Marantz 112 - Page 4

The
output
of
L205
is
led
to
the
additional
IF
amplifier
(H210)
for
intensifying
the
AM
IF
gain.
The
output,
in
turn,
is
led
through
the
coupling
capacitor
C253
to
the
input
(pin
@)
)
of
the
AM
IF
section
in
the |1C(H202),
where
the
output
is
amplifed
to a
sufficient
level
to
drive
the
detector.
The
fully
amplified
IF output
derived
from pin
@)
is
applied to the
diode H228
to
detect
audible
signal
through
the
detector
transformer
L206. The
detected
audio
signal
is
filtered
and
amplified,
and
the
final
audio
output
is
obtained
from
the
collector
of
H211
and
applied
to
the
output
jacks through
the
function
switch.
The
DC
component
of
the
detected
IF
signal
is
used
as
an
AGC
voltage
to
control
the
emitter
current
of
the IF
amplifier
H210
through
the
resistor
R273
and
R271,
and then, the
emitter
voltage
derived
from
transistor
H210
controls
the
collector
voltage
of
the RF
amplifier
H209.
A
part
of
the
IF
signal
output
is
also
applied
to
the
diode
H229
through
the
capacitor
C257
and
is rectified
to
obtain
DC
current
for energizing
the AM signal
strength
meter
MOQO1.
2.1 Suggestions for Troubleshooting AM Tuner
Check for broken AM bar antenna, next try to tune a station by rotating the 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 L206. Mext connect anoscilloscope to the test point @)
or J218 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 the oscillating voltage use an 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 the Model 112 consists of four functional blocks: FM Front End, |F
Amplifier & Detector, Muting Control and MPX Stereo Decoding Circuit.
An FM signal induced by the FM antenna is led to the FM antenna coil L101 through the
balun coil. The signal is then fed to the FET RF amplifier, which in turn feed its output to the
next Mixer H102 through the double-tuned high-selective circuit. The Mixer converts its input
signal into 10.7MHz intermediate-frequency signal and amplifies it at the same time. The H103 is
a local oscillator and its output is injected into the base of the Mixer, the injection voltage is
about 60mV. The 10.7MHz front end output is led to the next IF amplifier portion through the
coaxial cable.
The IF section consists of five IF amplifer stages and one sub IF amplifier stage. The second
and third stages of the five-stage IF amplifier, contained in the |C H202, serve as limiter. Three
pieces of dual element ceramic filters are also used to obtain high selectivity, two IC limiter stages
(H202) and a symmetrical diode limiter are also employed for the best limiting characteristics,
improved capture ratio and good AM suppression. 3
A part of the IF amplifier H201 output is rectified by the diodes H220 and H221 and its DC
output is fed back to the gate of the FET RF amplifier to decrease the gain with increase of the
signal strength.
The IF signal sufficiently amplified through every IF amplifier stage is finally fed to the
detector amplifier. The detected audio output is led to:(a) the Quadradial Jack on the rear panel
through the resistor R233 and (b) the MPX stereo decoding IC H215 through R289 and H212.
3.1 Audio Muting and Stereo Mode Auto-Selecting Circuit
The muting circuit consisting of all solid-state electrical switching is incorporated in the
Model 112. :