING A7
The primary winding of the second audio-frequency transformer e is connected in the plate circuit of the tube a. The secondary winding of this transformer is connected in series with the input circuit of the tube 6. Each winding has a .002-microfarad radio-frequency by-pass condenser across it.
The tuning condenser in the second stage is connected from the grid terminal of the tube 6 to the —C terminal, or, in other words, across the series combination of the secon- dary winding of the radio-frequency transformer and the secondary winding of the audio-frequency transformer e. As far as the tuning is concerned, it is approximately the same, whether the tuning condenser is connected across the extremities of the secondary winding of the radio- frequency transformer or as shown in the figure.
Considering the functioning of this type of circuit it is found that the radio-frequency input is amplified by the tube a, then passed on to the tube 6, where it is again amplified and passed on to the detector tube c. In the detector tube c the radio-frequency energy is changed into audio-frequency energy and as such it is applied back on the grid of the tube a, which amplifies this signal at audio frequency and passes it on to the second stage of audio-frequency amplification, which is effected by the tube 6. The audio-frequency output is taken out of the plate circuit of the second amplifier tube b. The phones or loudspeaker f are connected in the plate lead of the tube b and are shunted by a .002-microfarad radio-fre- quency by-pass condenser.
SUPERHETERODYNE RECEIVER
Principle of Operation.—The graph in Fig. 27 shows the fundamental circuits of the superheterodyne receiver. The name is derived from the fact that the incoming
48 “RADIOS R EE CHLY ERs
signal is heterodyned by a local oscillator. The beat fre- quency between the incoming signal and the local oscilla- tor is amplified before application to the detector which changes the signal into audio frequency.
It will be assumed that the wavelength of the received signal is 300 meters, which corresponds to 1,000 ke., one ke. (kilocycle) being the equivalent of 1,000 cycles. A local radio-frequency oscillator a is coupled to the input circuit of the high-frequency detector b, so that the signal frequency and the local oscillator frequency beat together to give a frequency that is equal to the sum of the two frequencies in question, and another frequency that is equal to the difference of the two frequencies in question. It is the latter that will be considered.
The local oscilla- tor a is so adjusted that the beat fre- | quency will be that frequency to which the intermediate-fre- quency amplifier c is resonant. In this case the intermediate frequency amplifier is resonant to energy having a frequency of 30 ke., which corresponds to a wavelength of 10,000 meters. Thus the local oscillator is adjusted to a frequency of either 1,030 ke. or 970 ke.; in either event the difference or the beat frequency is 30 ke. This gives the reason why it is possible with a superheterodyne receiver to tune in a particular station at two different settings of the local oscillator.
The function of the high-frequency detector 6 is to rectify the radio-frequency input so that the beat fre- quency will appear in the output circuit as an alternating current, having a frequency in this case of 30 ke. This
MTe 27,
AND SERVICING * 49
30-ke. signal is applied to the input circuit of the inter- mediate-frequency amplifier c. The intermediate-fre- quency amplifier is the heart of the superheterodyne receiver, for the fundamental principal involved is that a signal of the order of 300 meters can be amplified to a greater degree and with less chance of producing unstability in the receiver circuits if it is changed to a 10,000-meter signal and amplified at that wave length.
In the case of a radio-telephone signal from a broadeast- ing station, the fundamental frequency would be accom- panied by frequency bands 5,000 cycles wide on either side of the carrier. In order to produce an undistorted signal in the audio-frequency amplifier output it is neces- sary that the intermediate-frequency amplifier be broad enough to pass all the frequencies in the side bands, which means that the intermediate-frequency transformers should be capable of passing a frequency band 10,000 cycles wide, or from 8,570 meters to 12,000 meters. An intermediate- frequency transformer that will only pass wavelengths between 9,000 and 11,000 meters is too sharp, because it will chop off some of the side bands in the broadcast signal, which will be manifested by distortion in the audio-fre- quency output unit. This is the reason for the fact that some superheterodyne receivers produce a distorted signal. Their intermediate-frequency circuits are too selective and they exclude a vital part of the incoming signal. True sound reproduction is obtained only when all the side bands (and nothing more) appear in the loudspeaker output.
The function of the low-frequency detector d is to rectify the intermediate-frequency signal to produce one of the 5,000 cycle frequency bands for amplification in the audio- frequency amplifier e. The current in the output circuit of the detector tube d follows the audio-frequency varia-
50 CRADIO SRECEIVERS
tions in the amplitude of the 30,000-cycle current, which is due to the modulation frequency at the transmitting station.
Schematic Diagram.—A schematic wiring diagram of a superheterodyne receiver is shown in Fig. 28. This set has one stage of radio-frequency amplification ahead of the high-frequency detector, three stages of intermediate- frequency amplification ahead of the low-frequency detec- tor, and two stages of audio-frequency amplification. In this receiver there is a jack connection a that allows for plugging in either an antenna-ground system or a loop. There is also a change-over switch 6, which can be placed in either one of two positions. In one position of the switch the receiver functions as a tuned radio-frequency receiver with a feed-back in the antenna circuit, employ- ing four tubes; one radio-frequency amplifier tube, a detector, and two audio-frequency amplifiers. When the change-over switch is thrown to the other position, the receiver functions as a superheterodyne receiver.
The antenna-ground or the loop connections to the receiver are made through the medium of a plug to the radio-frequency input jack a. The rotor of the feed- back coupler c and the primary winding of the first radio- frequency transformer d are connected in series with the jack.
The secondary winding of the input transformer d is tuned to the broadcast wave band by means of a .0005- microfarad variable condenser. The input transformer d, in this case, is of the figure-8 type so as to minimize the possibility of picking up signals directly on this coil. A receiver of this type is very sensitive and if the solenoidal type of transformer is used in the input circuit, consider- able of the signal is picked up directly by the input trans- former. Thus, in this case, if a loop is used for external
AND SERVICING
AQfiAuly 4
ee ae ae ee Roe |e
oo Poe eae nid OFTHE
$°O pad
YAY J
QQVQ0 0
Bo “OI
—— mS
49C j5/
AOHPHISO
x
erat
52 ‘RADIO RECEIVERS
pick-up its directive properties will be impaired, owing to the pick-up within the set. The figure-8 type of trans- former minimizes the possibility of direct pick-up within the receiver at this point in the circuit. The grid return of the first radio-frequency amplifier tube e is brought to the contact terminal of the stabilizing potentiometer f.
The plate of the first radio-frequency amplifier tube e is connected to one end of the stator winding of the feed- back coupler c, the other end of this winding being con- nected to the primary winding of the second radio-fre- quency transformer g. The other end of this latter wind- ing 1s connected to the positive terminal of the B-battery supply for the radio-frequency amplifier tubes.
The secondary winding of the radio-frequency trans- former g is tuned by means of a .0005-microfarad con- denser. It is at this point in the circuit that energy from the local oscillator is introduced. This is effected by means of the coupling coil h, which is in the oscillating circuit of the separate oscillator and is inductively coupled to the secondary of transformer g. The grid-condenser (.00025 microfarad) and the grid-leak (2 megohms) com- bination 7 is put in series with the grid lead to the detector tube 7 to cause this latter tube to effect detector action.
The plate of the detector tube 7 is connected to the change-over switch b. In the superheterodyne position the plate lead is connected to one end of the primary wind- ing of the first intermediate-frequency transformer k. In the radio-frequency position of the switch the plate lead is connected to the detector B-battery supply through the contacts of the jack | and the primary winding of the first audio-frequency transformer m.
The lower end of the primary of the transformer k is connected to the positive B-battery supply for the detector
%
AND SERVICING 53
tubes. The secondary terminals of this intermediate- frequency transformer k are connected to the grid of the first intermediate-frequency amplifier tube n and the contact terminal of the stabilizing potentiometer f.
The output of tube n is coupled to the input of the second intermediate-frequency amplifier tube o through the medium of the second intermediate-frequency trans- former. The output of tube o is applied to the grid of the third intermediate-frequency amplifier tube p.
The output of the third intermediate-frequency ampli- fier tube p is applied to the grid of the second detector tube g through the medium of the tuned intermediate- frequency transformer r. The grid-leak (2 megohms) and grid-condenser (.00025 microfarad) combination s causes the tube q to function as a detector. The grid-return lead of the second detector tube q is brought to the negative filament terminal. The plate of the second detector tube q¢ is connected to the triple-pole double-throw switch b. In the superheterodyne position the plate of tube q is connected to the detector B-battery supply through the contacts of jack J and the primary winding of the first- audio-frequency transformer m. ‘The first, or uppermost, pole of the blade of the change-over switch 6 is connected to the positive filament terminals of the three intermediate- frequency amplifier tubes n, 0, and p, the detector tube q, and the oscillator tube ¢. The terminal that this pole engages in the superheterodyne position is connected to the positive A-supply lead through the rheostat wu.
The local oscillator tube ¢ has a tuned Hartley oscillat- ing circuit. The fact that the .0005-microfarad tuning condenser is connected from plate to grid insures the fact that the two elements of the tube in question will always be 180° out of phase, which is the condition for self-oscilla- tion. The grid excitation for the oscillator tube ¢ is that
54 SRADIO SRECEIVERS
voltage which exists across the grid coil v. The circuit through the inductive branch of the oscillating circuit might be traced from the grid of tube ¢ through the erid excitation coil v, the coupling coil h to the negative fila- ment terminal. ‘There is a .1-microfarad radio-frequency by-pass condenser wu from the negative filament lead to the positive radio-frequency B-battery lead. The circuit is traced from the positive radio-frequency B battery through the plate coil x to the plate terminal of oscillator tube t. In effect, the coils v and x are joined together at their inner ends by virtue of the radio-frequency by-pass con- denser w, and this point is at ground potential as far as radio frequency is concerned. The condenser w also func- tions as a blocking condenser in the oscillator circuit, allow- ing the plate potential to be supplied at the mid-point between plate and grid, the condenser w blocking the d.-c. potential from being applied to the grid of the oscillator tube 1.
When the change-over switch b is thrown to the super- heterodyne position, all the tubes shown in the figure are in operation. The condenser in the radio-frequency and first-detector circuits tune their respective circuits to the incoming radio-frequency signal and the oscillator con- denser is set to give the desired 30-ke. beat frequency, the intermediate-frequency amplifiers functioning at this frequency.
For local reception, the change-over switch b is thrown to the radio-frequency position, thereby cutting off the filament-current supply to the tubes n, 0, D4 OG, ae The plate of tube 7 is cut off from the primary winding of the first intermediate-frequency transformer k and is con- nected to the detector B-battery supply through the con- tacts of the jack J and the primary winding of the first audio-frequency transformer m, and the plate of tube q 1s
AND SERVICING 55
disconnected from its output circuit. Thus, only the tubes e, 7, y, and z are left in operation, functioning as one stage of radio-frequency amplification, detector, and two stages of audio-frequency amplification, respectively. This will be found to be adequate for the satisfactory reception of local signals.
List and Constants of Parts.—The parts used in the construction of the set shown in Fig. 28 may be purchased; some of them, however, may be readily constructed by the experimenter. The rotor and stator of the coupler c¢ are each wound with 30 turns of No. 24 d.s.c. wire, the stator on a 3-inch form and the rotor on a form to fit inside the 3-inch form.
The transformer d is of the twin-8 or double-D type. The primary has 15 turns No. 24 d.s.c., and the secondary has 50 turns No. 24 d.s.c. wire.
The transformer g has three windings. The primary winding consists of 18 turns No. 24 d.s.c. wire wound on a 23-inch form; the secondary, 50 turns; and the oscillator coupling coil h, 2 turns, all wound on the same form.
The intermediate-frequency transformers should have a working range between 8,000 and 12,000 meters.
The oscillator coils v and x are wound on a 23-inch form with No. 24 d.s.c. wire, the grid coil v having 27 turns, and the plate coil z, 28 turns.
The three variable condensers have each a capacity of .0005 microfarad. The condenser w has a capacity of .1 microfarad. The two detector grid condensers are .00025 microfarad each. The grid leaks are 2 to 3 megohms each. The potentiometer f is of the 400-ohm type. The rheo- stat u has a resistance of 6 ohms.
56 CRADIO SRECEIVERS
SHORT-WAVE RECEIVERS
Peculiarities of Short-Wave Reception.—The four most popular short-wave bands at the present time are the 20-, 40-, 80-, and 100-meter bands. The logical thing to do is to use a separate coil for each of the three first-men- tioned wave bands in the list of four here given. It will be found that the coil for the 80-meter band will also suffice for the 100-meter band.
In short-wave reception it is not desirable to use a tun- ing condenser having a maximum value of capacity greater than .00025 microfarad. Probably a tuning condenser saving a maximum capacity value below .0002 microfarad is still more desirable because the tuning in the short-wave band is far different from the tuning in the broadcast-wave band. This can be explained by the following facts that were received from actual practice.
A .00025-microfarad variable condenser is shunted across a coil of such an inductance value that the combina- tion tunes to 20 meters with the condenser dial set at 10. This combination tunes the circuit to 21 meters with the condenser dial set at 11. The frequency of a 20-meter wave is 300,000,000 +20= 15,000,000 cycles per second which is 15,000 ke. The frequency of a 21-meter wave is found to be 14,800,000 cycles, or 14,300 ke. Thus, by rotating the tuning condenser through | division of the dial, a 700-ke. frequency band has been covered.
Since the signals from a radio broadcasting station cover a frequency band about 10,000 cycles, or 10 ke. wide, the 20-meter broadcaster would be passed over with a rota- tion of the tuning control of ~;th of a dial division.
On the other hand, consider the tuning around 500 meters. With the same .00025-microfarad tuning con- denser and a coil of proper inductance the circuit is tuned
AND SERVICING 57
to a wavelength of 500 meters with the condenser dial at 80. This is approximately the dial setting for this wave- length in the standard type of broadcast receiver. Now, if the tuning condenser is rotated through 2.6 divisions to 82.6, the wavelength will be increased to 508 meters. The frequency at 500 meters is 600 ke. and the frequency at 508 meters is 590 ke., the difference being 10 ke., or the frequency band of a broadcasting station. Thus, it can be seen that the tuning condenser dial must be rotated through 2.6 divisions to pass through the signals from a 500-meter broadcasting staton. <A consideration of the foregoing analysis will afford a good idea of the reason why broadcast signals appear to afford much sharper tun- ing on the shorter wavelengths. In the realm of the extremely short waves the tuning condenser must not be too large or tuning will be very difficult.
The consensus of opinion, among the uninitiated, is that there are only radio telegraph signals on the short waves, but this idea is fallacious. It is true that most of the activity on the short waves is radio telegraph communi- cation, but the fact remains that there are also some broad- casting stations operating on the short waves. For instance, the Pittsburgh station KDKA broadcasts pro- grams on 26.38, 42.95, and 62.5 meters and WGY at Schenectady broadcasts on 35 meters. It is quite cus- tomary for both of the stations just mentioned to broad- cast the same program on a short wave that is being broad- casted on their normal broadcasting wavelength. There is less static on the short waves and often a broadcast pro- gram can be heard on the short wavelength of a station that can hardly be heard or not picked up at all on its normal broadcasting wavelength.
