Communications equipment schematic manual. [1st ed.]

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J.dP~ PHOTOFACT PUBLICATION

communications equipment schematic manual

Complete schematic diagrams and basic theory for modern equipment, including:

•

•

FM TRANSMITTERS

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DOUBLE SUPERHETERODVNE RECEIVERS

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POWER SUPPLIES

•

ALIGNMENT PROCEDURES

$3.95 Cot. No. CEM-1

communications equipment schematic manual

[

HOWARD W. SAMS & CO., INC. THE BOBBS - MERRILL COMPANY, INC. Indianapolis

• New York

FlRST EDITION

FIRST PRINTING -

AUGUST, l963

COMMUNICATIONS EQUIPMENT SCHEMATIC MANUAL

Copyright © 1963 by Howard W. Sams & Co., lnc., Indianapolis 6, Indiana. Printed in the United States of America. Reproduction or use, without express permission, of editorial or pictorial content, in any manner, is prohibited. No patent liability is assumed with respect to the use of the information contained herein. Library of Congress Catalog Card Number : 63- 191 03

PREFACE Commercial FM two-way radio is becoming increasingly popular as a sophisticated means of communcication for businessmen and blue-collar workers who need instant, semi-private contact with their offices or fellow workers. Each month the FCC issues several new licenses which permit operation of one or more base stations and several mobile stations. Generally, older license holders eventually apply for an expanded license (one which allows the holder to operate more stations). Radios enable businessmen to keep in constant touch with their offices while on the road; they are used by road construction crews to better coordinate their overall project efforts; and they are even used by doctors to receive emergency calls. For this reason, two-way radio owners are entirely unlike TV set owners. A TV owner uses his set for pleasure, but the radio owner uses his equipment for furthering his business interests; thus has a much more important reason to spend money for maintenance and repair.

July, 1963

Indeed, regardless of the fact that there are thous-ands of CB radios in use, the commercial FM two-way radio business is booming, and smart service technicians are riding this boom to increased profits and a larger business clientele. All two-way units need frequency and modulationdeviation checks every 'Six months, as provided by the FCC Rules. In addition, they must be kept in peak performance by alignment and tuning which can only be done by a qualified, licensed technician. Communications Equipment Schematic Manual not only presents schematics and circuit discussions of twoway radios now in actual use across the country, but it also helps familiarize you with the accepted techniques and practices of keeping these high-quality radios in peak condition. Also presented are methods for checking frequency and modulation deviation as well as aligning and tuning of transmitters. Several types of typical, modern FM two-way radios are used as examples.

CONTENTS CHAPTER 1

THE FM Two -WAY RADio- ..........................................................................................................................................7 Transmitter Block-Diagram Analysis-Receiver Block

Diagram Analysis

CHAPTER 2 Two-WAY RAD10

FM

TRANSMITTER............ ................. ......................... ................. ...............•...............•....•.............. 12

Aerotron 600 Series TransmitterBendix 16TS-1 and 16TS-2 TransmitterComco types 582-T and 582-T-N Transmitters-Comco Type 905-T-E Transmitter-General Electric

TPL Transmitter, Type ET-33-C,DPower Amplifier Model 4EF11A10,11The Motorola Transmitter- The RCA Super Carfone Transmitter

CHAPTER 3 TH E

FM

Two-WAY RADIO R ECE IVER . . .......................... . . ...........................•................................ .. . .•......•... ........ .. ....43

Electric TPL 130-174 me FM Receiver Type ER-31-C,D-The Motorola Motrac ReceiverThe RCA Super Carfone Receiver

Aerotron 600 Series Receiver Circuit Description-The Bendix 16RS· l Receiver-Comco Type 580-R, 582-R, Receivers--Comco Type 905-R Receiver-General

CHAPTER 4 THE TRANSISTORIZED POWER SUPPLY ... ................. . .... .....................................................•........................................ 87

905-D Power Supply-General Electric TPL 12-Volt Power Supply Model 4EPISB10-Motorola Motrac

Aerotron 600 Series Power SupplyBendix 15PT-1 Power SupplyComco Types 581-D and 582-D Power Supplies-Cameo Type

CHAPTER 5 MAINTENANCE AND ALIGNMENT P ROCEDURES..................................... ......................•....... ....................................... 103

