Heathkit SW-7800 - Synthesizer Circuit

Page
122
Heathkit
Active
Filter,
Audio
Amplifier,
and Mute
‘The
AM
audio
signal
coming
from
preamplifier
tran-
sistor
Q402
is
applied
to
mode switch
SW2.
When
the
switch
is
in
the
AM
Narrow
position,
the
signal
is
passed
to
the
active
filter
stages
of
U404.
In
the
AM
Wide
position,
the
audio
is
passed
only
to
switching
stage
U404D.
Sections
A,
B,
and
C
of
U404
form
a
three-stage,
active,
low-pass
filter.
This
filter
is
designed
for
unity
gain
and
has
a
roll-off
frequency
of
2700
Hz
at
6
dB,
The
narrowed
audio
bandwidth
helps
pre-
vent
stations
on
nearby
frequencies
from
interfering
with
the
desired
station. U404D
forms
an
electronic
switch which either selects
an
input
from
the
filter
stages
or
from
audio
preamplifier
Q402
(through
Mode
switch
SW2).
U404D
is
connected
as
an
inver-
ting
amplifier
so
it
can
be
cut
off
by
a
decrease
in
voltage
produced
by
the
mute
circuit
Capacitor
C473
couples
the audio
signals
coming
from
U404D to
Volume
control
R5
and
to
Recorder
output
jack
J1.
The
voltage
at
J1
is
unaffected
by
the
setting
of the
Volume
control,
so a
constant
level
is
present
at
the
jack
for
recording
purposes
The
audio
signal
from
RS
passes
through
audio
amplifier
U405,
which
increases
the
available audio
signal to
a
power
level
of
approximately
one
watt
for
an
8-ohm
load.
The
output
is
then
coupled
through
Phones
(headphones)
jack
J2
to
the
external
speaker.
Transistor
Q404
performs
a
muting
function.
When
the
mute
line
is
shorted
to
ground,
Q404
turns
on
and
places
11
volts
on
the
AGC
line,
which
cuts
off
U403.
The
grounded
mute
line also
drops
the
voltage
at
U404D
pin
2
through
D423,
R473,
and
R472.
This
turns
off
the
amplifier
to
prevent
audio
from
reaching
the
Volume
control.
D422
prevents
the
higher
AGC
line
from
driving
the
S
meter
to
full
scale
so
it
can
indicate
zero
during
muting
periods,
Product
Detector,
BFO,
and
BFO
Switch
In the
USB/CW
or
LSB
modes,
the
amplified
IF
sig-
nal
coming
from
U403
is
coupled
through
C463
to
buffer
amplifier
Q403.
The
output
of
this buffer
is
then
coupled
to
the
product
detector
where
it
is
mixed
with
the
BFO
signal.
Capacitor
C495
couples
the
resulting
audio
signal
to
the
active
filter, where
it
is
treated
the
same
as
an
AM
narrow
signal.
A
section
of
the
Mode
switch
activates
BFO
Q406,
BFO
switch
Q405,
and
product
detector
U406.
In
the
USB/CW
mode,
B+
is
applied
through
SW2
to
each
circuit.
Coil
L426
and
capacitors
C503
through
506
determine
the
frequency
of
the
BFO.
When
LSB
is
selected,
B+
is
also
applied
to
the base
of
Q405,
which turns
the
transistor
on
and
switches
C502)
in
parallel
with
L426.
This
additional
capacitor
causes
the
frequency
of
the
BFO to
de-
crease.
Diode
D431
prevents
Q405
from switching
whenever
USB
is
selected.
In
the
AM
modes,
the
same
section
switches
B+
to
audio
preamplifier
Qu03.
Power
Supply
‘AC
power
passes
through
power
switch
SW3,
which
is
part
of
the
Volume
control,
and fuse
F1
to
the
primary
of
power
transformer
T'1.
Capacitors
C1
and
C2
remove
any
RF
that
may
be
present
on
the
AC
line.
Resistor
R1
allows
any
static
charge
on
the
chassis
or
antenna
to
drain off
in
the
event
the
Re-
ceiver
is
not
connected
to
an
earth
ground.
The
AC
voltage
at
the
secondary
of
1
is
changed
to
DC
by
a
bridge
rectifier
formed
by
D425,
D426,
D427,
and
D428.
Capacitor
C511
filters
the
DC,
while
resistors
R509
and
R511
reduce
the
voltage
to
12-
and
11-volts
DC
respectively.
Capacitor
C509
further
filters
the
11-volt
line,
while
C508
filters
the 12-volt
line
A
second
set
of
contacts
on
switch
SW3
allows
power
switching
of
an
external
DC
source.
Fuse
F401
protects
the
external
DC
supply
and diode
1424
provides
reverse-polarity
protection
for
the
Re-
ceiver.
SYNTHESIZER
CIRCUIT
The
synthesizer
circuit
consists
of
the
display
cir
cuitry,
and
the
VCO
phase-locked
loop.
The
heart
of
the
display
circuits
is
U201,
which
is a
multifunc-
tion
device.
It
also
supplies
thg
time
base
for
the
phase-locked
loop.
LED
Displays
and
Display
Driver
Integrated
circuit
U201
contains
a
10
MHz
crystal
oscillator,
a
decade
time-base counter,
an
6-decade
data
counter
with
latches,
a
7-segment
decoder,
digit
multiplexers,
and
8-segment
and
8-digit
drivers
for
the
LED
display.
The
output
of
the
frequency
counter
is
coupled
directly
to
the
display
drivers
to
provide
an
all-in-one
counter package.