Tone Squelch Mode (TONE)

TM 11-5820-670-30

RADIO SET AN/ARC-131 INTERUNIT CIRCUIT DETAILS. (CONT)

SQ ADJ potentiometer R9602 allows front panel adjustment of the carrier squelch signal. Normal set-

ting of the SQ ADJ potentiometer is at a point just above the level where noise breaks squelch. The

squelch circuits can be disabled by pressing the SQ DIS switch S9602 on the receiver-transmitter.

This has the same effect as switching the SQUELCH control on the control unit to DIS position.

When an incoming signal is present, the level of rectified noise component applied to dc amplifier

Q5205, from noise rectifier CR5201, causes Q5205 to turn off, which in turn causes dc amplifier

Q5207 to turn on. When Q5207 turns on, the emitter current applied through contacts A2 and A3 of

relay K5501 energizes relay K5502 and +16 vdc is applied through contacts A1 and A2 of relay K5502 to

the receiver-audio amplifier output stage. This action causes the receiver-audio amplifier to un-

squelch and pass audio signals.

Tone Squelch Mode (TONE)

When SQUELCH control S9106 is set to TONE, the ground at contact 7 is connected through contact 8

and then to contact 9 on S9105A rear. From contact 9 through contact 10 of S9105A rear, the ground

is connected to pin X of control unit connector J9101 (this ground is applied to pin X only when the

mode control switch is set to T/R or RETRAN) and then through interconnecting wiring to pin 9 of

receiver-transmitter connector P9601.

From pin 9, the ground (squelch tone ground) is connected to pin 31 of audiofrequency amplifier

connector J5501. The ground is used to energize tone mode relays K5503, K5504, and K5505. Relays

K5503 and K5504 connect the squelch filter circuit so that it attenuates the 150 Hz tone. All other fre-

quencies are fed back to emitter follower Q5202 through the squelch filter. In this condition, the am-

plifiers are completely degenerative for all frequencies except 150 Hz and provide maximum gain at

150 Hz.

The 150 Hz tone signal is amplified as described for CARR squelch operation and is applied to tone

rectifier CR5202. The filter output of CR5202 is applied to dc amplifier Q5206 and causes Q5206 to

turn on. Relay K5505 is energized in TONE squelch operation and direct couples Q5206 and Q5207

through contacts B1 and B2. When Q5206 turns on, Q5207 also turns on which causes relay K5502 to

energize, and apply + 16 vdc through contacts A1 and A2 to the receiver-audio amplifier output stage.

This activates the audio stages and allows the audio signals to pass.

In the absence of a 150 Hz tone, Q5206 and Q5207 remain off and relay K5502 is not energized to un-

squelch the receiver-audio amplifier, and audio is prevented from passing. Amplified audio signals

are coupled through pins 2 and 8 of audiofrequency amplifier connector P5501, through pins 13 and

14 of receiver-transmitter unit connector P9601, and applied through interconnecting wiring to the

control unit.

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