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Recommended Test EquipmentAerotron Preventive MaintenanceReceiver Alignment ProcedureTransmitter Alignment ProcedureTVR Maintenance-Bendix 16TS

Alignment Procedure-Bendix 16RS Alignment Procedure-Comco 582 Series Receiver Alignment and Maintenance-GE ER-31 Alignment Procedures

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CHAPTER 1

THE FM TWO-WAY RADIO A two-way radio by itself comprises a complete broadcast and receiving station. The transmitter is quite similar to a commercial broadcast station except that it commonly uses a form of FM called phase modulation. Likewise, the two-way radio receiver is quite similar to a standard superheterodyne receiver. The two-way radio receiver, however, is far superior to the standard superhet in that it utilizes dual conversion to achieve greater sensitivity as well as better image rejection. This simply means that there are two oscillators and two IF's in the receiver. The combination of the transmitter, receiver, and self-contained power supply makes the two-way radio the most versatile means of communications in the world. TRANSMITTER BLOCK-DIAGRAM ANALYSIS

Since phase-modulation (PM) is used almost exclusively in commercial two-way radio, the function of each stage will be studied briefly. It is unnecessary here to discuss standard FM, because this system is practically nonexistent in commercial two-way radio. Oscillator

The prhnary function of the oscillator in Fig. 1-1 is to generate the fundamental RF signal necessary to

FUNDAMENTAL OSCILLATOR

PHASE

mitted energy, any changes here show up in the RF carrier. Therefore good frequency stability is essential in maintaining good-quality communications. If the oscillator were to drift off frequency , it might cause not only poor (or even complete loss of) communications, but interference on the adjacent channels as well. To provide this needed stability, the osciUator is crystalco.ntrolled. Many manufacturers go even one step further and enclose the crystal in a heated, thermostatically controlled oven to prevent any frequency drift caused by temperature variations. Audio Amplifier

The audio signal produced by the microphone is relatively weak and must therefore be amplified (by a conventional audio amplifier). From the speech amplifier (Fig. 1-1) the signal is fed into a limiter stage. The purpose of the limiter is to prevent any sharp peaks (like noise) from getting into the modulator and causing overdeviation of the RF carrier. ff the signal from the speech amplifier Is weak-due to an extremely weak microphone signal-the limiter will act like a conventional amplifier. However, if the signal is too strong, the limiter ceases to amplify anything over a predetermined amplitude. Without the limiter the RF carrier might be overmodulated. From the limiter the audio signal is again amplified in another conventional amplifier, after which it is fed into the phase modulator.

MOOULATO~

The Phase Modulator MICROPHONE

SPEECH ~w>L.IFtEA

l.lV.ITER A>.'!)

AMPLIFIER

Fig. 1-1. Block diagram of phase-modulated transmitter.

drive the transmitter into the production of an amplified RF signal suitable for transmission. As might be expected, since the osciUator is the source of this trans-

In the phase-modulator stage the RF signal from the oscillator and the AF signal from the audio amplifier are combined. Although the modulator here might look like a mixer or a low-level modulation stage of an AM transmitter, it actually is designed to hold the amplitude modulation of the RF to a minimum. The modulator is included at this point in the transmitter to simplify the design and redi.1ce the audio power required to drive it. Actually the modulator can be considered a mixer of frequencies in which two different signals are intermingled to create one composite output ready to be multiplied. 7

Buffe r- Doubler

The oscillator and modulator have a buffer crrcuit in their output to isolate them from the multipliers. Because frequency multipliers present a variable load to the preceding stage, nonlinear modulation could occur if the modulator were that preceding stage. A good buffer requires very little driving power for efficient operation. Also, the fact that its driving requirements are constant enables the modulator output to remain linear. If the modulator and oscillator worked into a changing load (like an RF amplifier or frequency multiplier), their output requirements would likewise change and thus reflect a changing impedance into their circuits. The buffer with its doubling action is only the first in a series of multiplier amplifiers. FM transmitters usually use doublers, triplers, quadruplers, or any combination of these to increase the frequency and power in steps toward the final transmitted frequency. The reason for using several stages to obtain the final frequency and power is obvious. The oscillator signal is a weak 0.2 watt or so, and there is no feasible tube that will put out a signal of 15 or 20 watts with only a 0.2 watt input. Therefore, one multiplier amplifier drives a second, the second a third, and so on. In this way the power, frequency, and deviation are gradually increased more stably and efficiently than by using only one tu be (if this were possible) . Tri pier

