Marantz 4270 - Marantz 4270 AM Tuner Section; Marantz 4270 FM Tuner Section

The
output
of
L153
is
led
to
the
IF
amplifier’s
input
(pin
@)
through
a
coupling
capacitor
C169
and
amplified
to the sufficient
level
to drive
the
detector.
The output of IF
amplifier
(pin
@®)
is
led
to
the
detector’s
input
(pin
@
)
through
IF
filter
L154.
The
detected
audio
signal
derived
from pin @)_
is
filtered and
amplified
and
final
audio
output
is
obtained
from the
collector
of
H152 and
applied
to
the
TAPE
OUTPUT
jacks
through
the function
switch.
The DC component of the detected IF signal is used as a AGC voltage to control emitter
current of RF and IF amplifier through the resistor R154 and R155. A part of the DC component
is also applied to the signal strength indication amplifier incorporated in the IC. The output
appears at pin (4 and is level adjusted by R152, indicated on the signal strength meter M004.
2.1 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 IF
transformer
L154.
Next
connect
a
oscilloscope
to
the
test
point
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 1.5
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 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
4270
is
divided
into
four
functional
blocks:
FM
Front
End,
IF
Amplifier
and
Detector,
Muting
Control,
and
MPX Stereo
Decoding
Circuit.
FM
signals
in-
duced
by
a
FM antenna
are
led
to FM antenna
coil
L101
through
a
balun
coil.
These
signals are
then applied:
to
the
FET
RF
amplifier
which
in turn applies its
output
to
the
next
FET
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 700mV.
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 IF amplifier and one stage of AGC amplifier.
Three pieces of dual elements ceramic filters are also used to obtain high selectivity, four stages
of
symmetrical
diode
limiters
are
also
employed
for
the
best
limiting
characteristics,
improved
capture ratio and good AM suppression.
A
part
of
FM
Front
End
output
is
applied
to
the
AGC
amplifier
H201
and
its
rectified
output
is
fed
back
to
the
gate
of
FET
RF
amplifier
to
decrease
the
gain
with
increased
signa!
strength.
The
IF
signal
sufficiently
amplified
through
every
stage
of
IF
amplifier
is
finally
applied
to
the
detector
amplifier.
The
detected
audio
output
is
led
to
the
buffer
amplifier
H208
and
its
buffer-
ed
output
is
led
to;
(a)
noise
amplifier
H310
through
resistor
R378
and
capacitor
C333,
(b)
QUADRADIAL
jack
on
the
rear
panel
through
resistor
R379,
(c)
MPX
stereo
decoding
!C(H321)
through
R301
and
H301.
3.1 Audio Muting and Stereo Mode Auto-Selecting Circuit
The
muting
circuit
consisting
of
all
solid-state
electrical
switching
has
been
incorporated
in
the
Model
4270.
Three
inputs
control
the
muting
function.
The
first
is
related
to
signal
strength,
the
second
to
the
noise
condition
at
the
detector
and
the
third
is
derived
from
the
DC
com-
ponent
of
the
detector
output.
These
inputs
are
properly
matrixed
and
gated
to
provide
muting
free
from
noise
and
transients.
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