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Marantz 112 - Page 6

Marantz 112
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The
input
stereo
composite
signal,
amplified
by
the
audio
amplifier,
is
delivered
to
the
phase
detectors
PD-1
and
PD-2.
A
part
of
the
stereo
composite
signal
is
also
delivered
to
the
stereo
decoder
section.
The
VCO
(Voltage
Control
Oscillator)
produces
a
free
run
oscillation
in
the
neighborhood
of
76kHz
with
the
time
constant
determined
by
the
capacitor
C305
and
resistors
R311
and
R312
set
on
the
outside
of
pin
(4.
The
VCO
output
has
its
frequency
divided
into
19kHz
through
the
two
frequency
divider
stages
(DIV-1,
DIV-2),
and
is
reverted
to
the
phase
detector
PD-1,
which
contains
two
input
terminals
designed
to
produce
an
output
in
proportion
to
the
product
of
the
two
input
signals.
The
signal
led
to
PD-1
input
one
is
a
19kHz
square
wave
formed
through
frequency
division
of
the
76kHz
VCO
output
signal
by
the
two
frequency
divider
stages
DIV-1
and
DIV-2,
and
the
19kHz
pilot
signal
included
in
the
stereo
composite
signal
as
a
reference
signal
is
led
to
the
other
PD-1
input.
Therefore,
the
output
of
the
PD-1
which
has
passed
through
the
low
pass
filter
LPF-1
provides
DC
output
voltage
in
proportion
to
the
phase
variance
between
the
two
inputs.
This
DC
output
voltage
is
amplified
by
the
DC
amplifier,
and
supplied
to
the
76kHz
VCO
as
a
control
voltage.
This
means
that
the
output
frequency
and
phase
of
the
VCO
have
been
phase-locked
to
the
input
pilot
signal.
The
38kHz
sub-carrier
reproduced
by
the
PLL
as
stated
above
is
delivered
through
the
stereo
switch
to
the
stereo
decoder
section
as
a
switching
signal,
thus
driving
the
decoder
stage.
One
PD-2
input
is
given
the
19kHz
resulting
from
the
frequency
division
completed
by
the
DIV-1
and
DIV-3,
whereas
the
other
input
gets
the
19kHz
output
contained
in
the
composite
signal,
and
the
output
is
provided
with
a
DC
output
in
proportion
to
the
amplitude
of
the
pilot
signal.
This
DC
output
is
furnished
through
the
LPF-2
to
the
trigger
amplifier
which
drives
the
stereo
indicator
lamp
and
stereo
switch.
Therefore,
insufficient
supply
of
the
pilot
signal
results
in
failure
to
light
the
stereo
indicator
and
to
turn
on
the
stereo
switch
located
in
the
path
of
the
38kHz
switching
signal,
thereby
avoiding
a
wrong
stereo
operation.
H213
attached
on
the
outside
of
pin
©)
is
a
switching
transistor
for
automatic
monaural-stereo
switchover.
When
the
intensity
of
an
incoming
signal
from
an
FM
station
is
weaker
than
a
predetermined
level,
this
H213
is
turned
on
and
pin
iS
grounded,
thereby
developing
a
condition
for
monaural
reception.
For
a
-forced
monaural
operation,
switch
the
MODE
switch
to
‘“MONO”,
and
H213
comes
into
an
‘‘On”
condition
with
the
positive
bias
voltage
applied
to
the
base,
and
pin
is
grounded,
thereby
establishing
monaural
operation.
The
transistor
H214
connected
externally
to
pin
is
intended
to
stop
the
76kHz
oscillation
of
the
VCO
which
interferes
an
AM
signal
during
the
reception
of
an
AM
station.
When
the
function
switch
is
set
to
‘“AM”
position,
a
positive
bias
is
charged
on
the
base
of
H214,
H214
is
turned
on,
and
pin
is
grounded.
Thus,
the
oscillation
of
the
VCO
is
stopped,
ending
the
interference
with
AM
reception.
3.3
Suggestion
for
Troubleshooting
FM
Tuner
3.3.1
Symptom:
No
FM
Reception
First,
turn
on
the
Power
switch
and
try
to
tune
FM
stations.
Rotate
the
fly-wheel
tuning
knob
slowly
and
observe
the
FM
tuning
meter.
If
the
tuning
meter
deflect
at
several
received
frequencies,
the
tuner
circuits
preceding
the
discriminator
circuit
may
have
no
failure.
When
no
reading
is
obtained
in
the
meter,
check
the
FM
local
oscillator
circuit,
using
an
RF
VTVM.
The
normal
local
oscillator
voltage
is
one
or
two
volts
(rms)
at
the
tuning
capacitor,
depending
on
the
tuning
capacitor
position.
If
the
local
oscillator
voltage
is
normal,
then
check
all
voltage
distribution
in
the
FM
Front
End
and
IF
amplifier
unit
and
compare
them
with
those
shown
in
the
circuit
diagram.
When
the
tuning
meter
deflects
but
no
sound
is
obtained,
check
the
audio
circuit,
using
a
high
sensitive
oscilloscope.
|
3.3.2
Symptom:
No
Stereo
Separation
First,
check
that
the
‘‘MONO”
swith
is
in
normal
out
position.
Connect
an
FM
RF
signal
generator
output
modulated
by
a
stereo
modulator
to
the
rear
FM
antenna
terminals,
and
check
whether
the
stereo
beacon
is
turned
on
or
not.
If
not
turned
on,
check
for
19kHz
VCO
output
signal
(test
point
@,R304),
using
an
oscilloscope
and
frequency
counter.

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