Following the buffer-doubler is the tripler (Fig. 1-1) . Because less power output is available from any multiplier-amplifier for the higher-order harmonic frequencies, low ones (usually no higher than the fourth) are utilized in multipliers. The frequency tripler operates essentially like the preceding buffer-doubler, except that its grid and plate circuits require more input power because of the greater output power needed. By operating the tripler (and all multipliers) in Class-C, maximum efficiency can be realized from the stage ( s). Second Doubler

The frequency and power in a PM transmitter are gradually increased in steps. Because the power is higher from one stage to the next, the design of each stage must be different. The larger the power, the higher the current-carrying capabilities of the tube need be. As an example, the oscillator in most transmitters is a miniature sharp-cutoff pentode, and the second doubler is usually a miniature power pentode. Except for the increased power requirements and capabilities, the second doubler in a power-pentode arrangement operates exactly like preceding multipliers. This Class-C 8

amplifier bas its plate circuit tuned to recognize the second-harmonic frequency of the grid input signal. The Doubler- Driver

The doubler-driver performs two important functions. First, it multiplies the signal frequency one last time before the latter is amplified by the final stage and transmitted. Secondly, the power delivered to it by the second doubler must be amplified high enough to drive the final RF amplifier. The twenty-fourth harmonic frequency of the original oscillator signal appears in the output circuit of this stage. By multiplying the order of multipliers we see that the buffer-doubler, tripler, second doubler, and finally the doubler-driver multiply the .frequency a total of 24 times (2 X 3 X 2 X 2 equals 24; and 24 X 6 me equals 144 me). Likewise, the original ±500-cycle deviation now becomes + 12 kc. At this time the signal is now at its final frequency, ready to be transmitted; however, the RF power is not sufficient to carry the signal very far. Different classes of power amplifiers will be encountered after the doubler-driver, depending on the type of service for whfoh the transmitter is licensed, and also on its design. The Final Powe r Amplifier

As shown in Fig. 1-1, the last stage of the transmitter is the power amplifier. Here the RF signal is boosted to the desired level for transmitting. The output power of this stage, however, is not the same in all frequency ranges, even if the same types of tubes are used. As the frequency increases, the power output capability of a tube decreases. The efficiency of the final stage is usually 50 % or better. This figure is computed by dividing the plate power consumed from the power supply (P=IE) by the measured output power in watts as read on an RF wattmeter. As an example, if a 6146 were used as a final with plate conditions of 600 voe at 200 ma, the input power would be 600 X 0.2 equals 120 watts. At 50 % efficiency, then, this stage would have an RF power output of approximately 60 watts (120 X 1:12 = 60). The output of the power amplifier is fed to the antenna where the RF energy is radiated into space. However, the antenna and amplifiers cannot merely be connected together because there would be a serious impedance mismatch between them. Instead, a coupling network is needed to properly match the impedance of the RF output stage to that of the antenna system. Only then can maximum power be transferred to the antenna. RECEIVER BLOCK- DIAGRAM ANALYSIS

A good-quality PM or FM communications receiver must be capable of reproducing the transmitted in-

formation as intelligible audio, with little noise, at the speaker. For this reason, amplitude modulation was mainly given up as a faithful means of two-way radiocommunications. AM suffers from all kinds of noise, both natural and man-made. The transmitted wave is subject to so many kinds of interference that, before it is received, it quite frequently has picked up enough extraneous noise signal to make reproduction unintelligible. Noise pulses produce variations in the amplitude radio signals and are therefore very troublesome in AM operation. Tbe bum of power lines, spark of the ignition system in an automobile, running of a motor, and even atmospheric disturbance are but a few of the sources of interference to these signals. However, the noises that bother AM do not cause interference in FM reception because they affect only the amplitude of the signal, not the frequency. In an FM transmitter, as previously explained, the audio modulation causes excursions of the frequency-not amplitude, as in AM. Therefore, any amplitude variations of the FM wave (and there are many, due to the same noise sources as for AM interference) are not reproduced in the receiver. Instead they are removed from the signal by the limiter stages. As a result, only frequency variations are converted into audio,

that will faithfully reproduce the greatest output with the smallest input signal. The next major requirement of a receiver is its ability to select a desired station and to reject all others.

