Noise Interference on Commodore SX64

I have drawn a blank on the specialist Commodore forums and I expect this is more of a general electronics issue anyway so hoping someone here could shed some light.

I have an old (c.1984) Commodore SX64 which has interference on the audio circuit - strange thing is if you drive the audio by changing voices (in synthesizer software) it suddenly mostly disappears until the next boot.

Here is a video of it:

https://www.youtube.com/watch?v=uFIeeDq4a8g

The noise appears to be in around 15kHz - but where is it coming from? Is it possible it is from the power supply?

I have already replaced all the electrolytic caps on the mobo as well as the SID and PLA chips.

My only course of action is to replace all the other caps in the system (which needs to be done anyway) but I would rather know where to source the problem.

Many Thanks
Jules

That sounds very much like video signal creeping into the audio stages, especially as it changes in time with the picture content. 15KHz is probably the picture scan frequency so it points to either a loose grounding point or a capacitor on the power feed to the monitor section being faulty.

Brian.

Hello julesgilson,
Try replacing the main filter capacitor in the units power supply.
I've had these in the past causing audio problems just as you've described, though on TV's and the odd VCR.
Regards,
Relayer

Thanks for the advice guys.

I also thought it might be something to do with noise coming from the display. So I removed it completely. In fact I took the main board out disconnected from everything except the power supply. By wiring an external amp into the audio circuit I could still hear the noise.

So, yeah, basically thinking power supply now. Just ordered all the electrolytic caps.

One thing.... there are a bunch of polyester film caps on the power board... don't know anything about them and proving hard to get. Should I replace them, if so will ceramics do?

Thanks again.

If they are across the supply lines and ground yes, you can replace them with ceramics but polyester ones are stable and have virtually unlimited life so I doubt they are responsible for your problem. Electrolytic capacitors on the other hand, contain liquid electrolyte which tends to dry out over time so they should be replaced.

Don't go nuts trying to kill the noise completely, audio circuits in computers of that era were nowhere near as well designed as modern ones and the sound was often added as a novelty feature for sound effects in games, they were never intended to be high quality.

Brian.

The noise is probably the 15.75KHz horizontal video scanning frequency getting into the SID audio in pin. You could try grounding the SID audio in through a 10K resistor and see if that makes a difference. I think it's left floating in all Commodore implementations so it picks up noise from everywhere. Make sure you know what you are doing. If you short pins out in error while powered up you will kill your SID.