Short-Wave Receiver With Interchangeable Coils.—In Fig. 29 (a) is shown a schematic wiring of a short-wave
58 CRADIO SRECEIVERS
receiver that can be used to receive signals between the
limits of 17 and 130 meters. is shown in view (0).
The arrangement of parts
The same reference letters are used
in both views to indicate corresponding parts.
Hel 4 Cena Re! ULLAL EEE izle
l | +B Amp.
+B Det:
=B+A -A
SS —
D
The 10-turn coil a is the coupling coil in the aperiodic,
or untuned, antenna circuit, through the medium of which the radio-frequency energy picked up by the antenna system is induced into the coil b, which forms part of the
tuned circuit.
The coupling between the coils a and 6 is
variable.
AND) {SERV ILCING 59
The ground lead is connected through to the low, or filament, side of the secondary coil b so as to reduce body capacity. However, it will be found that there will be less interference from local sources such as street lighting cir- cuits, subways, street cars, and elevated systems, and alternating-current induction from house lighting circuits if the ground is not connected through to the negative filament lead.
The feed-back coil c is closely coupled to the filament end of the secondary coil b. Tuning is effected by means of the condenser d. Regeneration is controlled by means of the condenser e. This method of effecting regeneration is quite similar to the capacity-controlled regenerative circuit and it is used because of the fact that the regenera- tion control varies only slightly with frequency, so that a single setting of the control in question may be used for a wide band of wavelengths. This feature is conducive to a small amount of variation in the tuning due to the adjustment of the feed-back circuit. It follows, then, that, since there are only the two controls d and e, if the latter is such as to require a very small amount of adjustment, the receiver becomes practically single control, which is quite a desirable feature for a short-wave receiver.
The grid leak f should be made as large as possible and the grid condenser g as small as possible. A satisfactory combination is 5 megohms and .0001 microfarad, respec- tively. The positive potential for the plate of the detector tube h is supplied through the primary winding of the audio-frequency transformer 7, the radio-frequency choke 7, and the coil c. The function of the radio-frequency choke 7 is to keep all radio frequency out of the audio- frequency circuit, where it might cause howling, and also to prevent the distributed capacity of the phones or the primary winding of the audio-frequency transformer from
4—)
60 SR ADIO SRECEIVERS
shorting the radio-frequency currents around the oscila- tion-control condenser e.
The tube & with its auxiliary apparatus provides a stage of audio-frequency amplification. The jack | constitutes a convenient means of connecting the phones or loud speaker in the output circuit. The filament current for both tubes is controlled by the rheostat m.
The constants that remain unchanged for all the wave- lengths between 17 and 130 meters are given herewith.
a—10 turns No. 24 d.c.c. wire, 3-inch diameter, sole- noid winding.
d—.00014-microfarad variable condenser.
逗.00025-microfarad variable condenser.
f—5-megohm grid leak.
g—.0001-microfarad fixed condenser.
h—UX-200-A detector tube and socket.
7—Low-ratio audio-frequency transformer.
j—200 turns No. 36 d.s.c. wire, 1-inch diameter, sole- noid winding.
k—UX-201-A amplifier tube and socket.
/—Output jack.
m—10-ohm rheostat.
From a consideration of the foregoing list it will be seen that the only elements that change for covering the dif- ferent wave bands within 17 to 130 meters are the secon- dary inductance 6 and the feed-back coil c. Their constants are as follows: 20-Meter wave Band _ 6—8 turns No. 18 bare copper wire, 3-inch diameter,
spaced the diameter of the wire.
c—2 turns No. 24 d.c.c. wire, 3-inch diameter, no spacing. 40-Meter Wave Band
6b—8 turns No. 18 bare copper wire, 3-inch diameter, spaced the diameter of the wire.
AN Deny ERY 1.1 NiG 61
c—4 turns No. 24 d.c.c., 3-inch diameter, no spacing. 80-Meter Wave Band
b—19 turns No. 18 bare copper wire, 3-inch diameter, spaced the diameter of the wire.
c—6 turns No. 24 d.c.c., 3-inch diameter, no spacing.
In Fig. 30 are shown the wavelength callibration curves for the 20-, 40-, and 80-meter bands, the respective coils designated for those particular wave bands being used.
20,000 18,000 16,000 ey 4 2/4000 & 5S) y at 4 % & 8/2000 = x & 8 /0,000 & : Ww Q Ny & 8000 G xX S , g 6,000 8 4000 2000 ana
LOE ZO NIO MFO SOM COM COMCOMmIOMIOO: Condenser Dial Setting
Fig. 30
These curves were made with a straight-line frequency condenser at d, Fig. 29; thus any straight-line-frequency condenser having the same maximum and minimum capacity limits will give practically the same shape of curve. If a condenser having a straight-line capacity curve is used, having the same maximum and minimum limits as the condenser used for obtaining the curves shown in Fig. 30, the range will be the same but the shape of the curves will be different.
With the 20-meter coil in operation it is possible to tune from 8,900 ke. (33.7 meters) to 18,000 ke. (16.7 meters).
62 SR ADIO CRECEIVERS
The majority of transmitters in operation on the 20- meter band will be tuned in between the two vertical lines on the curve in question, namely, between 18 meters (16,700) ke.) and 22 meters (13,600 ke.).
With the 40-meter coil in circuit, it is possible to tune from 4,300 ke. (69.8 meters) to 9,500 ke. (31.6 meters). The active part of this curve is that within the so-called 40-meter band, which is that part of the curve included between the two vertical lines, namely, between 36 meters (8,330 ke.) and 42 meters (7,150 ke.).
With the 80-meter coil in operation it is possible to tune from 2,200 ke. (1386 meters) to 5,400 ke. (55.5 meters). The active part of this curve is that portion which is designated as the 80- meter band and_ which includes the wavelengths between 77 meters (3,900 ke.) and 83 meters (3,620 ke:).
Short- Wave Throttle Tuner, 17 to 90 Meters. In Fig. 31 is the schematic wiring diagram of a receiver for covering the 20-, 40-, and 80-meter wave bands. As discussed in the case of the preceding receiver, it is quite essential that a method of regeneration be effected that has little effect on the tuning of the receiver input circuit, and it is also desirable that this feed-back be controlled in such a manner that it will not need a great deal of adjustment over a wide variation in tuning. In the preceding circuit arrangement a special method of regeneration control was effected having the desired characteristics. In this circuit the feature is the throttle method of regeneration control.
SF sf
HiGy.o1
WEAN DY SERV LOIN'G 63
There are only two tuning ranges with this receiver and one set of constants. The two ranges are effected through the medium of a single-pole double-throw switch. The following is a list of the receiver constants.
a—10 turns No. 18 d.c.c. wire, 34-inch diameter, no spacing, tapped at fifth turn.
b—4 turns No. 18 d.c.c. wire, 14-inch diameter, no spac- ing. .
c—.00001-microfarad variable condenser.
d—.00018-microfarad variable condenser.
e—.0001-microfarad fixed grid condenser. f—.00025-microfarad variable throttle condenser. g—5-megohm grid leak.
h—10-ohm rheostat.
i—Single-pole, double-throw switch.
j—UX-200-A tube and socket.
k—200-turns No. 36 d.s.c., 1-inch diameter, solenoidal winding.
The only thing about this receiver that needs particular explanation is the construction of the two coils a and b. They are both of the low-loss type. In constructing these coils, scribe a circle 34 inches in diameter on the surface of a piece of wood, the latter being preferably about > inch to 2 inch thick. Locate 11 equidistant points around the circumference of this circle, as shown / to 1/1, Fig. 32, and fasten a 2 by 53-inch stud at each one of these points. These studs can be made by hammering through a 23- inch nail at each of the eleven points.
In winding the coils begin at a and pass the No. 18 d.c.e. wire, on the outside of the first stud, on the inside of studs 2 and 3, on the outside of stud 4, etc., always passing the winding outside of one stud and inside of the succeeding two studs. The coil a, Fig. 31, is composed of 10 turns, with a tap at the mid-point. The coil 6 is composed of 4 turns.
64. RIAD T Om Rie CE diy Eeeas
The antenna is coupled to the receiver input circuit through the .00001-microfarad condenser c, Fig. 31. This little coupling condenser may be variable or it may be fixed. Possibly some advantage may be derived by having a variable con- denser.
The coil a is tuned by means of the .00018-microfarad variable con- denser d. It is possible to tune through two ranges of wavelengths by means of the double-throw switch 7 and the tapped coil a. With the switch 7 con- nected to the fifth-turn tap on the 10-turn coil a, it is possible to tune from 18 to 51 meters with the tuning con-
cept ead ewes ou eae
H1G.32
RG EIS, A Tekonl |
Wavelength in Meters
nian eer
30 40 Cond. bial is ttin
Dike, 6%}
denser d. With the switch 7 engaging the tenth turn on coil a it is possible to tune from 35 to 90 meters.
AND SERVICING 65
The coil 6 has a fixed inductive relation to the coil a, the former being located 2 inches from the grid end of the latter. This amount of coupling is sufficient to effect the minimum degree of regeneration, and, from this point up to the maximum, is controlled by means of the throttle condenser f.
The radio-frequency choke k prevents the distributed capacity of the phones or primary winding of the first audio-frequency transformer, when used, from shorting the radio-frequency energy around the throttle condenser f.
A chart showing the two wavelength curves for the two settings of the wave-change switch 7 is shown in Fig. 33. The lower curve covers the 20- and 40-meter band and the upper curve covers the 40- and 80-meter bands.
SINGLE-SIDE-BAND RECEIVER
In view of the outstanding advantages of the single-side band eliminated-carrier system for long-distance radio- telephone communication, the American Telephone and Telegraph Co. have established a commercial radio- telephone system wherein this system is used at the transmitting end, and a receiver designed for the reception of signals of this character is used at the receiving end. It seems logical to assume that this type of transmission and reception will be used more and more during the coming years and it is probable that broadcast programs will be sent out at some future time by single-side-band trans- mitters.
In the discussion concerning the transmitter, the circuit constants given were those of the transmitter that was used in the successful two-way transatlantic radio tele- phone tests, the transmitter being later used for commercial transatlantic telephone communication. In order to link up the receiver discussion with that of the transmitter,
66 (RADIO CRECEIVERS
the receiver-circuit constants given herein will be for the reception of signals emanating from the transmitter in question, it being the most powerful single-side-band eliminated-carrier transmitter in operation at the present time, having a radio-frequency output of 200 kw.
A schematic wiring diagram of a simple receiver of this type that was used to pick up the signals during the trans- atlantic tests is shown in Fig. 34. The circuit constants are as follows: pA
a—500-turn honeycomb coil.
b—750-turn honeycomb coil. c—300-turn honeycomb coil. d—25-turn honeycomb coil.
AND SERVICING 67
e—300-turn honeycomb coil.
f—500-turn honeycomb coil.
g—.0005-microfarad variable condenser.
h—.0005-microfarad variable condenser.
7—.0005-microfarad variable condenser. g—.00025-microfarad grid condenser.
k—.1-microfarad fixed condenser.
[—.002-microfarad fixed condenser.
m—12-ohm rheostat.
n—12-ohm rheostat.
o—3-megohm grid leak.
p—UX-200-A detector tube and socket.
g—UX-201-A amplifier tube and socket.
Considering the schematic diagram in Fig. 34, the antenna is connected to one side of the tuned circuit con- posed of the 500-turn coil a and the .0005-microfarad tuning condenser g. The other side of this parallel com- bination is connected to ground. It is by means of this circuit that the antenna-ground system is tuned to reson- ance with the incoming side band.
The energy in the antenna system is induced into the detector-tube input circuit by means of the inductive coupling between the antenna coil a and the grid coil b. The latter is a 750-turn coil and is tuned by means of the .0005-microfarad tuning condenser h. Detector action is effected by means of the combination of leak 0 and con- denser j7. Regeneration is obtained with the feed-back coilc. The .002-microfarad by-pass condenser / shorts the radio-frequency energy in the detector plate circuit around the phones or the primary winding of the first-audio frequency transforrher, if such is used. The receiver, as described so far, will function quite well in the reception of single-side-band signals if the degree of regeneration is increased to the point of oscillation.
68 SR ADIO CRECEIVERS
The single side band that is intercepted by the receiv- ing antenna has a frequency band of from 55,800 cycles to 58,500 cycles. In meters, this is 5,370 meters to 5,120 meters. It is to be remembered that the function of the transmitter in this case is to transmit good quality speech but not necessarily music or frequencies higher than the speech frequencies. Good quality speech can be trans- mitted with a frequency band of from 300 to 3,000 cycles.
The received signal only occupies a 2,700-cycle frequency band, whereas, in normal transmission on this wavelength where the carrier and both the upper and lower side bands are transmitted, the frequency band would be twice as wide.
If the constants of the receiving circuit are adjusted so that the detector sets up oscillations having a frequency of 55,500 cycles, this frequency will remodulate, or beat, with the received side band of 55,800 to 58,500 cycles, and a dif- ference-frequency band of 300 to 3,000 cycles, or the voice- frequency band will result. Thus, there is nothing compli- cated necessary in the reception of signals of this type, the old regenerative detector circuit being quite capable of giving satisfactory results.
It is possible to carry the development of this receiver a step farther and use a separate oscillator at the receiving station to beat with the incoming signal to produce the voice-frequency band. The application of the separate oscillator is also shown in the figure, but is not considered entirely necessary for satisfactory results.
The oscillatory circuit for the separate oscillator is of the tuned Hartley type, the grid of the oscillator tube q being connected to one end of the oscillatory-circuit inductance edf and the plate to the other end. The fila- ment-ground is connected to an intermediate point on the inductance in question. Tuning is effected by means of a
ANE Dag) Ee RV Eb CTEN G 69
.0005-microfarad variable condenser k shunted across the entire oscillatory-circuit inductance edf. The 500-turn coil f is the plate coil and the 300-turn coil e is the grid excitation coil. A 25-turn coil d is used for coupling the oscillator to the receiver input circuit.
Although the simple arrangement described in the preceding paragraphs is satisfactory for amateur work, it is not quite elaborate enough for a commercial installation, and a receiver of the superheterodyne type is used. The general outline of a commercial type of single-side-band receiver 1s shown in Fig. 35.
