09-10-2017, 08:41 PM
The problem could be in the FM1000 circuit or Z300, but it could be in the FM IF stages also.
The fact that you have -2.5 V at step 4 is a good sign, as it indicates the FM1000 and Z300 is oscillating. Keep in mind that this is an oscillating FM detector, where the oscillation frquency locks to the incoming IF signal and follows the FM modulation. This causes a variation in plate current of the FM1000 tube, which produces the demodulated audio signal. The microphonics is normal, as the frequency of the FM1000 oscillation is affected by the vibration as you tap the chassis and is an indication that the FM detector is working correctly.
In order to work properly, the FM1000 needs an adequate signal at test point D. If there is not enough IF signal, the frequency will not lock and you get no audio out.
If you have aligned Z300 according to the procedure and you have the -2.5 V at test point F, then the detector may not be the problem. Rather there may not be enough 9.1 MHz IF signal feeding the FM1000.
Have you aligned the 9.1 MHz IF stages per the procedure and do all tuned circuits peak properly? With a properly operating and aligned 48-480 you should hear loud rushing noise on FM when tuned between stations. If you don't hear noise, then there is an RF or IF gain problem, or Z300 is not tuned correctly.
The fact that you have -2.5 V at step 4 is a good sign, as it indicates the FM1000 and Z300 is oscillating. Keep in mind that this is an oscillating FM detector, where the oscillation frquency locks to the incoming IF signal and follows the FM modulation. This causes a variation in plate current of the FM1000 tube, which produces the demodulated audio signal. The microphonics is normal, as the frequency of the FM1000 oscillation is affected by the vibration as you tap the chassis and is an indication that the FM detector is working correctly.
In order to work properly, the FM1000 needs an adequate signal at test point D. If there is not enough IF signal, the frequency will not lock and you get no audio out.
If you have aligned Z300 according to the procedure and you have the -2.5 V at test point F, then the detector may not be the problem. Rather there may not be enough 9.1 MHz IF signal feeding the FM1000.
Have you aligned the 9.1 MHz IF stages per the procedure and do all tuned circuits peak properly? With a properly operating and aligned 48-480 you should hear loud rushing noise on FM when tuned between stations. If you don't hear noise, then there is an RF or IF gain problem, or Z300 is not tuned correctly.
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