Marantz 2270 - Page 4

2.2 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 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 loca! 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. 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 2270 is divided into five functional blocks: FM Front End, IF
Amplifier, Detector, Muting Control 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 FET Mixer H102 through the triple 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 six stages of IF amplifier, one stage of AGC amplifier and two
stages of multipath signal amplifiers. Eight pieces of ceramic filters are also used to obtain high
selectivity three 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 H207 and rectified its output
is fed back to the gate of FET RF amplifier to decrease the gain with increased signal strength.
The signals required for multipath indication are obtained from the three stages of IF
amplifiers through coupling capacitors C234, C236 and C238 respectively and rectified by three
pair of full wave diode circuits. Thus obtained AM components of the FM signal is amplified by
the transistor H208 and H209 and its output is again rectified to obtain DC current required for
actuating the Multipath indication meter.
The IF signal sufficiently amplified through every stage of IF amplifier is finally applied to the
IC limiter on the Detector Unit. The detected audio output is led 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.
Audio Muting and Stereo mode auto-selecting circuit
The muting circuit consisting of all solid-state electrical switching has been incorporated in the
Model 2270. 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 component
of the detector output. These inputs are properly matrixed and gated to provide muting free
from noise and transients.
The first input of DC voltage obtained by rectifing a part of IF output signal from the H206 is
applied to the base of H306 and turns on it, if the IF output is greater than predetermined level
(muting threshold level). When the H306 is turned on the H307 is turned off, allowing the
emitter-collector resistance increasing and the collector voltage rises about 9.7V. The increased