The radio waves are intercepted by the loop a, and this energy is applied to the input circuit of a high-frequency
Hire@ars)
detector b. Energy from a separate 90,000-cycle oscil- lator cis applied to the same input circuit. The difference- frequency band of 31,500 to 34,200 cycles that appears in the output circuit of the detector tube is passed through the band-pass filter d to the intermediate-frquency amplifier e, As before, there is only one side band, 31,500 to 34,200 cycles, the frequency of the 90,000-cycle supplied carrier, and the upper side band due to the beating of the 90,000- cycle frequency with the signal input having been elimi- nated by the band-pass filter.
_ The single-side-band output of the intermediate-fre- quency amplifier is passed on to a low-frequency detector f. Another oscillator g at this point in the circuit supplies a 34,500-cycle carrier, which beats with the 31,500- to 34,200-cycle side band, the result in the detector output
70 FRADIO SRECEIVERS
circuit being the voice-frequency band, 300 to 3,000 cycles.
From the second detector, the audio-frequency energy
passes to the audio-frequency amplifier h. AUDIO-FREQUENCY AMPLIFIERS
TYPES OF AUDIO-FREQUENCY AMPLIFIERS
After taking up the discussion of the different types of.
detector and radio-frequency amplifier circuits, consider- ation will now be given to the various methods of obtain- ing efficient audio-frequency amplification.
There are three fundamental types of audio-frequency amplifiers; namely, transformer coupled, resistance coupled, and impedance, or choke-coil, coupled. The writer has personally constructed each of the audio- frequency amplifiers that are discussed in the following pages, and a list of the circuit constants and the apparatus used, in each case, is provided.
TRANSFORMER-COUPLED AUDIO-FREQUENCY AMPLIFIER
The first audio-frequency amplifier circuit to be con- sidered is that of a transformer-coupled unit. When the proper parts are used in a properly designed circuit, this type of audio-frequency amplifier is paramount. The schematic wiring diagram for this amplifier is shown in Fig. 36 and the following is the list of material that was used in its construction, as well as the circuit constants.
a—High-ratio audio-frequency transformer (6 to 1).
6—Low-ratio audio-frequency transformer (2 to 1).
c—Output transformer (1 to 1).
d—.002-microfarad radio-frequency by-pass condenser.
e—live 1-microfarad audio-frequency by-pass con- densers.
f—6-ohm rheostat.
g—Two 500,000-ohm potentiometers.
Ce a
AND IS RIVE CEN'G 71
h—U X-200-A detector tube and socket.
1—UX-201-A amplifier tube and socket.
j—UX-171-A power amplifier tube and socket.
The output circuit of the detector tube h is shown in the drawing to facilitate the explanation of the method of coupling the output of the detector tube to the amplifier input. A variometer may be connected in the detector output circuit to tune the plate circuit to the frequency of the incoming radio-frequency signal and thus effect regeneration. In this case it is necessary to have a radio- frequency by-pass condenser d from the high side of the primary winding of the first audio-frequency transformer a
Fic. 36
to the negative filament terminal, in order to by-pass the radio frequency in this part of the circuit around the primary winding of the transformer a and the B battery.
The condenser ¢ in the plate circuit of the detector tube h functions to by-pass audio-frequency currents around the detector B battery. There is a 500,000-ohm potentio- meter g whose extremities are connected across the term1- nals of the secondary winding of the first audio-frequency transformer a. The contact terminal of this potentiometer is connected to the grid terminal of the first audio-frequency amplifier tube 7. This potentiometer functions not only as a volume control (because the value of the signal voltage
72 SRUA Dl Oe eRVE GLE tvanaRes
applied to the grid of tube 7 can be varied by moving the contact terminal from the low voltage end of the resis- tance element to the high-voltage end), but it also tends to flatten out the transformer characteristic and make the transformer amplify all frequencies alike.
There is an audio-frequency by-pass condenser e from the low side of the secondary winding of transformer a to the negative filament terminal, or, in other words, across the C battery for this tube. There is another audio- frequency by-pass condenser e from the low side of the primary winding of the second audio-frequency trans- former b to the negative filament terminal of the tube 7, or, in other words, across the B-battery supply for the tube 7.
The connections of tube j, or the second amplifier tube, are practically the same as for the tube 7. The output of the amplifier tube 7 is obtained through the medium of the output transformer c. This transformer has a one-to-one ratio and functions to pass the amplified audio-frequency signal on to the loudspeaker unit and at the same time keep the high-potential direct current in the plate circuit of the last tube from passing through the windings of the loudspeaker field coils. This latter effect is undesirable from the standpoint that the current, if in the wrong direc- tion, will tend to demagnetize the permanent magnetism of the speaker field-coil core, and will tend to bias the diaphragm of the speaker unit, thus effecting distortion.
If the tubes mentioned in the material list are used, the A-battery potential is 6 volts and the B- and C-bat- tery potentials are as indicated in Fig. 36. It is neces- sary to use some sort of power tube in the second audio- frequency amplifier stage to prevent distortion due to overloading of the tube in question; therefore, in this case a UX-171-A tube with 180 volts on the plate and —40.5 volt bias has been used.
AND SERVICING 73
IMPEDANCE-COUPLED AUDIO-FREQUENCY AMPLIFIER
Before the development of audio-frequency transformers for use in radio-broadeast receivers reached its present stage of perfection, the thought took root in some sections that transformer-coupled audio-frequency amplification was not conducive to good quality, although it was admittedly the best as far as voltage amplification was concerned. However, when the general broadcast public began to give up the quest for DX reception and expressed a desire for good quality reception, the transformer manu- facturers put their engineers on the problem of producing a better audio-frequency transformer that would allow for undistorted amplification. In the meantime the imped- ance-coupled and the resistance-coupled audio-frequency amplifiers sprang up as an answer to the problem of obtain- ing distortionless audio-frequency amplification. In both eases it was admitted that they were not as efficient as transformer-coupled amplifiers from a standpoint of voltage amplification, hence more stages were required, but it was claimed that they were capable of producing a better quality output than the transformer-coupled amplifiers that employed the audio-frequency transformers on the market at that time. This was probably true. The impedance-coupled and resistance-coupled amplifiers probably did procure a better-quality output signal than could be obtained from the coupling transformers that were available for radio broadcast fans in the early days of broadcasting.
A schematic wiring diagram of a choke-coil coupled audio-frequency amplifier is shown in Fig. 37. The fol- lowing is the list of the parts that were used by the author in the construction of a three-stage impedance-coupled audio-frequency amplifier.
74 fRADIOUCR Bic ETERS
a—Four 200-henry choke coils. _
b—.002-microfarad radio-frequency by-pass condenser.
c—Three 1-microfarad audio-frequency coupling con- densers.
d—Two 1-microfarad grid by-pass condensers.
e—Three 1-microfarad plate by-pass condensers.
f—Ouput transformer (1 to 1).
g—6-ohm rheostat.
h—Two 500,000-ohm potentiometers.
i—UX-200-A detector tube and socket.
4—Two UX-201-A amplifier tubes and sockets.
k—UX-171 power-amplifier tube and socket.
_ The amplifier shown schematically in Fig. 37 is capable of producing about the same output volume as that shown
eh
Fig. 37
in Fig. 36, thus one more tube is required in an impedance- coupled amplifier than in a two-stage transformer- coupled audio-frequency amplifier to obtain the same amount of volume.
If there is a feed-back for radio frequency in the plate circuit of the detector tube 7, Fig. 37, the condenser b is required to by-pass radio frequency in this circuit around the plate choke a and the detector B battery.
AND SERVICING 75
The maximum audio-frequency signal input is available across the first 200-henry choke coil a, hence across the points 7 and 2. There is another circuit between the points 7 and 2 besides that afforded by the choke coil a; namely, that circuit extending from point 7 through the audio-frequency coupling condenser c, the grid resistance h, the audio-frequency by-pass condenser d to the negative filament lead, the audio-frequency by-pass condenser e to point 2. If the combined capacity reactance of the coupling condenser c and the by-pass condenser e is small - relative to the grid resistance h, the signal voltage across the coil a is practically apparent in its entirety across the grid resistance h, owing to the fact that the voltage drop across the condensers named is negligible and the signal voltage across the coil a is impressed across the series combination mentioned.
Thus it can be seen that the audio-frequency by-pass condensers should offer a low resistance to the passage of audio-frequency currents. The value of the capacity reactance of a condenser is equal to $zfC, where 7z is the constant 3.1416, f is the frequency in cycles per second, and C is the capacity in farads. Since, for a given value of capacity, the lower the frequency the higher its reactance, in choosing the proper value of capacity to use as an audio- frequency by-pass condenser, it is well to consider the lowest frequency that it is called upon to by-pass, which is in the neighborhood of 50 cycles.
With the frequency term fixed at 50 cycles per second in the formula for capacity reactance, the capacity term can be varied and the change in reactance noted. It will be found that a condenser having a capacity of .01 micro- farad offers a reactance of over 300,000 ohms, whereas, a 1-microfarad condenser brings this reactance value down to 3,000 ohms, approximately. The resistance h, which
4—6 ;
76 FRADIO “RECEIVERS
is in series with the coupling condenser c as far as the signal voltage is concerned, has a maximum value of 500,000 ohms, so it can be seen that the reactance of condenser c is negligible if it has a value of 1 microfarad when signal frequencies of the order of 50 cycles are being put through the amplifier circuit.
The upper limit of the frequency band in the audio- frequency signal that is received from a broadcasting station is of the order of 5,000 cycles and a .01-microfarad condenser offers a reactance of only 3,000 ohms at 5,000 cycles and a 1-microfarad condenser, 32 ohms at the same ~ frequency. ‘Thus, while a .01-microfarad coupling con- denser is adequate for the higher frequencies in the received audio-frequency band, it is too small to be con- ducive to the satisfactory amplification of the lower fre- quencies in the received band, and if these low frequencies are omitted the signal quality is impaired.
The variable resistance h functions as a grid leak. It prevents the coupling condenser c from charging up to a sufficient degree to block the tube 7; in other words, it allows electrons to leak off as fast as they arrive. Having this element in the circuit, a variable, allows for getting the most volume out of the amplifier.
An output transformer f is used to supply the amplified audio-frequency signal to the loudspeaker field coils, at the same time keeping the high-potential direct current from passing through these windings.
If the tubes used in this amplifier are as listed in the material list, the A-battery potential is 6 volts and the B and C battery potentials are as indicated in the figure.
It is to be noted that the values of the bias-battery potentials for the different plate potentials are less in this case than in the case of the transformer-coupled amplifier, because there is a greater drop in potential through the
AND SERVICING hy
choke-coil winding in the plate circuit of an amplifier tube than there is through the primary winding of an audio- frequency transformer. This means that in the case of choke-coil coupling the effective plate potential will be lower, hence a lower value of biasing potential is required than in the case of transformer coupling. Bearing this fact in mind it will be noted that the bias-battery values in the case of a resistance-coupled amplifier are still lower for the same values of plate potential, than in the case of the transformer-coupled amplifier and the choke-coil coupled unit.
TRANSFORMER-RESISTANCE COUPLED AUDIO-FRE QUENCY AMPLIFIER
A resistance-coupled amplifier, like an impedance- coupled amplifier, is conducive to obtaining good-quality output signals. One stage of transformer-coupled and two stages of resistance-coupled amplification are quite a popular circuit arrangement. Such a circuit arrangement is shown in Fig. 38 and the following is the list of the mate- rial used, as well as the circuit constants.
a—First-stage audio-frequency transformer (6 to 1).
b—Output transformer (1 to 1).
c—.002-microfarad radio-frequency by-pass condenser.
d—1-microfarad audio-frequency by-pass condenser.
e—Two 1l-microfarad coupling condensers.
f—Two 1-microfarad grid by-pass condensers.
g—Two 1-microfarad plate by-pass condensers.
h—6-ohm rheostat.
i—Two 100,000-ohm variable resistances.
4—One 500,000-ohm variable resistances.
k—UX-200-A detector tube and socket.
I—Two UX-201-A amplifier tubes and sockets.
m—UX-171 power-amplifier tube and socket.
n—200-henry choke coil.
78 Fo SROACD LOR Te Orn Iver eRes
As shown in the schematic wiring diagram, the trans- former a is connected in the circuit in the same manner as previously described. In the resistance-coupled circuits the output resistances 7 perform the same function as the impedance coils in the impedance-coupled amplifier described in the preceding text. It is convenient to have the resistances in the resistance-coupled amplifier variable, as this allows for getting the optimum degree of efficiency possible, although .a properly designed fixed resistance gives excellent results.
Feed-bLack
LGN: [\ {\ CNIS | jae i # B ome cate
7 Bi. 50 CAEAIAS | ihr <atnh wieteathe the C
+B4S et ee eee +A-B (ies eee | —A
Fia. 38
If the tubes used in this amplifier are as listed, the A-battery potential is 6 volts and the B- and C-battery potentials are as indicated in Fig. 38. A relatively small amount of bias is required for the grids of the amplifier tubes whose plates are supplied with potential through coupling resistances, owing to the high potential drop through the resistances in question. This drop is so great
AND SERVICING 79
that, even though the plate-supply sources are 150 and 180 volts, the actual effective potential at the plate termi- nals is considerably less than this.
!
POWER AMPLIFIERS AND POWER PLATE SUPPLY
ADVANTAGES OF POWER AMPLIFIERS
The design of audio-frequency transformers for use in broadeast receiving sets has reached such a high degree of perfection that, if the right ones are used in the proper circuits, excellent reproduction of broadcast programs may be obtained.
If a signal of sufficient volume to operate a loudspeaker so that it can be heard all over a six-room home is taken out of the plate circuit of a tube of the UX-201-A type, there is bound to be distortion and it does not necessitate a very critical ear to notice it. This distortion is due to the fact that the output tube is being overloaded.
The UX-201-A type tube was designed for use as a radio-frequency amplifier, detector, and audio-frequency amplifier (up to a certain degree), but real power cannot be obtained from anything less than a power tube. It is true that tubes of the UX-171 type were designed to answer the requirements of a power tube in a receiving set, but if high-quality amplification is desired with plenty of volume, it is necessary to use a tube of the UX-210 type, which is a 7.5-watt power tube. This tube with 500 volts on its plate and a bias of 40.5 volts is capable of producing a high degree of distortionless output energy.
Tubes of the UX-201-A type are capable of producing a distortionless signal of relatively low value, above which value the signal becomes distorted, owing to the overload- ing of the tube. By overloading is meant that the ampli- tude of the signal voltage applied to the grid of the tube is
80 SReADIO SRECEIVERS
sufficient to produce a value of plate current that is off the straight part of the plate-current grid-voltage character- istic curve of that type of tube. As long as the voltage swing of the grid is low enough to keep the values of plate current along the straight part of the characteristic curve, a certain amount of positive potential on the grid of the tube will produce the same amount of plate- current change that the same amount of negative grid potential will cause. However, when the swing of the grid potential is boosted to an abnormal value by trying to obtain too great a signal output from too small a tube, the plate-current values are carried off the straight part of the characteristic curve, and grid swings in the positive direction will cause different changes in the plate current than are caused by equal swings in the opposite direction. This is manifested by distortion in the signal output.
