Marantz 4415 - FM TUNER OPERATION AND TROUBLESHOOTING

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.
If
the
local
oscillator
voltage
is
normal,
check
all
voltage
distribution
in
the
AM
circuits
by
usinga
DC
VTVM
and
compare
the
measured
values
with
those
given
in
the schematic
diagram.
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 tn 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 10.7 MHz 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
4415.
|
The DC voltage obtained by rectifying the sub IF output signal from the H109 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 is turned on, thus decreasing the
emitter
collector
resistance
to
near
zero
ohm
and
allowing
emitter
current
path
to
the
final
!F
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 keep H111 turn off, then no emitter current is supplied to the H108 and signals below
the 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 MOOQ.
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
(1)
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
(i) (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 (6)
(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.
Pin
(5) on the MPX IC H114 is connected through R163 to the collector of the autostereo switch
transistor
H112,
which
turns
on
or
off
according
to
the
incoming
FM
signal
strength,
thereby
automatically switching between the stereophonic and monaural operations. The H112 turns on
or off in accordance with whether the FM signal strength is more or less than approximately
25znV.
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