Marantz 2015 - FM Tuner; Suggestion for Trouble Shooting of FM Tuner

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 coi! L101. These signals are then
applied to the FET RF amplifier which in 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 10.7MHz front end output is led to the next IF section. The !F 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 2015.
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 IF 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 IF 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 merer MOOI.
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 C161 to input terminal pin @ 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 40 (right channel) and pin
(B) (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 6) (right channel) and pin D (left channel), respectively. The signals
amplified in the postamplifiers to the required levels (approximately 10 dB) are delivered to pin
© (right channel) and pin (left channel), and are then passed through C169 and C168 to pin
terminals J114 and J115, hence, to the function switches.
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
25uV.
3.3 Suggestion for Trouble Shooting of FM Tuner
3.3.1 Symptom: No. FM Reception
First turn on the Power switch and try to tune FM stations. Rotate the fly-wheel tuning knob
slowly and observe the FM signal strength meter. If the signal strength meter deflect at several
frequencies received, the tuner circuits preceding the discriminator circuit may have no failure,