Thus, a tube of the UX-201-A type is satisfactory for a certain amount of volume and beyond this value of volume it will distort the signal, no matter how good the audio- frequency transformers are and no matter how good the loudspeaker is. The thing to do is to use a power tube and put a high potential on its plate and an adequate bias on the grid.
It is not economical to obtain the 500 volts for the plate supply to a UX-210 tube from a dry-battery source. The logical manner in which to obtain this high d.-c. potential is to step a 110-volt a.-c. supply up to approximately 550 volts by means of a step-up transformer. This high- voltage alternating current can then be rectified and passed through a filter circuit, which will smooth it out to approxi- mate direct current to the extent that it can be used as a source of constant potential for the plate of the tube in question.
AND SERVICING 81
Thus, what is needed for the final stage of amplification to produce great volume with a high degree of quality in the course of reception of broadcast programs, Is a recti- fier for supplying a high d.-c. voltage to a UX-210 type tube, the latter being used in a properly designed stage of audio-frequency amplification. This constitutes what is termed a power amplifier.
POWER AMPLIFIER AND POWER SUPPLY WITH FULL-WAVE RECTIFICATION
In Fig. 39 is shown the schematic wiring diagram of the circuit arrangement for a power amplifier and power supply, or B-eliminator, which is supplied with high- voltage direct current from a full-wave rectifier circuit. The following is a list of the material used by this writer in the construction of the unit to be described, as well as the circuit constants.
a—200-watt power transformer with two 10-volt secon- dary windings, one 1,100-volt secondary winding with a mid-tap, and one 110-volt primary winding.
b—Two 30-henry choke coils.
c—Audio-frequency low-ratio transformer, (2 to 1).
d—Output transformer, (1 to 1).
e—2-microfarad, 750-volt, filter condenser.
f—4-microfarad, 750-volt, filter condenser.
g—6-microfarad, 750-volt, filter condenser.
h—Four 1-microfarad, 200-volt, audio-frequency by- pass condensers.
1—1-microfarad, 200-volt, grounding condenser.
4-—2-ohm, 2.5-ampere rheostat.
k—7-ohm, 1.25-ampere rheostat.
I—500,000-ohm potentiometer.
m—2,500-ohm variable resistance.
n—400-ohm potentiometer.
82 SRADIO CRECEIVERS
o—25,000-ohm heavy-duty resistor.
p—7,000-ohm heavy-duty resistor. -
g—8,000-ohm heavy-duty resistor.
r—10,000-ohm heavy-duty resistor.
s—25-ohm variable resistance.
t—Switch.
u—Two UX-216-B (or UX-281) tubes and sockets.
v—UX-210 power amplifier tube and socket.
w—Tell-tale lamp.
x—Two-single-contact jacks.
1 to 8—Terminals.
9 to 12—Terminals on double-terminal blocks.
The above material is the nucleus of a power amplifier that is very satisfactory for reproducing broadcast pro- grams. The 110-volt alternating current is supplied to the power input receptacle through the medium of an ordinary power plug. ‘The terminals of the input recept- acle are connected to the primary terminals of the power transformer, one of the leads in question being connected through a single-pole single-throw switch ¢. The func- tion of this switch is to turn the unit on and off. When the amplifier is in operation, there is approximately 1 ampere drawn at 110 volts from the a.-c. supply line. This means that the switch ¢ must have a current-carrying capacity of at least lampere. Thus, the ordinary filament switch will answer the purpose very nicely.
The rectifier filament-supply winding has a no-load terminal voltage of 10, so a 2-ohm rheostat 7 is connected in series with the filament supply to the rectifier tubes w. These tubes are of the UX-216-B type and draw a filament current of 1.25 amperes at 7.5 volts. The rectifier fila- ment-supply winding must be insulated for voltages of the order of 750 volts, for the mid-point of this winding is the source of the rectified but unfiltered d.-c. supply. There is
AND SERVICING 83
alternating current at 550 volts (R. M. 8.) applied to the rectifier plates, and the value of the rectified voltage is of the order of 525 volts. The letters R. M. S. mean root mean square, which in turn indicate that this voltage value, which is indicated by an a.-c. voltmeter, is equal to
SitO ee isi2
v)
QQOQ orerere 2 A-C Power Y 1 Bl y2pU7 4 rhe / a
Fie. 39
the square root of the mean of the square of all the instan- taneous values of voltage in one cycle of the a.-c. potential.
The effective value of the a.-c. potential is only .707 of the maximum, or the peak, value that is reached in every half cycle. ‘Thus, in order to calculate the maximum value
84 FRADIO SRECEIVERS
of potential that is applied to the plates of the rectifier tubes, it is necessary to multiply by 1+.707, or 1.41. This gives 550X1.41=775.5 volts, and this is the peak value of voltage that is applied to the rectifier plates when an a.-c. voltmeter indicates 550.
Now there is some drop in potential through the rectifier tubes, and if this drop is 25 volts, the resultant will be 525 volts of rectified voltage. This rectified voltage, at this point in the circuit is unfiltered; that is, it is not smoothed out, and has the characteristics of an a.-c. potential with the negative half of the cycle eliminated by
NS
a a
Fic, 40
the action of the rectifier tubes. With full-wave rectifica- tion there is a pulsating potential that looks like an a.-c. sine wave with all the negative half cycles inverted so that they are on the positive side.
The changes that take place in the rectifier and filter circuits are shown graphically in Fig. 40. Curve a shows the characteristics of the applied a.-c. voltage. After
passing through the rectifier, which has unilateral impe- —
dance (allows the passage of current in one direction only) the voltage has a pulsating characteristic as shown in curve 6. This pulsating voltage is applied to the filter circuit, which is composed of a large amount of inductance and capacity that tend to keep the voltage from dying down to zero in the course of its pulsations, owing to the
a
ENDS Ee RIV DCN G 85
charging and discharging of the condensers through the filter chokes. The output voltage from the filter circuit is thus of the nature shown in curve c. It is so close to a d.-c. voltage that it can be termed such.
The high-voltage secondary winding on the power transformer a, Fig. 39, has a potential of 1,100 volts between its extremities, and there is a mid-tap on this winding. The extermities of this high-voltage winding are connected to the plates of the two rectifier tubes u. The mid-tap of this winding is the source of the negative termi- nal of the high-potential direct current and in this case is connected to one side of all the filter condensers e, f, and g and to the output terminal 5. Lead 4 is grounded through the receiving set.
The source of the positive terminal of the mecunied but unfiltered d.-c. supply is at the mid-point of the rectifier filament-supply winding. A lead from this point is con- nected to the first filter condenser e and to one end of the first filter choke 6b. ‘The other end of the first filter choke 6 is connected to the second filter condenser f and one end of the second filter choke. The other end of the second choke is connected to the last filter condenser g and terminal q of the double-terminal block. There is normally a jumper between terminals 9 and 10.
Terminal 10 is connected to terminal 1/1 of the next double-terminal block. It is also connected to the plate of the power amplifier tube v through the primary winding of the output transformer d. There is a jumper between terminals 1/7 and 12. The function of the two double- terminal blocks is to afford an expedient means of inserting a milliammeter in the high-voltage supply circuit from the rectifier to determine the following:
1. The total d.-c. drain on the rectifier.
2. The current to the plate of the amplifier tube v.
86 ‘RADIO CRECEIVERS
3. The direct current through the B-eliminator output
resistances.
4. The maximum voltage available at the output of
the rectifier filter circuit.
All of the above values may be determined by taking two readings, one with the 50-milliampere milliammeter connected to terminals 9 and 10, and a second reading with the milliammeter connected to terminals 11 and 12. In each case when the meter reading is taken the jumper is removed from the two terminals to which the meter is connected, and after the readings have been taken the jumper is connected back again. The first reading men- tioned, indicates the total direct current drawn from the rectifier circuit. The second reading shows the current through the B-eliminator output resistances 0, p, qg, and r. It is to be noted that there should be no external con- nections to the terminals /, 2, 3, 4, or 5 when the current readings are being taken.
The difference between the two readings is the value of the current to the plate of the power amplifier tube ». The maximum voltage available at the filter output is equal to the product of the current through the output resis- tances, in terms of amperes, and the total value of the output resistance, in terms of ohms, the product being the voltage drop across the resistances in question, which is the output voltage of the rectifier filter circuit.
It is very important to check, quite often, the current from the rectifier circuit, as there is a great possibility of overloading the rectifier tubes, or of passing too much cur- rent through the filter chokes, or of operating the amplifier tube with too great a value of plate current. The maxi- mum d.-c. load current for a UX-216-B rectifier tube is 65 milliamperes and for a UX-281, 110 milliamperes. This means that with half-wave rectification it would only
AND. SERVICING 87
be possible to draw 65 milliamperes from the rectifier cir- cult of the UX-216-B and 110 milliamperes from UX-281 without overloading the rectifier tube. In full-wave rectification the two tubes are operated in parallel to supply direct current to the load, each tube supplying half the load; therefore, with two UX-216-B tubes it is possible to draw 130 milliamperes from the rectifier circuit without overloading the rectifier tubes, and with two UX-281 tubes, 220 milliamperes may be drawn.
There is something else to bear in mind, and that is also a limiting factor for the current in the case of full-wave rectification, as well as the capacity of the rectifier tubes. The standard types of filter chokes are designed to carry up to 60 miliamperes, but beyond this point they are being overloaded. ‘This overload is manifested in heat, which may become of sufficient temperature to melt the insulat- ing compound out of the chokes or burn out the wire.
Another point to bear in mind is the fact that the higher the value of the current through the rectifier filter chokes, the less the effective inductance, the greater the voltage drop through this part of the circuit, and the less the avail- able output voltage for the B-eliminator circuit and the plate of the power-amplifier tube. Standard chokes for this sort of circuit have a d.-c. resistance of the order of 600 to 1,000 ohms. If the minimum value of 600 ohms be multiplied by 2, since there are two chokes in series in the filter circuit, the effective d.-c. resistance will be 1,200 ohms. Now, if the current drain on the rectifier is kept down to 30 milliamperes, the drop across the filter chokes will only be .03 1,200, or 36, volts. But, if the current is raised to 60 milliamperes, the drop across the filter chokes is increased to 72 volts.
The UX-210 power-amplifier tube » with 500 volts on its plate and a 45-volt bias, should draw about 30 milli-
88 RADIO RUE OU ERAS
amperes, and care should be taken to see that the bias is sufficient to hold the plate current down to this value. The plate of the tube will get hot when drawing 30 milliamperes at 500 volts, because there is a plate dissipation of 500 x .03 =15 watts. The fact being kept in mind that the UX-210 is a 7.5-watt tube, it can be expected to show a little color when it is made to dissipate 15 watts. An analysis of the foregoing facts shows how important it is to have a milli- amineter in series with the lead from the rectifier circuit.
As long as the rectifier and filter must be provided to supply high-voltage direct current to the plate of the power amplifier tube v, it follows that this supply might just as well be made use of in effecting a B-eliminator circuit, since the latter simply means the connecting of the proper resis- tances across the rectifier output. In this case, the resis- tances 0, p, g, and r function as the B-eliminator resis- tances with a total resistance of 37,000 ohms. This limits the current through these output resistances to approximately 12 milliamperes. This is the current through these resistances when there is no external load to a receiving set. If the current from the rectifier circuit is kept low enough it is possible to maintain a potential of 500, 135, 90, and 45 volts at the terminals /, 2, 3, and 4, respectively, for supplying B-battery potential to a four- or five-tube receiving set.
The condensers hf in the output of the power supply are audio-frequency by-pass condensers across the plate supply to the receiving set that is connected to the B-eliminator terminals. If the power-amplifier B-eliminator unit is located at a great distance from the receiving set, as in another room, for example, the by-pass condensers in question should be located right at the receiver.
The filament of the power amplifier tube v is supplied with energy from a separate 10-volt winding on the power
AND SERVICING 89
transformer. This voltage is stepped down to the proper terminal voltage (7.5) for a UX-210 tube by means of the 7-ohm rheostat k, which must have a current-carrying capacity of 1.25 amperes, as this is the normal current to the filament of a UX-210 tube at 7.5 volts.
The tell-tale lamp w is connected across the 10-volt amplifier filament supply, in series with a 25-ohm resis- tances. This little light is mounted on the front panel and is a means of indicating when the amplifier is on or off.
The audio-frequency signal is supplied to the jack «x through an ordinary radio plug. The terminals of this jack are connected to the extremities of the primary wind- ing of the input transformer c. The secondary terminals of this transformer are shunted by a 500,000-ohm potentio- meter / the contact terminal of which is connected to the grid terminal of the power-amplifier tube v. This resis- tance not only functions to aid the transformer in being impartial to all the audio frequencies that are passed through it, but it also functions as a volume control by vir- tue of the fact that the grid can be connected to any point along the resistance element of the potentiometer]. Maxi- mum signal voltage is applied to the grid of the amplifier tube when the potentiometer‘contact terminal is moved to the upper end of the resistance element, and minimum signal voltage when the contact is moved to the lower end.
The biasing resistance m is connected from the lower side of the secondary winding of the input transformer c to the mid-filament point, the latter being effected by means of the potentiometer n whose extremities are connected across the a.-c. filament supply, and the contact terminal of which is maintained at the mid-point of the filament supply, There is a by-pass condenser h across the biasing resistance. The proper bias is applied to the grid of the power-amplifier tube v by the drop in potential across the resistance m,
90 fRADIO “RECEIVERS
owing to the plate current to tube v through this resistance. The resistance can be varied and in this manner the grid bias can be changed.
It might be noted at this point that the two condensers h (near m) and 7 are very important from a standpoint of eliminating the hum from the signal output, and increas- ing the volume as well as bettering the quality. When the
s . es S& y (| bes — ee e
q Ey” pm:
= = = LOTT \eseai/ SS Ne :
aS
GS
y
TG
amplifier is in operation, note should be taken of the amount of hum in the signal output with the power-supply lead inserted in one direction. Then the plug in question should be reversed and the hum again noted. Which- ever way of inserting the power-supply plug produces the least amount of hum in the signal output, is the way which maintains the lead to which the condenser 7 is connected, at ground potential. There is a decided
——
AND SERVICING gI
difference in the amount of hum in the signal output with this condenser in or out of the circuit.
Shunting an audio-frequency by-pass condenser h across the biasing resistance m increases the output volume and makes a decided improvement in the quality of the output signal. Shunting by-pass condensers from the ends of the potentiometer n to the contact terminal makes only a slight improvement, and they are not considered necessary.
The signal output is taken from the secondary wind- ing of the output transformer d through the jack z.
