Marantz 2270 - Page 5

collector voltage increases the gate bias voltage and turns on the switching FET H308, decreasing
the source-drain resistance to near zero ohm and allowing the audio signal applied to the source
to flow to the center of 38KHz switching transformer 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 H306, thus the H306 keeps its turn-off state and this makes H307 turn on,
decreasing the collector voltage and turning off H308. Thus no audio signals can pass through the
FET. This is the fundamental principle of the muting operation but for more elaborate muting
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 C551 and amplified by the noise amplifier transistor
H551 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 H522, decreasing the emitter-collector
resistance to zero. This means the collector of H307 is short-circuited to the ground, therefore
the H308 is turned off and any audio signals having excessive high frequency noises can not go
through the FET’s sourcedrain path.
The transistor H303 connected in series with the 19KHz pilot signal amplifier transistor H302
is also turned off (when the transistor H522 or H307 are turned on.) and no current flows in the
H302, resulting in turning off the streo beacon lamps. Thus misoperation due to undesirable
noises is also avoided.
The third input is obtained from the FM discriminator circuit. The DC output so called ‘’S’’
curve is applied to the gate of H558 through a resistor R523 and deviding network (R565 &
R566). 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 H558 makes the transistor H561 turn on, and this makes the H306 turn
off,....H307 turn on, .... H308 turn off, .... H303 turn off (this means no 19KHz pilot signal is
amplified and no stereo beacon is turned on.) When the DC output is increased to the negative
predetermined level, the decreased source potential turns off the H559 which in turn makes the
H560 turn on and the H306 is turned off. 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 19KHz switching transistors are operated
automatically and open the circuits.
With the station correctly tuned in, the bias current of the FET H558 is adjusted so that both
transistor H560 and H5671 are not turned on, giving no effect on the transistor H306.
MPX Stereo Decoding Circuit
The buffered and non-equalized audio signals are applied to the first amplifier H301 which
serve as a tuned amplifier for the pilot signal in the composite signals and as a buffer amplifier for
the audio signals. The amplified 19KHz pilot signal is led to the second 19KHz amplifier H302
and further amplified if switching transistor H303 in turned on by the controlling DC signal as
described in the preceding chapter. The final 19KHz pilot signal is rectified by the doubler circuit
consisting of the H315 and H316 to obtain synchronized 38KHz amplifier driving signal.
The H304 is the 38KHz tuned amplifier and supplies its output to the switching matrix circuit
consisting of four diodes. While the composite signals are applied to the center tap of switching
transformer 1/2 L302. The right and left stereo signals decoded by the switching circuit are led to
the crosstalk cancelling amplifier which utilizes complementary configuration with NPN and PNP