Hello, Just installed the SMSFM in my PAL SMS (VA3) and I'm not too happy with the result. There a very noticeable humming noise and the FM sounds distorted. I'm not sure if this distortion is due to the background noise or not. I have double checked the installation and the solder joints seem ok, and shield is connected to ground. The humming is not constant but seems to vary with what goes on on the screen. Any suggestions on what might be the cause of this? Prior to installation I had no noticeable noise in audio. I have an oscilloscope to play around with. Any points in the circuit that are worth checking?
Ok, I have been probing the sound signal from various points. The problem seems to be with the CXA chip. At pin 8 the signal looks fine and low noise. At pin 9 the amplitude of the signal is lower and the noise level has increased - they are both about 200mv p-p or thereabouts. When checking FM sound, the output from CXA (pin 9) seems to have been going through some kind of rectifier. We only have the positive going half of the waveform now and its noisy. The FM sound is heavily distorted and is a pain to listen to. The PSG sounds ok, except for background humming. What function does the CXA have for audio? All I find is that it contains and audio buffer. Is it a bad idea to just bypass it?
Solved! I put back the cap in series with the SMSFM output and it sounds great! Apparently the SMSFM does not have that filtering cap that it's supposed to? Anyway, thanks for listening.