(A)

FM.

(B)

AM.

Fig. 1-3. Block diagram of FM and AM receivers.

Unwanted adjacent-channel signals are rejected by the bandpass characteristics of the IF amplifiers, while most of the incoming image frequencies are lost in the RF amplifiers and mixer. The combination of sensitivity and selectivity makes for a very efficient receiver. RF Amplifier

Courtesy Motorola C. b E. Inc.

Fig. 1-2. Receiver of a typical two-way radio.

Tbe receiver of a typical two-way radio is shown in Fig. 1-2. Each coil can has a tunable slug inside it to permit periodic adjustment. Fig. 1-3A shows the block diagram of a basic FM receiver. In spite of the apparent similarity to the standard AM receiver in Fig. l-3B, the FM receiver is actually quite different. The requirements of FM are the same as for AM, only more stringent. Perhaps the most important is high sensitivity, which is determined by the minimum RF signal-voltage input required to produce a specified output at the speaker. The most sensitive receiver is one

The RF amplifier, being the first stage of the receiver, naturally affects all following stages. In today's modern narrowband communications receivers, the RF section must be critical in accepting the required signal and still reject all others. This is hard to do, since the resonant antenna circuit must be broad enough to be tuned over a complete band. Because so many stations operate on the same or on adjacent frequencies, the problems of intermodulation and desensitization are more acute than they were in the earlier days of FM. These two conditions occur during reception of undesired RF signals in the RF amplifier circuit. Desensitization is the condition whereby an off-channel signal causes the amplifier grid to draw current on the positive peaks. This increases the grid bias and thus reduces stage gain. Desensitization means the receiver has become "desensitive" and requires a much stronger signal to operate. This can happen if a strong adjacentchanoel signal gets into the RF circuit and is amplified. The signal will not pass all the way through the !F's because of their bandpass response; therefore the operator will probably be unaware that another station is interfering. However, if an "on-channel" signal is re9

ceived, it now must overcome the interfering signal or the set will be effectively dead. Intermodulation is also caused by undesired signals entering the RF stage, but does not kill the receiver. R ather, it is reproduced and heard as audio in the speaker. Providing it is strong enough, an "on-channel" signal will usually overcome this condition, and then only the desired information will be heard. Unlike desensitization, intermodulation occurs only when two signals of the proper frequency are present at the antenna at the same time. As an example, if a 52-mc receiver has two signals-one at 52.12 and the other' at 52.24 me-entering its antenna circuit, they will produce a resultant at 52 me. Since the second harmonic of 52.12 me created in the mixer stage is 104.24 me, the difference between it and 52.24 me is 52 me-the center frequency of the RF circuits. Any modulation on either original signal will be recognized by and heard in the speaker. Desensitization and intermodulation are strictly problems of selectivity; therefore, good RF amplifier design is important. In normal use the amplifier seldom develps trouble. When a component does need replacing however, the exact replacement should be 'used if possible, or the one closest to it. What a different value of component will do to help or hinder intermodulation and/ or desensitization in a receiver is impossible to accurately predict. Just keep in mind that the original component was put there for a specific purpose, and you should not try to work around RF amplifier problems by trying to rebuild the circuits. They have been designed at the factory for optimum performance, and seldom if ever need reworking. High-Fre.quency O sci llator

In a high-frequency oscillator, the crystal doesn't oscillate at the desired freq uency. Instead the oscillator operates on a crystal harmonic, and the desired working frequency is obtained by means of a multiplier stage similiar to those employed in the transmitter. This oscillator works like the one in the transmitter, except it develops much less power. If excessive, power might leak through the RF amplifier stage and into the antenna, where it would be radiated and cause interference with other stations. The power requirement of the receiver oscilJator is small because it mixes only with the incoming RF to produce the IF. In the transmitter, however, the oscillator must develop more power because it drives frequency multiplier-amplifiers. In the VHF range of frequencies where most two-way radios operate, the local oscillator must operate at a very high frequency. An overtone or a harmonic crystal oscil1ator is therefore necessary. The harmonic oscilla-

