CPS2 background sound / noise

[system11]

I have a CPS2 here which had a bad supercap. I replaced it and reset the volume with power on button holding, but there's a horrible amount of background noise.

It's very possible this existed before the supercap was replaced as it was defaulting to maximum volume and at dB reduction steps 0db to 8db there's no problem. As soon as you reduce the volume to 10db cut, you get background hisses, pops, crackles, whistling etc that comes and goes erratically. Turn it up to 8db again and the sounds immediately cease.

Any ideas? It's certainly analogue in nature.

 

[Nic034]

try to replace all caps on motherboard and perform inspection for leak ?
I’m not a specialist of sound issue, but maybe related to amp ?

 

[system11]

If anyone else sees this it's the DAC/volume control section which uses one control chip and 3 op-amps plus assorted capacitors. One of the op-amps had gone bad. The first replacement immediately went bad too. Reminder the first time you boot hold the volume button to reset the volume settings. Which one up or down seems to vary by game... It was chip 10B (UB10A/B on the schematics) - both time left channel. I did try the capacitors first but it made no difference whatsoever.

You can see this circuit here, the schematic was invaluable as it shows how they're connected together and therefore why one fault can manifest in multiple places and make it look like the volume chip is bad. If you pull all the capacitors for the affected channel it isolates things nicely. Had to use a scope to trace it as pieces were removed, once I'd worked out where the noisy channels where.
https://petitl.fr/cps2/93646A/#sound-dac

 

[system11]

As an aside I noticed the supercap was rusting but it had not spread to the PCB itself. I then checked a perfectly working one I have, rusted. Another, rusted. Probably best to assume they all need replacing at this point.