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Page Title: Section VII. SQUELCH AND AUDIO OUTPUT SECTION
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Section VII. SQUELCH AND AUDIO OUTPUT SECTION
42. Squelch Amplifier V801
on the control grid of V801A becomes neg-
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
be
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.


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