RECEIVER SQUELCH FUNCTION.

TM 11-5820-401-34-3/0967-LP-432-3060

1-29.

AUDIO AMPLIFICATION/A5000. (CONT)

When a positive going audio signal is applied to the cathode of CR5102, the reverse biasing causes the

voltage at point A to increase due to reduced current flow through R5110 and CR5102. The voltage

increase is coupled across C5104 and applied to the base of Q5102 for amplification.

When the audio signal swings negative, the voltage at point A drops due to the change in biasing at the

CR5102 cathode and increased current flow through R5110 and CR5102. The voltage drop at point A is

coupled across C5104 and applied to the base of Q5102 for amplification.

Transistor Q5102 receives 25 vdc as operating voltage; however, this is not the voltage which is

switched by the squelch relay to turn the amplifier on and off. As shown in F0-12, 16 vdc is switched

by the squelch relay. However, it is applied constantly when the SQUELCH switch is in either of the two

OFF positions.

Referring back to the simplified version of the biasing circuit, it can be seen that the absence of 16 vdc

removes the bias at point A and the anode of CR5102. Without the bias, positive going audio signals

are clipped by the diode, and no signal is coupled across Capacitor C5104. Therefore, the amplifier

cannot work.

POWER AMPLIFIER Q401

Q401 is a power transistor which is not mounted in the A5000 tray. A5100 amplified output is applied to

the power amplifier prior to application of the signal to Transformer T5001.

TRANSFORMER T5001

The transformer performs an impedance matching function between the power amplifier and external

audio accessories. One of its outputs, unmuted audio, is a volume-controlled signal to a set of

external earphones.

MUTING RELAY K5001 AND MUTING RESISTOR R5117

The muting relay is an electromechanical switch which applies one of the transformer's audio outputs

to the muting resistor or otherwise bypasses the resistor.

When the relay is not energized, audio signals bypass the resistor, resulting in full-amplitude loud-

speaker audio output. If the relay is energized by an external ground source, the audio signals are

applied to the resistor, which provides attenuation and resultant speaker quieting.

Speaker quieting prevents acoustic feedback from the speaker into the microphone of a transmitter

located near the R-442. The muting relay ground is provided by the transmitter when it is keyed.

RECEIVER SQUELCH FUNCTION.

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A block diagram showing a simplified layout of the A5000 tray along with related squelch components is

contained in F0-12. Refer to the foldout while reading the information in this paragraph.

OLD SQUELCH

The receiver is squelched when 16-vdc power to the audio amplifier is turned off. In the OLD SQUELCH

position of the SQUELCH switch, 16-vdc power is available through the squelch relay only when rf

signals of at least 0.5 microvolt are received.

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