Heathkit HW-12 - Circuit Description; Transmitter Operation; Receiver Operation

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GENERAL CIRCUIT DESCRIPTION
This brief circuit description is intended onlyto
familiarize you with the Transceiver, We suggest
that you read this information before proceeding
with assembly. A more detailed circuit de-
scription can be found on Page 57. Youmay want
to study the detailed circuit description as time
allows after assembly is completed,
When reading this circuit description, it maybe
helpful to follow the circuit on the Block Dia-
gram.
Inthe
Block
Diagram,
the
receiver
stages
are located across the top,
bug ane Shin
the aL ansmitter
Stages
across
the
bottom,
and
stages
common
to
both
the
receiver
and
transmitter
are
located
through the center.
TRANSMITTER OPERATION
There are two ways to actuate the Transceiver
relay to change from receive to transmit oper-
ation; first, by the PTT (push-to-talk) method
where a pushbutton switch in the microphone is
used to close the relay circuit; or second, by the
VOX (voice operated transmitter) method,
In VOX operation, the voice sounds which are
transformed into electrical impulses by the mi-
crophone are amplified by microphone amplifier
stage VIA. These signals go to VOX amplifier
V10 and to AF (audio frequency) cathode follower
VIB. These electrical impulses are amplified
by the VOX amplifier and then by relay ampli-
fier V2B to a level that will operate the relay,
When the relay is switched to the transmit po-
sition, it turns off certain tubes in the re-
ceiver sections and turns on certain tubes in the
transmitter sections, as indicated by the dashed
lines on the Block Diagram, The tubes in both
sections that are not switched by the relay
remain in operation for both transmitting and
receiving,
With the transmitter switched on by either the
PTT or VOX method, the amplified voice voltage
from ViA is sent to AF cathode follower ViB,
which properly matches the low impedance of the
diode type balanced modulator, The balanced
modulator mixes the voice frequencies with a
signal from carrier oscillator stage V11B. Tie
modulator uses these two signals to develop
two new Signals; one is the sum of these two
Signals, and the other is the difference between
these two signals, With the modulator balanced,
the carrier signai is cancelled out and the re-
sulting output of the modulator is a "double
sideband, suppressed carrier" signal at the
carrier frequency (IF).
The output of the modulator is fed through trans- -
former Ti, which is tuned ito aid in balancing
the modulator, From T1, the signals are ampli-
fied by transmitter IF amplifier VZA, From V2A,
the amplified signals go to the crystal filter,
wehink, waren em AT wee ne ot ee oe
which passes only the lower sideband signal and
rejects the Uppee seidehand signal, oe lower
sideahand sional erystal filter ae
fied by IF amplifier V3, This s
through IF transformer T2 to te nsmi ee mixer
stage V4, In the mixer, the signal is combined
with a gional from VEO ares hI aes maa nae
Wit 2 Signa: irom VsrO (var iable fr equency
oscillator) cathode follower stage V14, This
mixing results in a single sidehand sienal at tha
Bae ae eee SAL SAk OH ete MAO Dae tia AL Wad
correct transmitter output frequency,
The VFO signal is produced by tunable oscil-
lator stage V13, The VFO signal from V13 goes
to VFO cathode follower V14 for isolation, This
signal is used in the mixer stages of both the
receiver and transmitter, thus locking the trans-
mitter and receiver to the same operating fre-
quency,
The
signal
from
transmitter
mixer
stage
V4
is
fed
through
receiver-transmitter
bandpass
coil
L2,
and
then
is
amplified
by
RF
driver
V5.
Output
signal
from
the
driver
is
applied
through
bandpass
coil
L3
to
RF
power
amplifier
tubes
v6
and
V7.
Here
the
power
level
is
greatly
in-
creased
and
then
is
fed
to
the
antenna
through
a
section
of
the
relay,
To obtain maximum transmitter output without
overloading, a portion of the driver output sig-
nal is ted back to preceding stages to adjust
their
gain
automatically
as
needed,
This
is
called ALC (automatic level control).
RECEIVER OPERATION
For receiving, the antenna is connected through
the relay to receive-transmit bandpass coil L3,
From coil L3, the signal is applied io RF am-
plifier stage V8A. From V8A, the signal passes