Heathkit HW-12 - Page 58

BALANCED MODULATOR
When the audio signal from V1B and the RF
sienal from carrier oseillator V11B is applied
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to the 4-diode balanced modulator, two dif-
foroent frequencies are produced, This ring-
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type balanced modulator uses diodes CRi through
Qne of the two sign
modulator is the sum of the audio and carrier
frequencies; the other signal is the difference
between the ‘audio and carrier r frequencies, These
signals are the upper and lower sidebands.
The carrier signal is applied across the modu-
lator diode ring in a balanced circuit, consisting
of one winding of transformer T1, capacitors
C1 and C2, resistors R3 and R4, andthe Carrier
Null control, The Carrier Null control is used to
balance out the carrier signal in the modulator,
leaving only the upper and lower sideband sig-
nals at the modulator output,
The output from the balanced modulator is the
result of combining the audio and carrier signals.
Neither the audio or carrier signals appear in
the output, but the effect of the audio signals un-
balancing the nulled circuit at an audio rate
produces the sum-and-difference frequencies
called sidebands, With no audio, there is no
output from the balanced modulator,
With the Function switch in the Tune position,
a DC voltage is applied to the balanced modu-
lator through resistors R1 and R2, This voltage
can be adjusted with the Tune Level control, This
DC voltage is used to unbalance the modulator
to provide a steady output signal for transmitter
tuning purposes,
TRANSMITTER IF AMPLIFIER
The sideband signals from the balanced modu-
lator are coupled through transformer T1 and
then are amplified by transmitter IF (intermed-
iate frequency) amplifier V2A. These signals are
then applied to the crystal filter, consisting of
crystals Y2 through Y5, and coil L1. The crystal
filter eliminates the upper sideband, and permits
the lower sideband to pass through to common
IF amplifier V3 for additional amplification,
Stage V2 is turned off while receiving by apply-
ing additional negative DC voltage to its grid
through the secondary of transformer T1. This |
control voltage is impressed on the AL (auto-
Ol DOl an Adds CWO Gas veel ae at
matic level ‘control) line, which is also used to
eontrol the eain in a number of other trans-
OIE aail tuo OS Ule
mitter stages to prevent overloading, Overload-
ing can be detected by observing the action of
the meter, Normally the meter rests at or
Slightly helow zero; however, if the onerator
talks too loud or if ‘the Mic Gain control is set
too high, the transmitter section would overload,
This causes a change in ALC voltage which in-
creases the bias, reducing transmitter gain and
causing meter deflection to indicate ALC action,
COMMON IF AMPLIFIER V3
When transmitting, V3 amplifies the lower side-
band signal from the crystal filter and then
applies this signal to IF transformer T2, When
receiving, the receiver IF signal is amplified by
V3 and is then applied to receiver IF amplifier
v9.
TRANSMITTER MIXER V4
Transmitter mixer V4 receives two signals si-
multaneously; one is the SSB signal from T2, and
the other is the VFO (variable frequency eats
lator) signal from V13 and V14, through the
secondary of transformer T2. Tube V4 produces
the frequency sum of these signals, which is at
the proper operating frequency. This signal is
applied to the primary of coil L2, (Tube V4 is
cut off by bias voltage from the ALC line when
receiving,) Coil L2, which has two windings,
is
broad-tuned
to
cover
the
80
meter
band.
This
coil
is
alsoused
when
receiving,
andis
then
connected to receiver RF amplifier V8A and
receiver mixer V8B.
DRIVER V5
Driver
V5
receives
the
signal
voltage
from
coil
L2
through
parasitic
suppressor
resistor
R50.
The
ALC
line
is
also
connected
to
V5
to
control
transmitter
gain,
and
to
cut
off
the
driver
when
receiving,
Single-tuned
coil
L3
and
the
double-
tuned
coil
L2
form
a
bandpass
device
that
covers
the
frequencies
of
the
80
meter
band
without
the
necessity
of
tuning
the
driver
stage,
Voltage
for
bridge
neutralization
of
final
amplifier
tubes
v6
and
V7
is
fed
through
capacitors
C63
and
C64
to
the
bottom
of
coil
L3,
and
across
C55,
The
small
winding
of
coil
L3
is
used
for
the
input
signal
from
the
antenna
when
receiving,