Power Supply Circuits

TM 11 5820-509-35

than previously. The reduced conduction of Q28 causes

amplifier Q8 and the reference frequency is summed

the base of relay driver Q29 to become more positive

with the output of clippers CR9-CR15 in the adder. The

and it conducts. As Q29 conducts, relay K6 is operated

unshifted output of Q23 is also summed with the output

and a small dc voltage is applied to the dc error A and

of CR9-CR15 in the adder circuit associated with

dc error B inputs to the varicap control circuit of signal

discriminator amplifier Q11. Transistor pairs Q8-Q11,

data translator A3. Since this small voltage is used to

Q9-Q12, and Q10-Q13 are each carefully selected to

calibrate the hf oscillator initially, the frequency of this

provide identical gain in each discriminator channel.

circuit returns to a value within the capture range of the

When the hf oscillator signal from signal data translator

discriminator circuits. After capture is assured, antilock

A3 differs only slightly from the reference oscillator

relay K6 is deenergized (as the voltage again appears at

signal (by 5 kHz, or less), the envelope of the summed

the base of transistor switch Q28). The dc error

signals in these two discriminator channels will remain

voltages at the output of the discriminator regain control

out of phase by an amount dependent upon the

of the hf oscillator frequency-determining circuits and

frequency error between the two signals. The leading

adjust the output frequency accordingly.

edge of the summed signal envelope triggers frequency

discriminators Q10 and Q13 so that they operate like a

bistable multivibrator at a rate equal to the phase-

2-8. Power Supply Circuits (fig. 7-5 and 7-6)

difference between the two envelopes.

The error

voltage at the output of these two transistors may be

a. When the 115-volt, 400-Hz ac primary source is

either positive or negative, depending upon whether the

used to power the radio set, the cable is attached to the

hf oscillator signal is higher or lower than the frequency

POWER receptacle on the front panel of RT-671/PRC-

of the reference oscillator signal. The amount of

47 and this voltage is routed to the radio set by fuse F2.

frequency-difference

between

the

two

signals

A portion of this input voltage is applied to low-voltage

determines the amplitude of this dc error voltage. The

transformer T2 in power supply module A5 where the

dc error voltages are applied to voltage-controlled

low-voltage circuit steps down this potential, rectifies it,

capacitors in the frequency-determining circuits of the hf

and filters it in conventional bridge-rectifier, capacitor-

oscillator in signal data translator A3. As the frequency

input LC filter circuits. The resulting 24-volt dc output is

of the hf oscillator is corrected, the error frequency at

routed to audio frequency amplifier A1 and passes

the input to the discriminator is reduced and the

through the normally-closed contacts of B + relay K3 to

resulting dc error voltage at the output of this circuit

the signal data translator. A conventional series-type

again approaches zero Coarse frequency control of the

electronic voltage regulator, consisting of transistors Q1

hf oscillator is obtained by the adjustment of the

through Q3 and their associated circuits, is used to

frequency control knobs on the front panel of RT-

provide a regulated 20 volts dc output. This potential is

671/PRC-47. The 2 5- to 12.499-MHz hf oscillator

applied to the continuously energized circuits of the

signal is isolated and amplified by buffer amplifier Q21

receiver-transmitter, and to the voice-operated circuits

before being routed to follower Q2 and the subsequent

of amplifier-modulator A2 and signal data translator A3.

circuits. Amplifiers Q6-Q7 and associated follower Q8

An output from low-voltage rectifier CR26 through CR29

provide hf oscillator signals to the receiver-transmitter

provides 26.5 volts dc [unregulated and unfiltered) for

circuits; while amplifiers Q3-Q4 and associated follower

circuits in audio frequency amplifier A I, signal data

Q5 provide the injection signal for diode quad mixer

translator A3, and the panel lamps on the front of

CR11 in oscillator control A7.

RT671/PRC-47. A branch of this same circuit arms the

d. In some instances, transients may cause the

push-to-talk relay (A5K1) in the power supply module.

discriminator circuits of oscillator control A7 to lose

b. A portion of the incoming 115-volt primary

capture of the hf oscillator in signal data translator A3.

power is applied to step-down transformer T2, is

The automatic capture circuit consists of detector CR12-

rectified by bridge-rectifier CR3 through CR6, and

CR13, transistor switch Q28, and relay driver Q29 in

applied to ac power control relay K2. The contacts of

association wi th antilock relay K6 on the main chassis

this relay route the primary power to high-voltage

of RT671/PRC-47. As the loss of capture occurs, the

transformer T1 in power supply A5 whenever POWER-

voltage drop across emitter resistor R142 decreases; the

LIGHTS switch S1 on the

base of transistor switch Q28 becomes less positive,

and 28 conducts less

2-12