Marantz 4230 - FM Tuner; Muting and Auto-Stereo Switching Circuits; MPX Stereo Decoding Circuit

slowly
and
observe
the
SIGNAL
STRENGTH
meter
whether
it
deflects
or
not.
If
the
SIGNAL
STRENGTH
meter
gives
a
deflection
at
several
frequencies
received,
no
failure
may
exist
in
the
stages
at
least
preceding
final
IF
transformer
L117.
Next
connect
a
oscilloscope
to
the
pin
terminal
J118
and
check
for
audio
signals
with
the
tuning
meter
deflected.
If
the
SIGNAL
STRENGTH
meter
does
not
deflect,
check
the
local
oscillator
circuit.
Normal
oscillating
voltage
at
the
hot
end
of
the
oscillator
tuning
capacitor
is
about
2
or
3
volts,
varying
with
tuning
capacitor
position.
When
measuring
oscillating
voltage
use
a
RF
VTVM,
no
circuit
tester
gives
correct
indication.
3. FM TUNER
All components are mounted on a printed circuit board P100.
FM signals induced by a FM antenna are led to FM antenna coil L101.
These
signals
are
then
applied
to
the
FET
RF
amplifier
which
in
turn
applies
its
output
to
the
next
transistor
mixer
H102
through
a
high
O
tuned
circuit.
The
mixer
convert
its
input
signal
into
10.7MHz
intermediate
frequency
and
amplifies
it
at
the
same
time.
The
H103
is
a
local
oscillator
and
its
output
is
injected
into
the
base
of
mixer
transistor,
the
injection
voltage
is
about
40mV.
The
10.7MHz
front
end
output
is
led
to
the
next
IF
section.
The
IF
section
consists
of
five
stage
of
IF
amplifier
and
one
stage
of
sub
IF
amplifier.
Two
pieces
of
ceramic
filters
are
also
used
to
obtain
high
selectivity,
a
pair
of
symmetrical
diode
limiters
are
also
employed
for
the
best
limiting
characteristics,
improved
capture
ratio
and
good
AM
suppression.
A
part
of
IF
amplifier
H105
output
is
rectified
by
the
diode
H115,
H116
and
its
DC
output
is
fed
back
to
the
gate
of
FET
RF
amplifier
to
decrease
the
gain
with
increased
signal
strength.
3.1 Muting and Auto-Stereo Switching Circuits
The
muting
circuit
consisting
of
all
solid-state
electrical
switching
has
been
incorporated
in
the
Model
4230.
The
DC
voltage
obtained
by
rectifying
the
sub
IF
output
signal
from
the
H
109
is
applied
to
the
base
of
H110
and
turns
on
it,
if
the
sub
|
F
output
is
greater
than
predetermined
level
{muting
threshold
level).
When
H110
turns
on,
the
muting
switch
transistor
H111
turned
on,
thus
decreasing
the
emitter
collector
resistance
to
near
zero
ohm
and
allowing
emitter
current
path
to
the
final
IF
amplifier
H108.
When
the
input
signal
is
lower
than
the
predetermined
level,
the
DC
output
obtained
is
small
and
can
not
turn
on
the
H110,
thus
the
H110
keeps
its
turn
off
state
and
this
makes
the
switch
transistor
H111
keep
turn
off,
then
no
emitter
current
is
supplied
to
the
H108
and
signals
below
threshold
level
are
muted
out.
The
muting
threshold
level
can
be
varied
by
adjusting
the
trimming
resistor
R153.
The
DC
voltage
obtained
is
also
used
to
make
the
Auto-Stereo
switching
transistor
H112
turn
on
and
off,
and
used
to
energizing
the
SIGNAL
STRENGTH
meter
M004.
3.2 MPX Stereo Decoding Circuit
A
Non-equalized
audio
signal
from
the
FM
detector
is
applied
through
the
phase
adjuster
network
of
C148
and
R161
to
input
terminal
pin
@
on
the
MPX
decoder
IC
H114.
The
MPX
decoder
IC
consists
of
a
stereo
decoder
and
postamplifier
for
the
output.
The
right
and
left
channel
signals
decoded
by
the
stereo
decoder
H114,
appear
at
pin
(right
channel)
and
pin
@)
(left
channel),
respectively.
These
signals
are
passed
through
the
low-pass
filters
and
de-emphasis
networks
to
eliminate
undesirable
residual
switching
signals
and
are
then
delivered
to
postamplifier
input
pin
@
(right
channel)
and
pin
@
(left
channel),
respectively.
The
signals
amplified
in
the
postamplifiers
to
the
required
level
(approximately
10dB)
are
delivered
to
pin
©
{right
channel)
and
pin
(left
channel),
and
are
then
passed
through
C169
and
C168
to
pin
terminals
J114
and
J115,
hence,
through
the
SELECTOR
switch
to
the
audio
amplifier
section.