A rear view of the amplifier-eliminator unit is shown in Fig. 41. The resistances and a few of the by-pass con- densers can be seen mounted on the rear of the front panel. The filter condensers and the B-eliminator output resis- tances are mounted on the baseboard. ‘The power trans- former is mounted at the right of the baseboard. The tube sockets, filter chokes, double-terminal blocks, and audio-frequency transformers are mounted on the sub- panel.
POWER AMPLIFIER AND B AND C ELIMINATOR
In Fig. 42 is shown a schematic wiring diagram of a power amplifier that is supplied with plate potential from a half-wave rectifier unit. In this diagram, the detector and two stagés of amplification are shown, with the con- nections from the eliminator unit. The second audio- frequency amplifier tube is the power-amplifier tube.
The following is a list of the constants, and the mate- rial needed for the construction of the eliminator and the two stages of audio-frequency amplification shown.
a—Power transformer with a 110-volt primary wind- ing a, a 525-volt 60-milliampere secondary winding a, and two 8-volt 2-ampere secondary windings a3 and ay.
b—Rectifier tube.
aa
‘RADIO SRECEIVERS
Tote wn nn nn ee en FF
92
AY G+& O+S = Gg
SI I—UL
S8LALS ff OL
AIYDIAS
saad Sa ple ee eae YS is a Se el a ed
AQEDIAGS ER LGUN G 93
c—Two 60-henry choke coils.
d—2-microfarad 750-volt filter condenser.
e—4-microfarad 750-volt filter condenser.
f—2- to 8-microfarad 750-volt filter condenser.
g—Heavy-duty resistor in six sections, having resistances beginning from top of 9,000, 11,000, 4,500, 4,000, 3,500, and 9,000 ohms.
h—100,000-ohm variable resistance.
1—Two 2,500-ohm variable resistances in series.
4—2,500-ohm variable resistance.
k—Detector tube UX-200-A.
[—Amplifier tube UX-201-A.
m—Power-amplifier tube UX-210.
n—Radio-frequency choke coil.
o—Audio-frequency choke coil.
»—Audio-frequency transformer (3 to 1).
g—Audio-frequency transformer (4 to 1).
r—60-henry choke coil.
s—.001-microfarad by-pass condenser.
i—Four 1-microfarad by-pass condensers.
u—2- to 4-microfarad fixed condenser.
v—two 1- to ;45-megohm grid leaks.
The biasing potential for the grid of the last-stage amplifier, or power-amplifier tube m, is obtained through the medium of the 2,500-ohm variable resistor 7. The grid bias for the radio-frequency amplifier tubes is obtained from terminal 1/0 by means of the two 2,500-ohm variable resistors that are connected in series and designated as 7, with two variable contact arms.
The potential to the plate of the detector tube k is lowered below the value of potential available at tap T'-2 by means of a high-resistance variable unit h connected to terminal 4. The other connections will be apparent from a close inspection of the figure.
94 FRADIO SRECEIVERS
RECEIVERS WITH A.-C. TUBES
Desirability of A.-C. Operation.—In the operation of radio receiving sets the trend has been toward complete battery elimination. Many satisfactory plate-supply units operating from an a.-c. source have been developed, but filament operation from the same source has, for a time, presented more of a problem because of the larger currents required and increased expense in the rectifier and filter circuits.
The development of tubes that used raw alternating current in the filament circuit offered an excellent solution of this problem. The grid and the plate circuits do not offer any unusual problems. These are wired and con- nected in practically the same way as in any set that is operated by a B and C power unit. This discussion, will, therefore, be focused on the filament circuit.
Wiring Receiver for A.-C. Operation.—The characteristic features of the more common types of a.-c. tubes have been given in another Section. The manner in which the fila- ment circuits are wired is shown in Fig. 43. In the radio- frequency stages and in the first stage of audio amplifica- tion, radiotrons UX-226 are used. Radiotron UY-227 is used as a detector, and radiotron UX-171 or 171A asa power amplifier. Potentiometers or center-tapped resis- tors are connected across the filament leads to eliminate the a.-c. hum.
A double-wave rectifier tube UX-280 is connected in the high-voltage winding of the power transformer and the output of this tube is used in the grid and plate circuits of the receiver.
Grid Bias.—It is essential that the signal remain undistorted as it passes through the various tubes of the receiving set. In order to obtain quality reproduction the
5,
AUN@D 2, SsEeRey-L GIN’ G
‘oluny amg Olpny js/ A0f2ALIT
th “SI Boe oc ¢ AOU
edb te] te Be Ee OF. re
1L!-Xf) 92e-KN L£EE-A/) 92e-X77 92C-X7 eile 17 t
66 FRADIO “RECEIVERS
proper B and C voltages must be used. One method of biasing a five-tube receiver using a.-c. tubes is shown in Fig. 44. The correct biasing voltages are obtained from the output of the power unit. In the case of the detector tube, the cathode is positive (43 volts) with reference to the filament. This is done to prevent the filament from attracting any of the electrons released by the cathode.
In the case of the power tube UX-171, the drop of potential in the tube itself and between +C and —C gives the necessary grid bias of —40 volts. The drop of poten- tial between —B+C and —C is 45 volts, which is satis- factory for the first audio tube. The radio-frequency amplifier tubes require no biasing when the plate voltage is not in excess of 67 volts.
SOUND REPRODUCERS TELEPHONE RECEIVERS
Fundamental Form.—In its simplest form, the telephone receiver, as shown in Fig. 45, consists of a thin, soft-iron diaphragm P mounted close to but not touching one pole of the permanent magnet NS. A fine wire ( is coiled around one end of the magnet and the terminals of this coil are connected directly in the circuit in which the instrument is to be used. The diaphragm is rigidly supported at its outer edge, but the center portion is curved slightly toward the magnet because of the at- traction between the diaphragm and the magnet. If a current is sent through the coil in such a direction that the lines of force set up by it coincide with those of the permanent magnet, the strength of the magnet will be increased and the diaphragm will be drawn closer to the
AND? SERVICING 97
pole. If, however, a current is sent through the coil in such a direction as to set up lines of force opposing those of the magnet, the strength of the magnet will be dim- inished and the diaphragm will spring away from the pole.
If a current that varies in value but is always in the same direction is sent through the coil, the lines of force induced in the magnet will increase while the current is increasing, and decrease while the current is decreasing. Thus, a varying pull on the diaphragm will cause vibra- tions that will be in harmony with the changes in current, whether the lines induced by the coil are in the same direction as those of the magnet or not.
The telephone receiver is affected by the fluctuating currents corresponding to sound waves and _ translates these currents into distinguishable sounds. ‘The dia- phragm of this simple device, like the diaphragm of the reproducer of a phonograph, is capable of emitting the most complex sounds; in fact, it is capable of imitating with a fair degree of accuracy practically all of the sounds of the human voice, of musical instruments, or other sounds made up of many complicated wave combinations.
Standard Type.—A cross-sectional view of a standard type telephone receiver is shown in Fig. 46. A barrel or shell a is used to protect the component parts of the receiver, and may be made of some insulating material
98 ‘RADIO CRECEIVERS
or in some cases is made of metal finished with an enamel coating. The ear piece 6 screws onto the shell and serves to cover the diaphragm end of the receiver except for a small hole in its center through which sound waves are permitted to escape. The permanent magnet c is U- shaped, and has both poles projecting close to the dia- phragm d to give as strong a pull as possible. The pole projections carry the windings, which are equally dis- tributed between the two coils and which are connected to
(a) Fic. 47
the terminals at e, only one of which is shown. Bringing both poles of the permanent magnet close to the diaphragm increases the number of lines of force effective on the diaphragm and increases considerably the sensitiveness of the receiving unit.
Watch-Case Type.—The construction of a compact form of telephone receiver, known as the watch-case receiver, is shown in Fig. 47; view (a) shows a section and view (b) shows the end with the ear piece and diaphragm removed. When the receiver is equipped with a head band, as shown in view (a), it forms a head set, a name which is applied whether one or two receiver units are used.
AND SERVICING 99
Although the principle of operation is the same as in the larger hand receiver, the construction and design is necessarily varied to decrease both the size and the weight. The shell a consists of a case usually of metal, threaded externally to engage an internal screw-thread on the hard-rubber ear piece 6. The magnets are built up of flat steel rings c, so magnetized that the opposite sides of their circumferences are of different polarities. The L-shaped pole pieces d, which reach nearly to the diaphragm and carry the magnet coils, are attached to the north and south poles of the steel rings. In many cases the magnets are not made of complete rings, but are approxi- mately half circles. The extensions carry- ing the coils are then fastened to the ends of the permanent mag- net. The diaphragm is of thin iron and is clamped between the body a of the shell and the ear cap 0.
Radio Headset.—A set using two watch- case receiver units is shown in Fig. 48. This particular type has a piece of soft iron mounted between the poles so that it is acted on by their magnetic field. The coil of wire carrying the received current is so located that it moves the iron armature in a manner corresponding to the current changes. ‘The armature is connected by levers to a light diaphragm, often of mica, which is controlled by the armature to produce sound waves in the air. The
Shee penton i way mite — —T
“sD 7727779
7: 99"
Fic. 48
100 ‘RADIO SRECEIVERS
lightness of the moving parts causes this receiver to be responsive to very weak signals.
In some instruments of this general type an adjusting screw is provided so that the sensitiveness of the signals may be controlled to a certain extent. For instance, if it is desired to weaken the signals, the armature is withdrawn from the magnets by the screw arrangement. Also, the tone or sound of the two receiver units may be exactly balanced for best operation. If no screw adjustment is furnished, the two receiver units should be adjusted at the factory so that the tone of each is the same.
SPEAKERS
Horn-Type Speaker.—Where it is desired to make the signals or message loud enough for a large group of people to hear, it is necessary to use a special type of receiver unit known as a loud speaker, or simply speaker. The operating unit of one type of horn speaker is shown in cross- section in Fig. 49, in which the magnet coils are shown at a and the metal diaphragm at b. The two coils are mounted on extensions of a permanent magnet. The diaphragm is in the form of a circular disk carefully mounted between rubber gaskets and so suspended that its surface rests but a small fraction of an inch above the extension pole pieces of the magnet.
When a signal current passes through the coils, it causes variations in the pull of the permanent magnet on the diaphragm. Accordingly, the diaphragm vibrates, and in doing so, sets up sound vibrations which emanate from
Fic. 49
ANID 3 SERVICING IOI
the mouth of the speaker. The horn, of whatever type it may be, is attached to the piece c.
Cone-Type Speaker.—The principle of operation of a cone-type speaker may be seen in Fig. 50. This speaker consists essentially of a set of coils a, which when energized actuate the armature 6. The armature is centered on the bar c and is free to swing in either direction. ‘To one end of the armature is fastened a drive pin, d, the other end of the pin being connected to the thrust lever e, the connection being made with soft solder. The thrust lever is, in turn, connected to the apex of the cone f. Signal currents flowing in the coils a cause the armature b to vibrate. The vibration is trans- ferred through the pin d and thrust lever e to the cone f. The vibration of the cone reproduces the music or speech transmitted from the broadcasting station. No perma- nent magnet is shown in the figure; in actual practice it is neces- sary to have it.
Electro-dynamic Speaker With Power Amplifier.—In Fig. 51 is shown in schematic form an electro-dynamic reproducer a combined with a socket power unit containing a stage of power amplification. B and C voltages are also provided for the receiver that is used to drive the speaker. ‘The reproducer a has two. coils. The coil b, known as the field coil, forms part of the filtering system of the power supply. A movable coil c, known as the cone coil, inasmuch as it is rigidly fastened to the cone d,
So +
LO+tF
O6 +
ES Oi
f
SS ee ee ee eed
102
AND SERVICING 103
moves in the strong magnetic field of the coil b in accor- dance with the modulation of the received signal.
The entire unit is designed to operate from a 105- to 125-volt 60-cycle supply. A two-way switch e is used to regulate the input of the power transformer. Two UX- 281 radiotrons are used as rectifiers and are connected in a full-wave rectifying circuit. The output of the rectifier tubes is smoothed out by means of the two 4-microfarad condensers f and the field coil 6 of the reproducer. The filtered energy is applied to the plate of the power amplifier tube UX-250 through the primary winding of the output transformer g. The open terminals of the input trans- former h are connected to the output of the receiving set with which this unit is used. The secondary winding of the output transformer g is connected to the cone coil c of the reproducer a. The interaction between the field of coil b and that of coil c produces a movement of the cone, which, in turn, reproduces the broadcasted music or speech.
SERVICING OF RADIO RECEIVERS
GENERAL INSTRUCTIONS*
CLASSIFICATION OF RECEIVING SETS
Classification of Circuits.—Instructions covering the servicing of broadcast receivers are supplied by the manu- facturer. These instructions, however, usually apply to individual sets and are of little value in general application. In this Section the service problems will be grouped under general specifications, followed by methods of diagnosing and correcting them.
In general, there are four basic pick-up circuits in use today: the so-called regenerative detector, the untuned radio frequency, the tuned radio frequency and the super- heterodyne. Any set on the market may be classified as using one of the foregoing types or possibly a combination of one or more. There are two additional types of pick-up circuits which have fallen more or less into oblivion and will not be found in general use in the broadcast receivers of today. They are the crystal detector and the straight audion detector, which employs no form of regeneration whatsoever.
Receiving sets consist of a pick-up circuit, a detector circuit, and an audio-frequency amplifying circuit. In the pick-up circuit radio-frequency amplification may be incorporated. The detector may be either a tube or a crystal. In the audio-frequency circuit from one to
*Abstract of article on “Servicing of Broadcast Receivers,” by Lee Manley and W. E. Garity, in the Proceedings of the Institute of Radio Engineers.
ACN Weta ER Vil CLIN, G 105
three tubes are generally used. In multi-tube sets employ- ing radio-frequency amplifiers, some arrangement of circuit is made to suppress or control the tendency of the tubes to oscillate, when the circuits are tuned to resonance. Any set on the market today may be grouped under one of the foregoing classes as to the circuit employed.
Classification of Failures.—In much the same way that receivers may be grouped under circuit classifications, their failure to operate may be grouped under certain | general classes, namely:
Lack of operating experience on the part of the user.
Location.
Defective accessories.
Open circuit.
Short circuit.
High-resistance connection.
Lack of operating experience may be the result of not following out instructions carefully enough, or, as is some- times the case, the instructions are not complete enough and are not entirely clear to the novice. It may be the result of insufficient instruction on the part of the service man who made the installation. Then, too, it may be the result of impatience on the part of the customer.
Under the caption of location many factors must be con- sidered. The type of building in which the installation is made, the proximity to steel buildings, power lines, trolley and railway lines, and the geological and topographical conditions surrounding the installation are all important factors.
Under defective accessories may be included defective tubes, power units, batteries, loud speakers, antenna and ground installations, also improper battery connections. Many sets fail or are returned to the dealer as unsatis- factory because of poor antenna and ground installations.
