Marantz 2245 - FM Tuner Operation; FM Tuner Signal Path

Suggestions
for
AM
Tuner
Trouble
Shooting
Check
for
broken
AM
bar
antenna,
next
try
to
tune
station
by
rotating
fly-wheel
tuning
knob
slowly
and
observe
the
AM
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
1F
transformer
L154.
Next
connect
a.oscilloscope
to
the
pin
terminal
J162
or
J157
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.
!f
the
local
oscillator
voltage
is
normal,
check
all
voltage
distribution
in
the
AM
circuits
by
using
a
DC
VTVM
and
compare
the
measured
values
with
those
given
in
the
schematic
diagram.
3. FM Tuner
The
FM
Tuner
section
of
Model
2245
is
divided
into
five
functional
blocks:
FM
Front
End,
IF
Amplifier,
Detector,
Muting
Contro!
and
MPX
Stereo
Decoding
Circuit.
,
FM
signals
induced
by
a
FM
antenna
are
led
to
FM
antenna
coil
L101
through
an
attenuator
switch
and
a
Balun
coil.
These
signals
are
then
applied
to
the
FET
RF
amplifier
which
in
turn
applies
its
output
to
the
next
EET
Mixer
H102
through
the
double
tuned
high
selective
circuits.
The
FET
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
source
of
the
FET
Mixer,
the
injection
voltage
is
about
700mvV.
The
10,7MHz
front
end
output
is
led
to
the
next
IF
amplifier
unit
through
a
coaxial
cable.
:
The
IF
amplifier
unit
consists
of
five
stages
of
1F
amplifier
and
one
stage
of
AGC
amplifier.
Six
pieces
of
ceramic
filters
are
also
used
to
obtain
high
selectivity
four
stages
of
symmetrical!
diode
‘limiters
are
also
employed
for
the
best
limitting
characteristics,
improved
capture
ratio
and
good
AM
suppression.
A
part
of
FM
Front
End
output
is
applied
to
the
AGC
amplifier
H206
and
rectified
its
output
is
fed
back
to
the
gate
of
FET
RF
amplifier
to
decrease
the
gain
with
increased
signal
strength.
The
IF
signal
sufficiently
amplified
through
every
stage
of
1F
amplifier
is
finally
applied
to
the
IC
limiter
on
the
Detector
Unit.
The
detected
audio
output
is
fed
to
the
buffer
amplifier
H502
and
its
buffered
output
is
led
to;
(a)
noise
amplifier
H551
through
resistor
R551
and
capacitor
C551,
(b)
Quad
Radial
Jacks
on
the
rear
panel
through
resistor
R564,
(c)
MPX
stereo
decoding
circuit
through
R563.
The
DC
current
caused
at
the
third
windings
of
the
discriminator
transformer
is
directly
applied
to
the
FM
center
tuning
meter.