Marantz 4270 - FM Tuner Audio Muting and Stereo Mode Auto-Selecting Circuit; FM Tuner MPX Stereo Decoding Circuit

The first input of DC voltage obtained by rectifying a part of IF output signal from the H205
and H206 is applied to the’base of H308 and turns on it, if the IF output is greater than pre-
determined level (muting threshold level). When the H308 is turned on the H309 is turned off,
allowing the emitter-collector resistance increasing and the collector voltage rises about 9V. The
increased collector voltage increases the gate bias voltage and turns on the switching FET H301,
decreasing the source-drain resistance to near zero ohm and allowing the audio signal applied to
the source to flow to the pin @) of decoding IC through the source-drain path.
When the input signal is lower than predetermined level, the DC output obtained is small and
can not turn on the H308, thus the H308 keeps its turn-off stage and this makes H309 turn on,
decreasing the collector voltage and turning off H301. Thus no audio signals can pass through
the FET. This is the fundamental principle of the muting operation but for more elaborate mut-
ing operation the second and the third inputs are necessary.
The second input is used to protect the muting operation and MPX stereo beacon lamps from
misoperation due to undesirable noises. The high frequency noises included in the detected
audio signals are separated by a small capacitor C333 and amplified by the noise amplifier tran-
sistor H310 and its output is rectified by the two diodes. The rectified DC output is proportional
to the noise components in the audio signals.
When there are excessive noises in the audio signals such as obtained with a station incorrectly
tuned in, the rectified DC output turns on the transistor H311, decreasing the emitter-collector
resistance to zero. This means the collector of H309 is short-circuited to the ground, therefore
the H301 is turned off and any audio signals having excessive high frequency noises can not go
through the FET’s source-drain path.
The transistor H317, also, turns off when transistor H309 or H311 turns on, and turns on
the transistor H303 connected to pin of the MPX stereo decoding IC. Pin is therefore
grounded equivalently to set the iC in the monaural mode of operation. This prevent misoperation
due to undesirable noises when the FM tuner is out of tuning.
The third input is obtained from the FM discriminator circuit. The DC output so called “‘S”
curve is applied to the gate of H312 through a resistor R273 and. dividing network (R361 &
R362). The DC output is zero with a station correctly tuned in, but will vary from negative to
positive values or vice versa when the tuning point is deviated toward either plus or minus
frequency from the correct tuning frequency.
When the DC output is increased to a greater level than that of predetermined, the increased
source potential of H312 makes the transistor H315 turn on (this means the collector of H309 is
short-circuited to the ground) ... H301 turn off, ... H317 turn off ... H303 turn on (This grounds
pin (8) of the MPX stereo decoding IC, therefore the decoder is set in the monaural mode of
operation and the stereo indicator lamp turns off}. When the DC output is increased to the neg-
ative predetermined level, the decreased source potential turns off the H313 which in turn makes
the H314 turn on (this means the collector of H309 is short-circuited to the ground). The
subsequent changes are exactly the same as that just described above.
Thus when the tuning is shifted-or-deviated to the certain frequencies in which undesirable
noisy side-audio signals are produced, both muting and MONAURAL/STEREO switching
transistors H303 are operated automatically and open the circuits.
With the station correctly tuned in, the bias current of the FET H312 is adjusted so that both
transistor H314 and H315 are not turned on, giving no effect on the transistor H308.
3.2 MPX Stereo Decoding Circuit
The stereo composite signal from the buffer amplifier undergoes a phase compensation by
R301 and C301, is applied through the muting switching FET H301 to the input terminal, pin
@, of the MPX stereo decoding IC H321 on a PLL (Phase Locked Loop) basis, and decoded
into the left and right stereo signals, which become available at pins @ and () respectively.
These decoded left and right stereo audio signals are introduced through a low pass filter compos-