(from TM 11-866)
18. AVC AMPLIFIER AND RECTIFIER.
a. The control grid of the AVC amplifier tube (V11) is connected
in parallel with the control grid of the 3rd IF amplifier (V7) which
is driven from the tap on the secondary coil of L31 of IF transformer
T3. Amplified IF voltages present in the plate circuit of tube V11
are impressed on the AVC rectifier tube (V12) by means of
transformer T6. Transformer T6 has a tuned primary and
closely coupled untuned secondary (L34). The untuned secondary
is connected to both diode plates of tube V12 and to the diode
load composed of resistors R53, R54, and R55. AVC control voltage
is obtained from the high end of resistor R53 and connected to the
AVC-MANUAL switch (SW4) through an IF filter consisting of a
1 megohm resistor (R52) and a .05 microfarad by-pass capacitor (C56).
This resistor-capacitor combination also determines the time constant
of the AVC system for the reception of modulated signals.
b. When AVC is used for CW code reception, a longer time
constant is desireable, and this is secured by adding a .25 microfarad
capacitor (C17) in parallel with the .05 microfarad capacitor (C56).
This extra timing capacitor is connected to one pole of the SIGNAL-
MOD-CW switch (SW3) which controls the beat oscillator. Therefore,
when the beat oscillator os turned on for CW code reception,
capacitor C17 is automatically added to the AVC system.
c. The low-potential end of the AVC rectifier diode load and
the paralleled cathodes of the diode itself (V12) are returned to the
-3 volt point on the "C" bias voltage divider. This provides the
minimum recommended grid bias for the controlled RF and IF
amplifier tubes (V1, V2, V5, and V6) without regard to any
negative bias furnished by diode V12.
d. When the AVC-MANUAL switch (SW4) is thrown to AVC,
the "S" meter (M1) is connected in shunt with the 1,000-ohm
adjustable resistor (R41). Since resistor R41 is in series with the
AVC diode load, some of the rectified d-c current flows through the
meter. The amount of the current depends on the strength of the
IF voltage impressed on the plates of the AVC diode V12. This
voltage in turn depends on both the strength of the incoming signal
and the accuracy of tuning. The meter reading varies as the receiver
is tuned through a signal, being the highest at exact resonance. Strong
signals produce higher meter readings than weak signals, therefore
the actual meter reading at resonance is an indication of the strength
of the incoming signal carrier. The setting of the variable resistor
(R41) controls the degree of meter deflection on any given signal,
and is usually adjusted to produce a reading of "S9" on a 50 microvolt
signal at 3.5 mc. When so adjusted, a change of one "S" number
on the meter indicates a change in signal strength of approximately
two to one. The SENSITIVITY control (R56) must be turned full on (10)
for maximum "S" meter accuracy.