Heathkit HW-12 - Automatic Volume

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with both signals present at the grid of V10,
the signals cancel each other, preventing the
speaker from tripping the VOX circuit, Speaking
into the microphone produces signals not present.
in the speaker circuit, permitting the VOX cir-
cuit to function normally. The VOX control not
only adjusts for proper anti-trip voltage, but also
determines the sensitivity of the VOX circuit.
PUSH-TO-TALK CIRCUIT
With the Function switch in the PTT (push-to-
talk) position, V10 is disabled by grounding its.
grid, V2B is made to operate and close the relay
by grounding its grid (shorting the bias) with the
push-to-talk switch in the microphone, The
Function switch in the PTT position also makes
the VOX delay circuit inoperative by removing
C105 from ground, Push-to-talk operation with
the Function switch in the VOX position uses
the VOX delay circuit, causing the receiver
to "delay" before coming on after the micro-
phone button is released, The Function switch
in the Tune position turns the transmitter on by
grounding the grid of V2B.
RECEIVER SECTION
RADIO FREQUENCY AMPLIFIER V8A AND
RECEIVER MIXER V8B
The incoming signal is connected to RF ampli-
fier V&A through coil L3, The amplified signal
from V8A is then coupled through coil L2 to
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receiver mixer V8B. During receiving, cutoff
bias is removed in the receiver section to permit
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tubes V8A, V8B, V9, and V12A to operate, V8A
is controlled by. bias from the AVC (automatic
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volume contr ol) circuit, AVC in the €iv
similar to ALC in
at
in the receiver is
e transmitter, in that it
ant rcececiver output
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is controlled by section C of the relay,
CRYSTAL FILTER
The signal is coupled from mixer V8B to the
erystal filter through C80, which is small in
value to avoid upsetting the input impedance of
the filter, The crystal filter exhibits the same
characteristics in receiving as in transmitting;
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it shapes the IF passband to have steep sides, a
flat ton, and a narrow bandwidth, This permits
good selectivity for SSB reception in crowded
amateur bands.
COMMON IF AMPLIFIER V3 AND RECEIVER
IF AMPLIFIER V9
Signals from the crystal filter are amplified by
common IF amplifier V3 and then fed to re-
ceiver IF amplifier V9, The cathode and screen
of tube V9 are connected directly to those of
tube V2A. The meter, which is connected inthis
circuit, indicates received signal strength in
S units, as the AVC voltage changes the current
in V9, The meter functions as an ALC indica-
tor when transmitting without any switching, The
gain of V9 is controlled by the AVC voltage
applied through resistors R91 and R92,
PRODUCT DETECTOR ViIi1A AND AUDIO FRE-~-
QUENCY AMPLIFIER V12B.
The amplified signal from V9 is coupled through
IF transformer T3 to the grid of product de-
tector V11A. Also, a signal from carrier oscil-
lator ViiB is fed to the cathode of ViiA. A
heterodyne mixing action takes place in V114,
resulting in an output signal which is the differ-
ence frequency of these two signals: an audio
signal, Capacitors C111 and C112 bypass any
RF signal coming from V11A, but permit the
audio signal to pass through to AF amplifier
V12B. The output from V12B is fed to the AF
output amplificr V12A through the AF Gai
control, and to the AVC circuit.
AUTOMATIC VOLUME CONTROL
Audio voltage is coupled to diodes D120 and
D121 through resistor R128 and cavacitor C128
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The diodes and capacitor C129 form a voltage
doubler, producing a negative DC voltage pro-.
portional “to the signal strength, Full AVC volt-
age is applied to the grid of receiver RF ampli-
fier V8A to prevent “overloading by strong re-
ceived signals.
Capacitor C129 in the AVC circuit charges
quickly to furnish a fast AVC response time,