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tor js tbe lesser used because of the very high harmonic content in its output circuit. 1f these harmonics are allowed into the mixer, serious difficulty from spurious responses may arise. Therefore, a special crystal is used which is made to oscillate on other frequencies very close to odd harmonics of their fundJmental. The main advantzge of this crystal oscillator is its relative freedom from harmonics below its oscillating frequency. To realize this feature, special circuitry must be employed that has some means of feeding back part of the output signal into the i11put circuit. This feedback is not used for oscmating purposes, but rather to bold the rate of oscillations steady at the desired overtone. The necessary energy is generally developed in the output resonant circuit and fed back either inductjvely or capacitively. M ixer

The mixer stage in an FM receiver beats the applied RF amplifier signal against the inj~cted RF oscillator signal in order to produce the intermediate frequency ( IF). This mixing action produces sidebands at the sum and difference frequencies of the two applied signals. Generally, the difference signal ( lower sideband) is used as the IF, and since the local-oscillator signal is stronger than the RF amplifier signal, development of spurious sidebands is kept to a minimum. However, if the oscillator signal happens to contain harmonics, they will also beat against the incoming RF signal and produce sidebands. To prevent these unwanted sidebands from feeding into the following stage, the output circuit of the mixer is designed to discriminate against all but the desired sideband. Generally this dfacrimination is accomplished by two or three very critically tuned ( high-Q) resonant circuits following the mixer. High-IF Amplifier

Considerable IF selectivity is required in communications receivers in order to prevent adjacent-channel interference. However, the IF gain must also be increased when higher IF selectivity is desired. Because of difficulties arising from instability and feedback in an IF system, there is a limit to the amplification-and therefore. the selectivity--obtainable. Besides this, a high-frequency IF is needed in order to provide for good image rejection, but the high IF furt.her lowers the selectivity obtainable. Therefore, the problem is met by using two TF frequencies to help reduce instability. The high IF provides the necessary image rejection, and the low IF the necessary selectivity. To gain the full advantages of FM reception, the IF system must have a selectable response that does not introduce objectionable distortion into the sideband

system. At the same time, it must have sufficient adjacent-channel rejection. If the selectivity curve is not symmetrical about the center frequency, considerable distortion will appear in the receiver output because some sidebands are amplified more than others. To correct such distortion, the response curve must be made as flat-topped as possible. High-Gain, Low-IF Amplifier

Since maximum overall gain in the IF amplifier permits the discriminator to operate at the high level required, the amplifier gain must be increased. One way is to decrease the ratio of capacitance to inductance in the IF transformers. Yet, whenever the grid-circuit capacitance is reduced to too low a value, the varying transconductance of the amplifier changes the effective input capacitance sufficiently to detune the stage and cause distortion. When sharp impulse noise bursts increase the signal at the input of the last IF stage, these bursts momentarily swing the signal far into the cutoff region. This detunes the IF transformer because the input capacitance can change as much as 2 mmf. The sudden detuning spoils the symmetry of the amplifier response curve and thus produces distortion. Most of the amplitude variation produced by this type of distortion is removed in the limiter, but phase distortion remains. Since the gain of an IF amplifier increases as the product of the transformer capacitance decreases, it is possible to overcome this distortion. This is done by using the stray capacitance to tune the output circuit (and thereby increase the ratio of inductance to capacitance) while employing a fairly high input capacitance. Limiter

A limiter is in reality an IF amplifier so arranged that, beyond a certain point, a further increase in input signal wiJI produce no corresponding increase in output signal. If the gains in the low-IF amplifiers are such that a strong signal is delivered to tb.e limiter, amplitude variations in the signal will be removed. Since the discriminator responds to frequency variations only,

the removal of these excessive amplitude vananons does not adversely affect the signal reprcxluction. Any overall gain in the limiters is undesirable because it will increase the problem of high-gain intrastage feedback. Amplification in the limiter takes place only when there is insufficient signal to drive it into saturation. Since about 2 volts of signal is required to saturate the limiter, an overall gain of 2,000,000 must be realized ahead of it with an input of 1 microvolt in the antenna. Discriminator

Following the limiters in the FM two-way radio receiver is the discriminator. It is the purpose of this stage to convert the IF signal into audio variations. Generally a discriminator works on the double-tuned transformer principle. That is, the input of the stage has two separate transformer windings that are critically tuned (one above and the other below the IF). The incoming IF signal variations cause current flow in either of these windings according to the polarity of the signal at any given instant. This current flow is rectified by a pair of diodes and smoothed by one or two resistors and capacitors. The discriminator is not designed for hi-fi. In radiocommunications it is sufficient to reproduce frequencies from approximately I 00 to I 0,000 cps. In fact, many units won't even reproduce audio this good. Audio Amplifier

