Audio Frequency Amplifier AM -3506/PRC-47 (A8A1)-continued

TM 11-5820-509-35

all of the supply voltage appears across C26 so that the

C21, Hartley oscillator Q5 is enabled and an output of

base of Q6 is near ground potential and it remains cut

approximately 800 Hz is obtained. The oscillator output,

off. When the audio input level exceeds the threshold

or the voice audio from microphone amplifier Q1, is

value, however, the base of vex amplifier Q7 is biased

applied to audio amplifier Q3. The resulting amplified

more positively and this transistor conducts.

The

output from Q3 is limited by CR5-CR6 and then applied

amount of conduction is limited only by the amount of

to output amplifier Q4 through microphone amplifier

negative feedback in the loop from the collector of Q7

gain control R27 This control is a screwdriver

through C26, through the base-emitter junction of Q6,

adjustment that establishes the maximum transmit audio

and back to the base of Q7. As Q7 begins to conduct,

output level at the secondary of transformer T2 and

the negative charge on the base of Q6 rapidly

connector P1-22. A portion of the output from Q4 is

disappears as C26 discharges through the low forward-

also applied to vex detector CR7-CR8 through vox gain

resistance of CR9. At a point on the discharge curve of

control R40. The resulting positive voltage controls the

C26, however, transistor Q6 reaches cutoff, the

conduction of vox amplifiers Q6 and Q7. When a

negative feedback path to Q7 is opened, and transistor

minimum threshold voltage is available at the output of

Q7 immediately switches to full conduction. As this

CR7-CR8, as determined by the output level of Q4 and

occurs, vex relay K1 is operated through the ground

the setting of vox gain control R40, transistors Q6-Q7

supplied by the collector-emitter junction of Q7. When

conduct and relay Kl is energized. The time required to

audio input disappears, or the level drops below the

close the vox relay following application of an audio

threshold established by the circuit, a more negative

signal to the input of the vox detector is called the attack

potential is applied to the base of Q7, and it is driven

time. Similarly, the time required for the vox relay to

toward cutoff. Total and immediate cutoff is prevented

release after removal of the audio signal from the input

by the presence of a small positive voltage in the

of the vex detector is called the release, or dropout,

feedback loop, but as C26 charges, a point is reached

time. A fast attack, slow release characteristic is

on the charge curve where nearly all the supply voltage

obtained by suitably selecting capacitor C26 and resistor

again appears across this capacitor. The base of Q6

R43, in association with diode CR9 When the audio

again approaches ground, transistor Q7 is again cut off,

input level is below the vox threshold, C26 charges

and vox relay Kl releases.

through Q6 and R43. When fully charged, nearly

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