106 CR ADIO RECEIVERS
Many a set of good quality and capable of delivering satis- factory results fails because the loud speaker that is used with it does not have the proper electrical characteristics to operate satisfactorily in conjunction with the receiver. Tubes will also cause trouble as they are subject to certain defects incidental to fragility.
Open circuits are generally found in the movable con- nections of the set such as a condenser pigtail, loop leads, loud speaker leads, and any other connection that is subject to movement or vibration in the normal operation of the set. Open circuits may also result from burned-out transformers or from mechanical failures in telephone jacks, rheostats, and switches.
If a set has been once tested and found to be O. K., short circuits rarely occur. When they do, it is the result of a mechanical failure of the moving parts or of tinkering with the mechanism of the set. It will sometimes happen that the pigtail of a moving element of the receiver will break and fall in such a way as to cause a short circuit of that element. This is particularly true of the pigtails of varia- ble condensers. The principal cause of short circuits that occur in the normal operation of a set is in the tubes. If the filament of the tube should break there is a possibility of its falling in such a way as to cause a short circuit between itself and the plate and grid elements of the tube. When such a fracture of the filament occurs the B or C battery, as the case may be, is short-circuited through the conduc- tors involved. This type of short circuit is generally of very brief duration as the filament will generally burn out as soon as the short circuit occurs. A contact between the grid and plate element of a tube is a more serious type of short circuit, resulting in the rapid deterioration of the B and C batteries, and may possibly cause a burn-out of the transformer windings in the circuits involved.
AGN Diy E Revi Lic LG 107
The foregoing troubles are relatively easy to check up as they are immediately apparent or can easily be located by a continuity of circuit test.
The most difficult type of failure to locate is that caused by a high-resistance connection. It is not only difficult to locate, but it is difficult to determine. This condition will cause the set to operate indifferently with rather unsatisfactory results. This condition is sometimes mis- taken as location trouble. A high resistance is possible at any connection in the receiver. Soldered connections that are soldered with a corrosive flux that has not been properly treated after the soldering operation are probably the worst offenders. Weak mechanical springs in telephone jacks and switches may also introduce high-resistance connec- tions.
| PRECAUTIONS
All sets should be tested before they are sold. This takes but a few minutes and will surely pay well in avoid- ing dissatisfaction as well as time that is sometimes neces- sary to service a defective set. A radio receiver that is working properly today does not as a rule go bad tomorrow, and if such an installation does fail, the dealer may feel that the trouble is due to a defective accessory rather than the set itself. When the service man is called on to service such a set he has the confidence that the set is O. K. and he will immediately be able to concentrate on the real proba- bility of failure rather than imaginary ones.
Then, too, if the dealer would acquaint the customer with the limitations of radio reception, what to expect and what not to expect, service problems would be minimized. Acquaint the customer as to the probable length of time his batteries and tubes will last. This is quite important, and if followed out, will avoid some very disagreeable service jobs,
4-8
108 ‘RADIO SRECEIVERS
SERVICE-SHOP EQUIPMENT
In order that satisfactory and efficient service may be given to patrons, the radio dealer must have a properly equipped service shop. The equipment should include the following items: Long-nose pliers; combination pliers; diagonal cutting pliers; 3-inch screwdriver; 6-inch screwdriver; heavy-duty screwdriver; insulated screw- driver; loud-speaker armature spacing tools; test leads with springs clips; testing outfit with a.-c. and d.-c. meters; a set of socket wrenches; 4- and 8-inch crescent wrenches; small claw hammer; dust brush; electric soldering iron and stand; spare-parts shelves; movable battery box or power unit; indoor antenna; antenna terminal; ground terminal; calibrated oscillator; headphones and cords; tool rack; series test lamp with leads and test points; hand drill with a selection of drills; small compass saw; small ball peen hammer; wire; socket contact adjusting tool; friction tape; rosin-core solder; polishing oil and cloth; pocket knife, and any other instruments and tools that will enable the man responsible for the service to do rapid and efficient work.
PORTABLE TOOL KIT
From the standpoint of service it is usually advisable to make all repairs on receiving sets directly on the owner’s premises. In order to do this efficiently, the serviceman must have a well-equipped tool kit, so that he may be able to make all tests and repairs with as little delay as possible. A well-stocked tool kit should have the following items: Portable test set with a.-c. and d.-c. meters and the neces- sary test leads; ear piece with head band; loud-speaker armature spacing tools; set of tested tubes; long nose pliers; diagonal cutting pliers; friction tape; electric solder- ing iron and rosin-core solder; large and small screwdrivers :
ANDY VER VilO1N GC 109
insulated screwdriver; pocket knife; 4-inch and 8-inch crescent wrenches; small hammer; large piece of canvas; flashlight; and a selection of nuts, screws, wire, etc.
SERVICEMAN’S CONDUCT
When a service man goes into a customer’s home he is usually going there as a representative of a commercial establishment. He should be instructed to be courteous and considerate. If he must take a set out of the cabinet for adjustment he should use a piece of cloth provided in his kit to protect the surface of the table he works on. He should answer all questions asked him no matter how absurd they may appear to him. The customer generally has one question that he would like to have answered, and in his mind the service man must be an expert, in order to be able to do such work, and so he unburdens his mind. The service man should respect this attitude on the part of the customer and should do his best to point out the fallacies tactfully and set the customer right in his ideas about radio. The service man should make the customer enjoy his visit and if this is done the service man becomes a valuable asset to a business and is a potential salesman.
OBTAINING INFORMATION FROM CUSTOMER
The service man, before he starts to make any adjust- ments other than turning on the set and trying the various controls, should question the customer as to how it happened, the time, place, and conditions surrounding the failure. He should have the customer re-enact the con- ditions at the time of failure. By getting all the symptons an astonishing amount of time may be saved in running down the difficulty. If sufficient questions are asked, the customer will generally give the real cause of trouble or he will suggest something in the course of inquiry that will point out just what the cause of failure was. Sets as a
1IO SR ADIO SRECEIVERS
rule do not go bad of themselves. The failure usually occurs while some operation is taking place, such as plug- ging in the loud speaker, turning the condensers ormaking a change in the battery connections.
The length of time that a set has been in operation will be an indication of various types of trouble. <A set that has recently been installed is subject to a certain type of failure, whereas a set that has been in operation for a year or more, 1s subject to other types of failure.
RELATION BETWEEN LENGTH OF SERVICE AND FAILURE
If a set has been installed for a period of two weeks or less, outside of the inability of the customer to procure the desired results, there are only a few reasons why the set should fail. They are:
A defective tube.
Defective battery, battery connection, or power unit.
Loud-speaker connection loose in telephone plug.
Burn-out of transformer.
Of course, there may be other reasons, but these are the most common and are given in the order of their probability of occurrence.
If the set has been in operation for a period of six months or a year, the possibilities of trouble will increase. If the failure in this type of installation has been gradual, the first thought would be that the tubes were becoming de- activated through continual use.
If the breakdown was sudden, a mechanical failure might be expected in one of the movable connections or pigtails. A burned-out transformer could be expected in difficulties of this sort. If the trouble is due to a noise condition, the failure might be ascribed to dust or dirt accumulations on the condenser plates or other important parts of the receiver. The defect might also be due to a
AND SERVICING III
soldered connection. It will require, as a rule, a rather long period of time for a soldered connection to corrode to such a degree as to cause this condition. The local atmospheric conditions under which the set has been operating may have some bearing on the cause of failure. If the set has been operating near the seashore and has been subjected to the action of salt atmosphere it may have caused sufficient corrosion of the connections or other metallic parts to introduce high resistance or leakage path.
If a set has been operating for a long period of time and has given satisfactory results and then develops noises and scratching sounds, one should not look for a loose connec- tion in the wiring of the set, but rather look for an open circuit in the moving parts. Worn mechanical parts are often mistaken for loose connections in the wiring. The wiring is absolutely stationary and it is not at all likely that it will be disturbed in the ordinary use of the set so as to cause a failure due to a loose connection. Vernier drive shafts and vernier plates will wear loose, and while apparently they are making perfect contact to the metal surfaces of the condenser when the set is brought into a critical condition, as is the case when receiving distant stations, will cause noises that might be thought due to a loose connection in the wiring.
PROBABLE SOURCES OF TROUBLE
TROUBLE IN ACCESSORIES
When trouble is experienced in radio reception the general tendency seems to be to blame it on the set. However, in a large number of cases the trouble may be traced directly to a defective installation or to defective accessories. This includes the aerial and ground, the A,
112 ‘RADIO SRECEIVERS
B, and C batteries or substitutes, the tubes, and the loud speaker.
The aerial may be grounded; it may be too long or too short; it may touch foreign objects; its connections may be corroded; its lead-in may be broken inside the insulation. The ground wire may be broken; it may be corroded at connection to water pipe or other ground; there may be an inefficient source of ground. The lightning arrester may be leaky or short-circuited.
The A battery may be discharged. This is indicated by a gradual dying out of the signals during reception. The electrolyte should be tested with a hydrometer. The connections at the terminals of the battery may be corroded. This results in noisy, intermittent, or weak reception. The terminals should be scraped bright and then coated with vaseline to prevent further corrosion. If an A-battery eliminator is used, the trouble will be found either in the rectifier or filter circuit. In all such cases it is best to report such trouble directly to the manufacturer.
Dry B batteries when run down cause reception to be weak and noisy. When the detector voltage is too low, the result may be a steady whistle. It is advisable to replace a 45-volt battery unit when it drops to 34 volts. The B-battery units may be wired incorrectly, resulting in wrong voltages being applied to the plates of the tubes.
Storage B batteries have the same peculiarities as storage A batteries. They should be tested periodically with a hydrometer and inspected for corrosion at the terminals.
The most common source of trouble in a B power unit is a defective rectifier, especially if it is a tube. There is also the possibility that the filter condensers have broken down, the filter chokes or the resistors in the voltage divider are shorted or open.
AND SERVICING 113
Defective tubes are another source of trouble. A tube may light and yet be dead so far as reception is concerned. Such a tube may sometimes be brought back to normal by reactivation. Sometimes the elements are shorted, which makes the tube unfit for reception. It happens also that the wrong type of tube is used.
The loud speaker has also peculiarities of its own. It may rattle, howl, or fail to produce any sound at all. Replacing the loud speaker with a pair of headphones will immediately determine whether the loud speaker or the set is at fault.
OUTSIDE INTERFERENCE
Sources of Outside Interference.—In addition to the complaints which the serviceman may find to be due to faulty equipment or installation, there are also many cases where unsatisfactory reception is due to interference originating outside the installation. These interferences manifest themselves in various ways. In order to deter- mine whether a particular disturbance comes in from the outside it is necessary to disconnect the aerial and ground from the set and turn on the power. If the noise disappears or is very much reduced in strength, then the disturbance is undoubtedly due to something outside the set. Among the many outside sources of interference may be mentioned static, nearby oscillating receivers, radio telegraph sta- tions, and defective power installations.
Much of the work in mitigation of electrical interference results in an improvement in the operation of the electrical devices or supply lines and is thus a double gain. There are, however, some electrical devices which, even when in perfect working order, cause disturbances that result in interference with radio reception. In many cases it is possible to provide filters, shields, chokes, etc., either at
114 SR ADIO CRECEIVERS
the source of disturbance or at the receiving set, which do much to relieve the difficulties.
Part of the disturbance from electrical devices is prae- tically inevitable and must be regarded, like atmospheric disturbances, as part of the inherent limitation of radio reception. In other words, the limitation upon radio reception is not only the distance and the power of the transmitting stations and the sensitiveness of the receiv- ing set, but also the omnipresent background of slight electrical disturbances which drown out signals below a certain intensity. This background of electrical dis- turbances is the underlying reason why reception from local stations is inherently superior to reception from dis- tant stations.
Power-Line Induction.—A frequent cause of interference is the presence of alternating-current power wires near the antenna or receiving set. Low-frequency voltages (usually 60 cycles) are induced and the resultant current flowing in the receiving circuit causes a “humming” sound in the telephone receivers. The low pitch of the hum will usually identify this source of interference. A method of eliminating or at least reducing the magnitude of this interference is to place the antenna as far as possible from the wire lines and at right angles to them. When the interference can not be eliminated by such means, the proper choice of a receiving set may help. An inductively- coupled (two-circuit) receiving set is less susceptible to such interference than a single-circuit set. The use of one or more stages of radio-frequency amplification should also help to filter out the audio-frequency interference. It has been suggested that audio-frequency interference might be shunted around a receiving set having a series antenna condenser by connecting between the antenna and ground terminals of the set a high resistance, which will
AND SERVICING Il5
offer lower impedance to the audio frequency than will the receiving set itself.
Sparking Apparatus.—Sparks are produced in the normal operation of many types of electrical apparatus, such as motors, doorbells, buzzers, gasoline engines, X-ray apparatus, violet-ray machines, some forms of battery chargers, rural telephone ringers, and heating-pad thermo- stats. Sparks are also sometimes produced at defective insulators, transformers, ete., of electric wire lines. Sparks usually give rise to electric waves which travel along the electric power wires and by them are radiated out and are then picked up by radio receiving sets. The noise thus produced in a radio set may come from a disturbance that has traveled several miles along the electric power wires.
One remedy for such types of interference is to eliminate the spark. This is possible if the spark is an electrical leak and not necessary: to the operation of the machine in which it occurs. Many very useful electrical machines, however, require for their operation the making and break- ing of electrical circuits while they are carrying current, and whenever this happens a spark is produced. It is impossible to eliminate these machines, so that it is neces- sary to make the spark of such nature or so arrange the circuits that the radio-frequency current is reduced or prevented from radiating.
To prevent the radio-frequency current produced by a spark from getting on to the lines connecting the sparking apparatus, some form of filter circuit is necessary. A tested condenser, 1 microfarad, more or less, connected across the sparking points will short-circuit a considerable amount of the radio-frequency current, or a condenser connected from each side of the line to ground will serve the same purpose. A choke coil in each side of the line in
116 ‘RADIO CRECEIVERS
addition to the condensers connected to ground forms a simple filter circuit that should prevent frequencies in the broadcast range from getting on the line. A high inductance, or choke coil, or a high resistance connected in each side of the line changes the characteristics of the circuit so as to reduce the amount of power radiated. If such a filter circuit is not effective or is impractical, the apparatus may in some cases be surrounded by solid metal sheet or wire screen that is thoroughly grounded. The screen should completely surround the apparatus. This may be difficult. For example, in shielding the igni- tion system of a gasoline engine the spark coils and all wires and other parts of the system must be enclosed in metal shields, and these must be very well grounded. Location of Source of Interference.—The first thing to do in tracing the source of trouble is to make sure that it is not in the receiving set itself. The next thing is to open the electric switch at the house meter; if the interfering noise is still heard in the radio set, the source is then known to be outside the house. It is then desirable to report the situation to the electric power company. Many of the companies have apparatus for the purpose of following up complaints of this kind. Usually a sensitive receiving set with a coil antenna is used to determine the direction from which the interfering noise comes, and this outfit is taken from place to place until the source is found. The location of such sources is often a very difficult and baffling undertaking. The trouble sometimes comes from a spark discharge over an insulator to ground, or between a pair of wires, or it may be that the wire is touching some object - such as a tree, pole, guy wire, etc. Such a spark discharge is a loss of power to the operating company and a potential source of serious trouble, and for these reasons the company is probably more interested in finding and eliminating
AGNES REV OLN: G 117
this type of trouble than the radio listener. Large leaks and sparks may often be observed at night, especially in hot weather. However, sparks that are too small to be readily noticed may cause serious interference to radio reception.
