Section VII. SQUELCH AND AUDIO OUTPUT SECTION

Section VII. SQUELCH AND AUDIO OUTPUT SECTION

42. Squelch Amplifier V801

on the control grid of V801A becomes neg-

(fig. 13)

a t i v e . Tube V801A is cut off and plate

current no longer flows through resistor

a. The squelch relay circuit deenergizes

R812; thus the cutoff bias on V801B is re-

the audio stages when no signal is being

moved. Tube V801B conducts and the dc

r e c e i v e d or when the input signal falls

p a t h from terminal 10 of relay K801 to

below the threshold level of the squelch

ground is completed, and causes squelch

c i r c u i t . The SQUELCH control, on the

front panel, determines what input signal

relay K801 to energize.

amplitude will deactivate the squelch cir-

f. When squelch relay K801 is ener-

gized, the following occurs:

cuit.

b. SQUELCH control R711 (fig. 23) con-

(1) Contacts 6-13 K801 close.

trols the gain of the receiver by applying

(2) A dc path from CALL LIGHT 1701

a negative voltage to the if. avc line. As

through terminal P of J8 and P801

the control is turned clockwise, more neg-

and through relay contacts 6 and 13

ative bias is applied and the receiver gain

to +26.4 volts is completed.

is decreased further. The control grid of

(3) If Radio Control C-1439/U is used,

V801A is connected to the diode load (fig.

a dc path from CALL LIGHT I1501

12) through resistor R803 (fig. 13). When

to +26.4 volts is completed through

no rf signal or noise is present at output

terminal h of J1501 and P1601, ter-

transformer T501 (fig. 12), the dc voltage

minal h of J1401 (for AN/TRC-68)

on pin 3 of V801A is approximately +5.6

o r J1701 (for AN/VRC-24), ter-

volts (fig. 13).

m i n a l h of P1, terminal P of J8

c. When a small rf or noise signal ap-

and P801, and through closed con-

pears at transformer T501, the voltage on

tacts 6-13 of K801.

pin 3 of V801A will be less positive by an

(4) If auxiliary equipment is used, 26.4

amount equal to the +5.6 avc delay voltage

volts is applied through R184,

less the dc voltage developed across the

t e r m i n a l R of P801 and J8, and

terminal C of P1.

d i o d e load by rectification of the rf or

noise by detector CR501 (fig. 12). The volt-

g. The SQUELCH control is normally

age on pin 3 of V801 with no signal is ap-

adjusted for threshold at the frequency of

proximately +5.6 volts dc, and, with signal

minimum received signal strength. Weak

signals or noise may cause squelch relay

o r noise, the voltage is some value less

K801 to operate intermittently (the CALL

positive than +5.6 dc, depending on the sig-

LIGHT lamps flicker on and off). To de-

nal strength.

termine whether noise or signals are caus-

d. When received signals are of suffi-

ing the lamps to flicker, SQUELCH DIS-

cient magnitude to produce if. avc thresh-

ABLE

PUSH

switch

S703

should

old bias (para 39), the grid voltage on pin

p r e s s e d . This switch provides a ground

3 of V801 is zero. When signals of greater

r e t u r n for squelch relay K801 through

magnitude are received, the grid voltage

r e s i s t o r R713, thereby energizing K801.

on pin 3 goes negative. When no signals

The audio output will permit identification

(or weak signals) are received, the voltage

of the input signal.

on pin 3 is positive or zero, and V801A

conducts. Plate current flows through re-

h. When the rt unit is controlled from

sistors R809, R812, and R811. The voltage

Radio Set Control C-1439/U, microswitch

drop across resistor R812 produced by this

S705E is operated and transfers control of

plate current biases V801B to cutoff. There

the squelch amplifier to Radio Set Control

is no ground return from terminal 10 of

C-1439/U squelch control circuit.

squelch relay K801 through V801B, and the

S Q U E L C H switch S1502 and SQUELCH

squelch relay is deenergized.

D I S A B L E PUSH s w i t c h S 1 5 0 1 p e r f o r m

e. When stronger rf signals are present

s i m i l a r functions t o those described for

at transformer T501 (fig. 12), the voltage

local operation.