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3.1 AUDIO MUTING AND STEREO MODE AUTO-SELECTING CIRCUIT
The muting circuit consisting of all solid-state electrical switching is incorporated in the Model
2325. Three input 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 rectifying a part of the IF output signal from the
H205 and H206 is fed to the base of H308 and turns it on, if the IF output is greater than a
predetermined level (muting threshold level). When the H308 is turned on, the 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 the switching
FET H301 on, decreasing the source-drain resistance to near zero ohm and allowing the audio
signal applied to the source to flow to the decoding IC pin (2) through the source-drain path.
When the input signal is lower than the predetermined level, the DC output obtained is small
and can not turn on the H308, thus H308 keeps its off state. This turns H309 on, decreasing the
collector voltage and turning H301 off. 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 C333 and are amplified by the noise amplifier transistor
H310. 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 turned in, the rectified DC output turns the transistor H311 on, decreasing the
emitter-collector resistance to zero. This means the collector of H309 is grounded, 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 the transistor H309 or H311 turns on, and turns on
the transistor H303 connected to pin (8) on the MPX decoding IC. Pin (8) is therefore grounded
equivalently to set the IC in the monaural mode of operation. This prevents 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 the resistor R281 and voltage divider network
(R361, R362). The DC output is zero with a station correctly tuned in, but will vary from
negative to positive value, or vice versa, when the tuning point is deviated toward either higher or
lower frequency from the correct tuning point.
When the DC output is increased to greater level than the predetermined one, the increased
source potential of H312 turns the transistor H315 on. (This means the collector of H309 is
grounded, H301 turns off, H317 turns off, and H303 turns on.) This grounds pin (8) at 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 predetermined negative level,
the decreased source potential turns the H313 off, which turns the 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 undesirable noisy
side-audio signals are produced, both muting and monaural/stereo switching transistors H303 are
automatically operated 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 H309.
3.2 MPX STEREO DECODING CIRCUIT
Digitized in Heiloo Netherland
5/30/2017
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