Commutators.—Where d.-c. motors are in operation near a radio receiving set interference is sometimes caused, especially when the brushes on the motor are sparking badly. The sparking should be reduced as much as possible by cleaning the commutator and by proper setting of the brushes. The remaining interference is sometimes overcome by placing two condensers, about 2 micro- farads each, in series across the power-supply line and con- necting their mid point to a good ground system.
Bell Ringers.—Another source of interference is the ringing machine used in rural telephone exchanges. Tele-
phone engineers can reduce or eliminate interference by connecting a filter between the machine and the ringing keys.
Precipitators.—Many cases of radio interference have been caused by electrical precipitators that are used to prevent smoke and noxious fumes or material from leaving the chimney. The precipitator operates by establishing a highly charged electric field, inside the chimney, of such a nature and direction that particles going up the chimney are charged and driven against the walls where they stick. Precipitators cause interference for the reason that the high voltage used in their operation is obtained from a rectifier that produces sparks and generates radio-fre- quency alternating current as well as the direct current which the precipitators need. If the precipitator is so designed and arranged that the distance between the rectifier and the chimney is only a few feet or if the entire apparatus including all leads is housed in a metal build-
118 SR ADIO SRECEIVERS
ing, there is usually no trouble. But if the rectifier is separated from the chimney the wire which joins them forms a good antenna that will radiate and cause inter- ference for 20 miles or more. Interference from these precipitators can be eliminated by placing a grounded wire screen entirely around these wires and thoroughly ground- ing the wire screen and the rectifier. If screening of the various parts is impracticable, damping resistances can be inserted at various points in the wire line, which will reduce the amount of power radiated. Tuned circuits connected across the spark gap of the rectifier will assist by absorbing the radio-frequency power.
TESTING OF RECEIVING SETS WESTON A.-C. AND D.-C. TESTER
Purpose of Tester.—The Weston a.-c. and d.-c. tester, known as model 537, is designed for testing any type of receiving set, whether operated from a direct-current or an alternating-current source. It will measure the various voltages used in the radio set; it will test continuity and condition of circuits, and test the tubes under the same conditions as exist when the tubes are in their sockets. All tests can be made by using the regular voltages normally supplied to the set by its batteries or socket power, so that no auxiliary power supply is required.
A.-C. Voltmeter.—The test set, shown in Fig. 1, has two instruments, an a.-c. voltmeter a and a d.-c. volt- milliammeter b. It is provided with various switches, plugs, binding posts, cords, and adapters for properly connecting the instruments to the circuits under test.
The a.-c. voltmeter a has three ranges ; hamely, 150, 8, and 4 volts. Hither of the two lower ranges is connected directly across the filament terminals when the switch cis set to A.C. The particular range depends on the position
AND SERVICING 119
of the range selector switch d, whether turned to 8 or 4, or away or toward the operator, respectively. These ranges are for the purpose of measuring the filament volt- ages of tubes when the filaments are heated with raw alternating current. This voltmeter may be allowed to remain in the circuit during the tests for plate voltage, plate current, grid-bias voltages, and tube tests described later. This will enable one to follow any changes that may occur during tests due to changes in line voltage. The 150-volt range is provided for measuring the line voltage and is available only at the two binding posts e. These are marked 150 and + on the instrument. This range is entirely insulated from all other circuits in the test set, and, therefore, while only one range can be used at a time to obtain correct readings, no damage to the set can result if both high and low ranges are connected simultane- ously, and it may remain in the circuit during any other test, regardless of connections, without damage or error. The low ranges are also available at the three binding posts f; these are stenciled 8, 4, and X. The low-range binding posts must not be used when the plug g is connected in the radio set, on account of possible interconnections. D.-C. Volt-Milliammeter.—The d.-c. volt-milliam- meter 6, Fig. 1, has four voltage ranges; namely, 600, 300, 60, and 8 volts, and two current ranges, 150 and 30 milli- amperes. The 600- and 300-volt ranges are for plate voltage, the 60-volt range for grid bias, and the 8-volt range for filament voltage measurements. These ranges are all available at the plug g for tube-socket tests, by properly setting the dial switch h. The 600- and 60-volt ranges are also available at the three binding posts 7 shown on the right of the instrument, when the dial switch h is set to Vm. B. P. (voltmeter binding posts). All volt ranges have resistances of 1,000 ohms per volt, so that
120 Ravio SRECEIVERS
they may be used for measuring the voltages from socket power devices.
The 30- and 150-milliampere ranges are available at the plug for plate-current measurements by setting the dial switch to Plate MA., and the range selector switch 7 away from the operator for 150 milliamperes, and toward the
oo i 4 ONT RAS
operator for 30 milliamperes. The 150-milliampere range is provided for measuring higher plate currents than 30 milliamperes and the output of rectifying tubes. The 600-, 300-, and 8-volt ranges may be read directly on the scales provided for them. The 60-volt and 30-milli- ampere ranges should be read on the 600 and 300 scales, respectively, dividing the indications by 10. The 150-
AND aj5 ERVECEN G I21
milliampere scale should be read on the 300 scale divided by 2. The 150-milliampere range is also available at bind- ing posts k by setting the dial switch h to MA. B. P.
Dial Switch.—The dial switch h, Fig. 1, is a bipolar switch, and connects the d.-c. volt-milliammeter 6 to the various circuits designated on the dial. ‘Two positions are available for plate-voltage tests, namely, 600 and 300 volts.
A.-C.—D.-C. Switch.—When the switch c, Fig. 1, is set toward the mark A. C., or to the left, it connects the low ranges of the a.-c. voltmeter a across the filament terminals of the plug g for measuring filament or heater voltages of tubes supplied with raw alternating current. When the switch is set to the mark D. C., or to the right, the low ranges of the a.-c. voltmeter are disconnected and the 8-volt range of the d.-c. voltmeter 6 can be used for fila- ment voltage measurements of tubes supplied with direct current or rectified alternating current.
This switch has only the one function referred to, and all tests, either for a.-c. or d.-c. tubes, except filament’voltage, are made on the d.-c. meter, including plate and grid- bias voltages and plate current, regardless of the position of this switch. When d.-c. filament voltage are to be measured, the switch c should be set to D. C. for the reason that both instruments will indicate and the a.-c. meter requires more current for its operation than the d.-c. meter, which may cause an error in the voltage indications. No harm, however, can result if the switch cis left on A. C.
Binding Posts.—Binding posts are provided for making voltage or current measurements directly on batteries or power units, or for any purpose for which the cord and plug are not adapted, as, for example, testing the heater voltage of tubes when the heater terminals are not in the base of the tube. The plug should be removed from the radio set when the binding posts are used.
122 RADIO SRECEIVERS
Adapters.—The adapters are shown in the foreground of Fig. 1, on the right and left of the plug g. The set is provided with a plug and socket of the UX type. If the set 1s equipped with tubes or sockets other than the UX type it is necessary to select the proper adapters to make the test. A plug with cord and terminals attached is also provided for connection to a lamp socket for line- voltage tests.
TESTING BATTERY-OPERATED SETS
Preliminary Adjustments.—First see that the A, B, and C batteries are connected to the radio set and all tubes in place except in the socket to be tested. Then insert the plug g, Fig. 1, into the empty socket in the set, and the tube into the socket J of the tester. Set the switch c to D.C. and the switch 7 to 30 and proceed with the tests. In new radio sets not previously tested, before any tubes are inserted it is preferable to make a preliminary test of each socket in succession. This will reveal defects, if any, in manufacture and those due to shipment, and possibly save tubes.
In radio sets having phone or speaker vee it is neces- sary to plug in either a headphone or speaker before tests can be made on certain tube sockets, especially in the last stage, as the plate voltage is connected to the plate of the last tube through the phone circuit. In some sets the filament voltage also can not be applied to a tube until a plug is inserted in the corresponding jack.
Testing A Battery.—Turn the switch dial h, Fig. 1, to A or A Rev., whichever gives a positive indication, and adjust to the filament voltage for the tubes, using the 8-volt scale on the instrument. If no indication can be obtained on the instrument, then there is either a broken or disconnected circuit or the A battery is entirely run
AND SERVICING 123
down. If a slow indication results, then either the A battery is low or the contacts are poor in the sockets, or in the rheostat, or in the circuit connections. If an unsteady indication results, then some connection is loose. Loose or variable contacts are frequently found in the spring contacts in the tube sockets and in rheostats.
In the above tests, the voltmeter indicates the voltage across the filament. In order to measure the total A-bat- tery voltage from the socket it is necessary to remove all the tubes from the set and tester and set the rheostat at or near its maximum position. Then the voltmeter indi- cates the A-battery voltage directly. With storage A batteries a more reliable test is obtained with a hydrom-. eter.
Testing B Battery.—Turn the switch dial h, Fig. 1, to either B 300 or B 600, depending on the voltage to be measured, and then read the indication on the correspond- ing scale. If the circuits are good, this indication will give the B-battery voltage less the drop in the primary of the transformer. For radio-frequency transformers this drop is negligible and for the average audio-frequency transformer, on account of the high sensitivity of the instrument, the indicated voltage will be within 1 per cent. of the actual B-battery voltage.
If no indication results, the following should be looked for:
(a) Disconnected battery.
(6) Run down battery.
(c) Spring contacts in socket out of place.
(d) Open circuit in primary of transformer.
(e) Open connection in some part of the plate circuit.
If low indication results, then look for the following:
(a) Partly run down battery
(6) Loose or corroded connection.
4-9
124 INASD VOPe RE cr ievenenes
If a variable indication results, then look for a loose connection in some part of the circuit.
Testing C Battery.—Turn the switch dial h, Fig. 1, to C or C-A Rev.; depending on whether it was necessary to use A or A Rev., respectively, when testing the A battery. The C-battery voltage is indicated on the 60-volt range and is read on the 600-volt scale divided by 10. The voltage read will be less than the actual C-battery voltage by the amount of the voltage drop in the secondary of the transformer. For radio-frequency circuits, when no grid resistors are used, this drop is negligible. For audio- frequency transformers the voltage indicated will be about 90 per cent. of the battery voltage. The C-battery circuit should be tested in the same manner as the B-bat- tery circuit, it being remembered that the secondary of the transformer, instead of the primary, is in circuit.
Locating Circuit Defects——When a defective circuit is indicated in the foregoing tests, a simple method for locat- ing the defective part is to remove the plug g, Fig. 1, from the radio set, and connect the two cables furnished with the set to the binding posts on the instrument, one to — and the other to 600, the switch dial h being set to Vm. B. P. Then if the B-battery circuit is to be tested, the free end of the cable that is connected to the — bind- ing post should be connected to the negative terminal of the B battery on the radio set, and with the free end of the remaining cable, the plate terminal or contact spring in the socket should be touched where the defective circuit was indicated. This circuit should be followed from one connection to another. When the defective part has been passed, the voltmeter will give-an indication, showing that the defect is in the part of the circuit just passed.
The A- and C-battery circuits can be tested in the same manner as described for the B-battery circuit. It is
AN Ds SERV. UCUNG 125
better to use the high-voltage range for all of these circuits for the reason that if-a high-voltage circuit is touched accidentally when tracing a low-voltage circuit, then no damage to the instrument or to the radio set will result.
Testing Batteries Directly.—To measure the voltage of a battery or battery substitute directly at its terminals, connect them to binding posts — and 60 or — and 600 on the tester, Fig. 1, depending on the voltage to be measured. Turn the switch dial h to Vm. B. P. This connects the binding posts to the instrument and makes it an ordinary double-range voltmeter.
Testing of Tubes.—Remove a tube from the radio set and place it in the socket J, Fig. 1, on the tester. Insert the plug g into the socket from which the tube was removed. _ For comparative tests of ordinary tubes it is desirable to select a socket having a B battery of about 90 volts and a C battery of 4.5 volts. If other sockets are also controlled by the same rheostat that controls the socket selected for the test, then these sockets must also contain tubes.
Power tubes, such as UX-120, UX-112A, UX-171A, UX-210, and UX-250 preferably should be tested from the socket in which they are regularly used on account of the higher voltage B and C batteries require.
If it is desired to test all the tubes directly from the sockets in which they are to be used, then all the tubes may be in place in the radio set except the one to be tested, and the plug in the tester inserted in the socket belonging to this tube. Then, by interchanging the tubes in succes- sion, all can be tested. In this test it is preferable not to use the detector-tube socket.
The first test is to determine if the filament or grid is touching the plate. This is accomplished as follows:
1. Set the switch dial h, Fig. 1, to B and then to C0 to make sure that the B and C batteries are connected and
126 RADIO SRECEIVERS
are of the correct values. The filaments may be lighted or not as is found necessary or convenient.
2. Then set the switch dial h to Plate MA., and the range selector switch 7 to 30, and insert the tube to be tested into the socket J. If the pointer on the instrument deflects violently to the right beyond the scale, it indicates that the filament or the grid is touching the plate. The tube should be immediately removed from the tester. In testing power tubes, the plate current resulting when tested without the proper grid bias will often be greater than the full-scale value on the instrument. This com- paratively mild slamming of the pointer must not be mistaken for the violent slamming resulting from a defec- tive tube.
If, after the foregoing tests, the plate-current test indicates approximately normal values on the scale, then the filament and grid are not in contact with the plate and further tests should proceed as follows: Set the switch dial h to A or A Rev., as is found necessary, and adjust the filament voltage, for which the tube is designed by means of the proper rheostat in the receiving set. Change the switch dial h to B and read the B-battery voltage. It is necessary that all tubes be tested at the proper plate voltage, in order to obtain comparative readings. Then set the switch h to Plate MA., and the range-selector switch j to 30 or to 150, as is found necessary. This changes the instrument into a milliammeter having a full-scale value of 30 or 150 milliamperes. Read the plate current on the 300 scale divided by 10 for the 30-milliampere range, or divided by 2 for the 150-milliampere range.
