04-09-2014, 10:24 PM
Wow, now you are really making progress! PC401 looks real easy to duplicate. Its only 3 resistors and a cap. Why not give it a try?
Q402, Q403, Q405 and PC401 have nothing to do with the audio itself. These parts serve to regenerate the 38 kHz subcarrier from the 19 kHz pilot signal transmitted along with the audio, and therefore only function with a MPX stereo signal. So its not surprising that you still have a good mono audio signal with a bad PC401.
The actual audio passes through Q401 (emitter follower), then through the double bridge demodulator. When in mono, DC bias from R416 passes through the bridge diodes, turning them on and allowing mono audio to be fed through to both channel outputs. In stereo, the regenerated 38 kHz subcarrier from transformer Z403, alternately turns on each bridge, separating the audio into right and left channel outputs.
What makes no sense is the voltage at the emitter of Q406, which should be grounded. Something is definitely funny there.
Q402, Q403, Q405 and PC401 have nothing to do with the audio itself. These parts serve to regenerate the 38 kHz subcarrier from the 19 kHz pilot signal transmitted along with the audio, and therefore only function with a MPX stereo signal. So its not surprising that you still have a good mono audio signal with a bad PC401.
The actual audio passes through Q401 (emitter follower), then through the double bridge demodulator. When in mono, DC bias from R416 passes through the bridge diodes, turning them on and allowing mono audio to be fed through to both channel outputs. In stereo, the regenerated 38 kHz subcarrier from transformer Z403, alternately turns on each bridge, separating the audio into right and left channel outputs.
What makes no sense is the voltage at the emitter of Q406, which should be grounded. Something is definitely funny there.
