Marantz 2275 - Page 6

The first input of DC voltage obtained by rectifing a part of the IF signal from H205 and
H206 is fed to the base of H308 and turns it on. This level is predetermined by the muting
threshold level control. When H308 is turned on, H309 is turned off, allowing the
emitter-collector resistance to be increased and the collector voltage to be raised to about 9V.
The raised collector voltage increases the gate bias voltage and turns on switching FET H301,
decreasing the source-drain resistance to near zero and allowing the audio signal applied at the
source to flow to the decoding IC, pin@).
When the input signal is lower than the predetermined level, the DC output obtained is small
and can not turn on H308, thus H308 remains off. This turns H309 on, decreasing it’s collector
voltage and turning H301 off. Thus, no audio signals can pass through FET H301. This is the
fundamental principle of the muting operation but for more elaborate muting, the second and
third inputs are necessary.
The second input is used to protect the muting operation and MPX stereo lamps from
misoperation due to undesirable noise. High frequency noise included in the detected audio signal
is separated by a small capacitor, C333, and amplified by noise amplifier transistor H310. Its
output is rectified by the two diodes. The rectified DC output is proportional to the noise
component in the audio signal.
When there is excessive noise in the audio signal such as obtained with a station incorrectly
tuned, the rectified DC output turns transistor H311 on, decreasing the emitter-collector
resistance to zero. This lowers H309 collector voltage to 0. Therefore, H301 is turned off and any
audio signal having excessive high frequency noise can not go through the FET’s source-drain
path. Transistor H317, also, turns off when transistor H309 or H311turns on. This turns on
transistor H303 connected to pin (@) on the MPX decoding IC. Pin (8) is therefore grounded and
puts the IC in the monaural mode of operation. This prevents stereo misoperation due to
undesirable noise when the FM tuning is incorrect.
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 resistor R281 and voltage divider network (R361 &
R362). The DC output is zero with a station correctly tuned in, but will vary from a negative toa
positive value, or vice versa, when the tuning point is deviated toward either a higher or lower
frequency from correct tuning.
When the DC output is increased to a greater positive level than predetermined, the increased
source potential of H312 turns transistor H315 on. (This means the collector of H309 is
grounded, H301 turns off, H317 turns off, and H303 turns on. This grounds pin 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 a greater negative level than
predetermined, the decreased source potential turns off H313 which turns H314 on. (This means
the collector of H309 is grounded). The subsequent changes are exactly the same as that just
described above.
Thus, when the tuning is shifted or deviated to certain frequencies at which undesirably noisy
side responses are produced, both muting and monaural/stereo switching transistors H303 are
operated automatically to provide muting
With the station correctly tuned, the bias current of FET H312 is adjusted so that both
transistor H314 and H315 are not turned on, resulting in no effect on transistor H309.
3.2 MPX Stereo Decoding Circuit
The stereo composite signal from the buffer amplifier undergoes a phase compensation by
R301 and C301, is fed through the muting FET H301 to the input terminal pin @) of the MPX
stereo decoding IC H321. This 1C uses PLL (Phase Locked Loop) technology and decodes the left
and right stereo signals, which become available at pins @ and (5), respectively. These decoded
left and right stereo audio signals are introduced through a low pass filter consisting of L301 to
L304 and C311 to C320 for elimination of undesirable residual switching signals and then
through a de-emphasis network consisting of R325, R326, C321 and C322 to a npn-pnp direct