Circuit Description; Receiver Signal Flow; Transmitter Signal Flow - Heathkit HW-9 User Manual

Page 98
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CIRCUIT DESCRIPTION
Refer
to
the
Schematic
(fold-in)
while you
read the
following information.
The component numbers
are
arranged
in
the following
groups
to
help
you
locate
specific parts
on
the schematic,
circuit
boards,
and
chassis.
i
1-99
Parts
mounted
on
the chassis.
101
-
299
Parts
mounted on
the oscillator circuit
board.
301
-
599
Parts
mounted
on
the
T/R
circuit
board
and
the
RF
probe.
RECEIVER SIGNAL
FLOW
During
receive,
the
incoming
signals that are
present
at
the
antenna connector pass through
a
low-pass
filter
on
the
T/R
(transmit/receive) circuit
board.
The
Band
switch
selects
the
proper
filter,
which
rolls off
the
high frequencies
on
each band.
A
diode switch
routes this filtered signal
through
a
bandpass
filter.
The
resulting signal
is
mixed
with
a
premix
signal
to
produce an
8.83
MHz
IF
signal.
The
8.83
MHz
IF signal
is
routed
through another
diode
switch;
then
it
is
amplified
and
passed
through
a
4-pole IF
filter.
The
signal
is
amplified
again before
it
is
routed
to
the
product
detector.
An
AGC
(automatic gain control)
voltage
is
produced
at this
point
to
keep
the
2nd
IF
amplifier operating
linearly.
The
product detector converts the 8.83
MHz
IF
sig-
nal
to
audio frequencies,
which
then pass through
either a
wide
or a
narrow
audio-active
filter.
An
audio frequency amplifier
then amplifies
this
fil-
tered signal so
it
can
drive
an
8-ohm
speaker or
a
set
of
headphones.
TRANSMITTER
SIGNAL
FLOW
Although
the transmitter signals
flow
in a direction
that
is
opposite
to
the receiver, the stages for
both
modes
are
very
similar.
During
CW
transmissions,
an 8.8307
MHz
signal
is
coupled
to
the
2nd
mixer
where
it
is
mixed with
a
premix
signal
from the
oscillator circuit
board
to
produce
the desired transmitter
signal.
This
signal
is
then
filtered
by
the
same
bandpass
filters
that are
used during
the receive
mode
before
it
is
pre-
amplified
and
applied
to
the
power
amplifier.
The power
amplifier
is
made up
of
two
transistors
in parallel
to
produce an
RF
output
signal.
This
sig-
nal
is
filtered
by the
proper low-pass
filter
for the
selected
band
to
reduce
harmonic radiation before
it
is
applied
to
the antenna.
The
following sections describe the operation
of
each
circuit in greater detail.