Heathkit SW-7800 - Circuit Description

Heathkit
Page
119
CIRCUIT
DESCRIPTION
Refer
to
the
Schematic
(fold-in)
and the
Block
Dia-
grams
(Illustration
Booklet,
Pages
40-42)
while
you
read the
following
information.
The
component
numbers
are
arranged
in
the
following
groups
to
help
you
locate
specific
parts
on
the
Schematic,
cir-
cuit
boards,
and
chassis.
1~
99
Parts
mounted
on
the
chassis.
101-199
Parts
mounted
on
the
front
panel
circuit
boards.
201-299
Parts
mounted
on
the
synthesize
circuit
board.
301-399
Parts
mounted
on
the
controller
circuit
board.
401-599
Parts
mounted
on
the
receiver
cir-
cuit
board
GENERAL
CIRCUIT
DESCRIPTION
REC
IVER
CIRCUIT BOARD
RF
signals
from
either
50-ohm
and
high-impedance
antennas,
or
the
built-in
collapsible
antenna,
are
passed
through
the
Local/DX
switch
to
the
receiver
input
filters.
A
DC
voltage
from
the
controller
circuit
board
selects
the
proper
filter
for
the
band
in
use.
The
filtered
signal
then
passes
through
an
RF
at
tenuator
diode
to
the
first
mixer.
The
first
mixer
mixes
the
received
signal
with
a
local
oscillator
signal
(LO
#1),
which
comes
from
the
con-
troller
circuit
board,
to
produce
a
difference
fre-
quency
of
50.455
MHz.
This
first
IF
signal
then
pass-
es
through
a
crystal
filter
to
the
second
mixer.
‘The
second
mixer
mixes
the first
IF
signal
with
a
local
oscillator
signal
(LO
#2),
which
comes
from
the
synthesizer
circuit
board,
to
produce
a
difference
frequency
of
455
kHz,
This
second
IF
signal
then
passes
through
a
ceramic
filter
and
buffer
to
the
IF
amplifier.
The
amplified
signal
now
passes
to
the
AM
and
product
detector
circuits
‘The
output
from
the
AM
detector
drives the
AGC
amplifier,
which
controls
the
gain
of
the
IF
amplifier
and
drives
the
S
meter.
The
output
signal
algo
passes
through
the audio
preamp
circuit
where
it
is
amplified
and
coupled
to
the
Mode
switch.
In
the
AM
modes,
the
detected
audio
signal
passes
through
an
active
filter
circuit.
In
the
AM
Wide
mode,
the
signal
passes
through
only
one
of
the
stages.
In
the
AM
Narrow
mode,
however,
the
signal
is
processed
by
four
filter
stages.
The resulting
fil-
tered
signal
then
passes
through
the
Volume
control
and
the
audio
output
amplifier
to
the
speaker