To determine whether the grid of the tube is in operat- ing condition and to indicate roughly the condition of the tube as an amplifier, set the dial switch h on Plate MA., the range switch 7 at 30, and press the key m. When the
SAND. SERVICING 127
key m is up in its normal position the grid is connected to the C battery in the set, and the current indicated on the instrument is the normal plate current of the tube. When the key m is depressed, the grid is connected to the —A terminal, which gives zero grid voltage, with a consequent change in plate current. If the grid is functioning properly, the plate current will be increased upon pressing the key, and the increase in current, when properly inter- preted, is a rough measure of the condition of the tube as an amplifier.
If no change in plate current occurs upon pressing the key, then the following may be the cause:
1. The radio set has no C battery.
2. The C battery, if used, is run down or disconnected.
3. The grid connection may be broken in the tube, or the grid. may be touching the filament. In either case the tube should be replaced. The approximate plate current values for the different types of tubes are given in the printed circulars that accompany the tubes.
To measure the total current drain on the B battery connect the 150-milliiampere binding posts k on the tester in series with the battery circuit at the —B terminal and set the dial switch h to MA. B. P.
TESTING A.-C. OPERATED SETS
Sets Using Raw A. C. for Filament Heating.—All radio sets, whether a.-c. or d.-c. operated, must have direct current for the plate and grid circuits. Therefore, if a.-c. operated, the plate and grid voltages must be obtained by rectifying and filtering the alternating current by means of suitable power units. All tests on plate and grid voltages and plate current must be made on the d.-c. volt-milliam- meter b, Fig. 1, just as on sets operated by batteries or battery substitutes. |
128 SR ADIO SRECEIVERS
The filaments on a.-c. operated sets may be heated either directly by raw alternating current or by rectified and filtered alternating current. To test a set equipped with tubes using raw alternating current, the switch c should be set to A. C.; care should be taken to see that the power supply is properly connected to the set and that all tubes are in place except in the socket to be tested. Then the plug g should be inserted into the socket of the set and the tube into the socket /. Then the same procedure should be followed as is described for battery-operated sets, except for the following: |
Read filament voltages on the 4-volt or the 8-volt range of the a.-c. meter a, and all other voltages and currents on the d.-c. meter b, using the dial switch h as previously described. A low or no reading means a defect in the power unit or in the connections.
To test the five-prong U Y-227 detector tube by voltages from its own socket in the radio set, two adapters must be used; one to adapt the four-prong plug g to the five-hole socket in the set, and the other to adapt the five-prong tube to the four-hole tester socket J. The plug adapter is so designed that the cathode circuit in the radio-set socket is not connected to the tester. The heater current is supplied to the heater element in the tube through the usual filament connections in the plug g, and the cathode is connected to one of the filament terminals in the tube adapter. With this connection, the plate voltage, fila- ment voltage, and plate current as they exist when the tube is in use are measured as for any other tube, but since no C voltage is available in the detector socket, a definite grid test cannot, in general, be made under these conditions.
As the currents required for the UY-227 and UX-226 tubes are comparatively large, there will be a slight drop in the connecting leads when their voltages are being tested.
ANDI SERVICING 129
To obtain the true values of the filament voltages at the transformer terminals add .16 volt to the indication of the U Y-227 tube and .1 volt to the UX-226 tube.
Sets Using Rectified A. C. for Filament Heating.—Sets using rectified alternating current for heating the filaments may be divided into two general classes; namely, those in which the filaments are connected in parallel and those in which the filaments are in series. The sets in which the tubes are connected in parallel are tested in the same way as battery-operated sets.
In series-filament operated sets it is preferable to select a socket having about 90 volts plate potential, and con- nected in the radio-frequency or intermediate-frequency circuit, depending on the type of radio set. Insert the plug g, Fig. 1, into this socket, and the tube into the socket | of the test set. For the complete test the other tubes should be in their respective sockets.
Set the switch h to A or A Rev., as is found necessary to give a positive deflection. Note the voltage, which is the voltage across the filament. Then test for plate current, plate and grid voltage and make the grid tests as for bat- tery-operated sets. Try each tube in succession, removing each one from its socket and placing it into the socket on the tester, remembering to insert the tube just tested into the vacant socket.
If all the tubes are equally low in filament voltage, it is possible that the trouble lies in the power supply, or that in one or more of the tubes the grid is touching the fila- ment. This can often be discovered by gently tapping the tube. If, however, voltages differ among the tubes, then the fault is most likely to be in the tube circuits. While changing tubes in making this test it is preferable to have the power supply shut off to prevent possible excessive voltage from being applied to other tubes.
130 FRADIO SRECEIVERS
No hard and fast rules can be given for these tests, as the sets differ so much in their construction and type of circuits. Anyone, however, familiar with the circuitsof any one particular set can work out methods for using the tester, by following the general directions given herein.
The foregoing instructions have been prepared by the Weston Electrical Instrument Corporation and apply directly to the use of the test set just considered. These same instructions, however, are applicable, with certain modifications, when testing radio receivers with the ordinary a.-c. and d.-c. meters.
SPECIFIC TROUBLES AND ADJUSTMENTS
SIMPLE TESTING EQUIPMENT
Continuity Tester.— When a more elaborate instrument is not available, the simple arrangement shown in Fig. 2 may be used to test the continuity of circuits, windings of transformers, coils, etc., or to locate defective condensers,
FiGa2
short circuits, and grounds. The tester consists of a pair of headphones a, a 43-volt battery b, and the test points c. In place of the headphones one may use a voltmeter with voltage sufficient to give a full-scale deflection when con- nected directly across the battery terminals. The use of the voltmeter is very convenient in checking the voltage drop in the circuits of a receiver. The intensity of the click in the phones or the indication of the voltmeter, whichever may be used, shows approximately the condition of the circuit under test.
AND SERVICING 131
Resistance Measurement.—In a large number of cases the serviceman is confronted with the problem of determin- ing the values of resistors used in receivers and power units. According to Ohm’s law, the resistance R of a circuit, in ohms, is equal to the electromotive force E, in volts, divided by the current J, in amperes, or
h=L=] When the current is given in milliamperes, the formula becomes ee 1,000H + T
From the foregoing one can justly reason that the resistance can be very readily calculated when the electo- motive force and the current are known. A simple scheme for measuring these quantities is shown in Fig. 3. The measuring unit consists of a 6-volt battery a, a 30-ohm
TGA
rheostat b, a voltmeter V having a working range from 0 to 8 volts, a milliammeter MA with a scale of 1 to 250 milli- amperes, and a pair of test points c, all connected as shown in the figure. To determine the voltage and current values, the test points c are placed on the terminals of the unit the resistance of which is to be measured and the rheostat adjusted until satisfactory readings are obtained. The resistance is then calculated as explained in the preced- ing paragraph.
Modulated Oscillator——For certain radio adjustments it is necessary to have a source of modulated high-fre- quency energy to energize the radio-frequency circuits of
4—10
132 FRADIO SRECEIVERS
the receiving set and produce an audible note in the phones or in the loud speaker. The most satisfactory generator of high-frequency energy is a vacuum-tube oscillator, a convenient type of which is shown in Fig. 4. The coil- condenser combination, L and C, respectively must be designed to cover the frequency range of the receiving set. For broadcast receiving sets the approximate frequency range is from 500 to 1,500 kilocycles.
The coil to be used with a .0005-microfarad tuning condenser to cover the broadcast range may be wound on
=I
Fic. 4
a 25-inch bakelite tube with fifty turns of No. 20 double- silk-covered wire. The coil is tapped at the twenty-fifth turn and a connection made to the negative terminal of the A battery.
Type UX-199 tube is recommended for the oscillator, although the general purpose tube UX-201A may also be used. With a UX-199 type tube, a 44-volt C battery may be used to light the filament, or act as the A battery. A 30-ohm rheostat is used in the filament circuit. Two small 223-volt B-battery units may be used in the plate circuit. In this way it is possible to make the oscillator a self-contained unit.
The grid condenser may have a capacity of .00025 microfarad. The resistance value of the grid leak deter- mines the pitch of the audible note produced by the oscil-
AIN'D® J fiRV ECIN.G gk.
lator. A 4- or 5-megohm leak will probably give the desired tone. If a higher tone is desired, a lower value of grid leak is used, and vice versa.
VARIABLE-CONDENSER TROUBLES
Possible Troubles.— Variable condensers are the tuning units in most of the present-day radio receiving sets. They are thus exposed to a lot of wear and tear, and, unless ruggedly constructed and well mounted, they will in time cause considerable trouble and inconvenience. The dials may slip out of position; the movable contacts, when not protected by pig-tail connections, may become dirty or loose, and introduce noises; the plates may become covered with dust, which also results in noises and lessened efficiency; the plates may be bent or the entire rotor or stator laosened, so that the condenser is shorted through part or the entire tuning scale; one of several condensers, when all are controlled by one tuning dial, may slip out of position and detune the entire assembly for all wavelength settings. The remedies for some of these troubles are quite obvious. The location and adjustment of many of these troubles, however, are not easy and should not be atempted by any one but those familiar with the correct procedure.
Shorted Plates.—Shorted plates are in some cases evi- denced by a scraping noise when the condenser dial is turned. In most cases, however, it is necessary to test the condenser electrically to determine the nature of the trouble. The simple test set shown in Fig. 2 may be used for this purpose. Disconnect the condenser from its circuit and connect the test points c to the terminals of the condenser with the rotor plates entirely out of thestationary plates. A short circuit in the plates will be evidenced by a click in the phones when the condenser dial is turned.
134 CRADIO “RECEIVERS
A visual inspection will then reveal the difficulty. Bent plates can sometimes be straightened with the ordinary long-nose pliers. Wear at the bearings can be com- pensated by tightening the adjustments, when such are provided.
Body Capacity.—Sometimes the tuning of a set is affected by the operator’s hand in contact with the condenser dial. This is known as body capacity. The most probable cause of this trouble is a reversal in the stator and rotor connec- tions. The stationary plates should be connected to the grid terminal of the tuning transformer, and the rotary plates to the grid-return terminal. Shielding is also effective in removing this difficulty. |
Misalinement of Multiple Condensers.—A large num- ber of present-day receiving sets are equipped with single- control dials. In some of these there is no form of com- pensation for discrepancies in the tuning circuits, and all adjustments must be made at the main condensers. In other cases, vernier condensers are employed to bring the circuits to resonance when minor discrepancies in tuning develop.
When radio reception is very weak and it is positively known that the batteries, or power units, tubes, trans- formers, by-pass condensers, and their connections check O. K., the trouble may generally be ascribed to a misaline- ment of the tuning condensers. Wide discrepancies in the. positions of the condenser plates can be determined by inspection. Minor discrepancies, however, can be deter- mined only by suitable electrical tests.
When there is no provision for changing the position of either the rotary or stationary plates, the only remedy is the replacement of the entire condenser assembly. Condenser units with adjustable rotors or stators can be brought to resonance as follows:
AND SERVICING 135
Set the oscillator, Fig. 4, in operation with the dial set near one end of its scale. Place the receiving set also in operation with the aerial and ground disconnected. Tie one end of a 20-foot insulated wire around the grid coil of the detector tube and place the other end near the oscillator. Remove all the radio-frequency tubes and tune the receiver to maximum signal, setting the vernier controls, if used, in their mid-positions. The position of the dial for maxi- mum signal is marked in a convenient location.
Remove the pick-up wire from the tuning coil of the detector tube, place it around the tuning coil of the last radio-frequency tube, and replace the tube in its socket. Tune the set as before, noting whether the position for maximum signal corresponds with that previously obtained. If there is a discrepancy, see whether it can be corrected with the vernier dial, if used. Otherwise, shift the tuning- condenser rotor or stator, whichever is adjustable, until the positions for maximum signal intensity correspond. Pro- ceed in the same manner with the remaining radio-fre- quency circuits, working backwards from the detector tube to the first radio-frequency amplifier tube.
Then set the oscillator at the other end of its scale and repeat the foregoing tests and adjustments. Generally, when the set has been adjusted at one frequency it will be found satisfactory on all other frequencies, but it is well to check it and make readjustments when necessary.
TESTING FIXED CONDENSERS
The simplest way to test fixed by-pass or filter con- densers is by the charge and discharge method. Discon- nect the leads from the terminals of the condenser and connect the condenser for a brief interval across a suitable source of d.-c. potential, such as a B battery or the output of a power unit. Disconnect the condenser from the
136 SRADIO SRECEIVERS
power source and connect a piece of wire across its termi- nals. If the condenser is in good condition a discharge spark should take place the instant the condenser terminals are shorted. If no discharge spark or a sharp crack takes place, the condenser is open, short-circuited, or leaky, and should be replaced.
ADJUSTMENT OF NEUTRALIZING CON DENSERS
In practically all radio receiving sets some form of balancing is employed to reduce the tendency to self- oscillation. When this adjustment is unbalanced, the receiver has a tendency to oscillate at practically all settings of the station selector dials. Before attempting to balance such a set it is well to check the filament plate and grid voltages, examine the grid circuits for opens, and test the tubes. The trouble is manifested by poor-quality signals and whistling and howling in the loud speaker.
There are two common methods of stopping oscillations; namely, by the use of grid resistors and by the neutrodyne system. Where grid resistors are used, the procedure is quite simple. Remove the resistor from its clips and test it with the unit shown in Fig. 3. If it is open, short- circuited, or of the wrong value, replace it with one of the correct resistance value.
In sets employing the neutrodyne system, small adjust- able condensers are used to effect balancing. Should these get out of adjustment, they may be readily readjusted as follows: Procure a tube similar to those used in the radio-frequency stages and saw off one of the filament prongs. Place the receiving set in operation with the aerial and ground connected, and the oscillator, Fig. 4, near the aerial wire. Tune the oscillator and set to a low reading on the dials, adjusting the set to maximum loudness. Remove the first radio-frequency tube from
AND SERVICING 137
the set and insert in its place the special tube. Now adjust the neutralizing condenser until the signal is minimum or disappears entirely. The adjustment of the neutralizing condenser is quite critical and should be done with care. When the first stage has thus been neutralized, remove the special tube and reinsert the good one. The remaining condensers are adjusted in the same manner. The adjust- ment is checked with the oscillator and set tuned to a high reading on their respective dials.
SERVICING OF POWER UNITS
Determining Whether Power Unit Is At Fault.—When radio reception is unsatisfactory and it is suspected that the B power unit is at fault, it is advisable first to check up on the other accessories, such as tubes, A battery, C battery, aerlal and ground connections, and the loud speaker. If all these seem to be in good condition, it is well to substi- tute a set of B batteries for the power unit and note the difference in operation. If this test shows that the B unit is at fault, the first thing to do is to make sure that the socket power is on and to try a tested rectifier tube in place of the one that is in the power unit. If the new tube improves the operation of the set, obviously, the trouble has been corrected.
Testing the Power Unit.—If the new tube does not improve the operation, the power unit should be tested for opens, short circuits, and grounds. With