The weak audio signal appearing at the discriminator output is usually amplified by two stages of audio amplifiers before it is delivered to the speaker. Besides being fed directly into the audio amplifiers, however, the audio is also fed into the squelch circuit. The squelch circuit, when it receives an audio signal, opens the AF amplifier and allows the audio to be amplified and sent to the speaker. Normally the squelch circuit holds the AF amplifier cut off when there is no audio present at the discriminator. This is done to prevent the irritating noise developed in the extremely high-gain receiver from reaching the speaker when there is no incoming message.

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CHAPTER 2

TWO-WAY RADIO FM TRANSMITTER All FM two-way radio transmitters are operated on the same basic principle-indirect FM, or phase modulation. This is the process of first developing an RF signal in an oscillator, and then modulating it with amplified audio from the microphone. The signal is then further amplified and multiplied in a series of voltage amplifiers (doublers and triplers) until it hns sufficient frequency (after multiplication) and strength (after amplification) to transmit. AEROTRON 600 SERIES TRANSM ITTER

The AEROTRON 600 series transmitter (Fig. 2-1) consists of a high-stability crystal oscillator followed by a phase-modulator stage. Several stages of frequency multiplication and amplification are required to reach the desired carrier frequency and provide a nominal output of 15 watts in the 148- to 174-mc band. When operation is required at ± 5-kc deviation, a frequency multiplication factor of 12 is used. If more than ± 5-kc deviation is required, a multiplication factor of 24 is used in order to reduce the possibility of excessive modulator distortion at the higher deviation levels. The pentode section of a 6BR8 ( V202A) is used in a Colpitts-oscillator circuit with fundamental-mode crystal Y201. C207, which has a nominal value of 22 mmf, may actually be any value from 18 to 27 mmf, depending on the cumulative effect of stray capacities and component tolerances on the crystal frequency. This i s a selected value that is chosen in final production test to produce the exact carrier frequency at the approximate midrange setting of C-204. C-204 is therefore used only for very small adjustments in frequency to compensate for tube aging or component changes. The output from the oscillator is coupled to the triode section of a type 6BR8 tube (V202B), which is a conventional phase modulator. The phase-modulated output of the oscillator is applied to buffer, or first-frequency multiplier, V203A, the triode section of a 6AW8A. The plate circuit of V203A is tuned by L211 and C218. The output of 12

V203A is applied to the pentode section of V203B, which operates as a frequency doubler. The signal is then applied to one half of a 6360 twin tetrode (V204) operating as a frequency tripler. L209, in the plate circuit of the frequency tripler, is coupled through a very small capacitance to L208 to form a critically coupled, double-tuned transformer that further attenuates spurious crystal-oscillator harmonics from previous stages. This leaves an exceptionally clean signal to feed the final frequency-multiplier stage. The second section of V204 operates as a mediumpower frequency doubler with its plate circuit (L206) tuned to the carrier output frequency. A link-coupled, double-tuned circuit consisting of L206, L207, L205, and L204 offers additional protection against spurious responses and also couples the unbalanced output of V204 to a balanced input at the grids of final RF amplifier V205. Final RF amplifier V205 is a 6360 used in a highefficiency, push-pull, class-C circuit. Due to the mechanical and electrical symmetry of the circuit components and the internal construction of the 6360, no external neutralization is required over the entire operating range of the equipment from 148 to 174 me. The output of V205 is coupled to the antenna by L203. C235 is used to cancel the reactance in the antenna circuit and to provide a proper match for the coaxial cable to provide optimum power transfer. A low-pass filter is included in the cabinet of the 6Nl5/U to provide the additional harmonic attenuation necessary in the special installations needing this feature. A small DC voltage is provided for the operation of a transistorized, controlled-reluctance microphone at the junction of R 201 and R209. Audio from the microphone is coupled through a 6-db per octave preemphasis network consisting of C202 and R203 to the triode section of a 6BN8 (V201). The output from this stage is applied to the two independent diode sections of V201, which operates as a symmetrical clipper. The output from the clipper is applied to the phase modulator through a dual-section, 12-db per octave, low-pass, audio-filter network consisting of R-

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