Yamaha CLP123 intermittent power and crackly sound

I picked up a cheap, but faulty, Yamaha Clavinova CLP123 recently thinking I might be able to fix it. The power module only clicks on intermittently so I assumed there was a broken solder joint somewhere. However, nothing is visible and there is continuity throughout the power board.
What should be my next step?

there is a dead short in the power supply or somewhere on the mainboard. most likely a tantalum capacitor. they fail with no external signs so you have to test all capacitors for shorts.

these units are about 30 years old so may be faulty or time expired electrolytic caps.

To start, find out which caps are dead and replace them with brand name parts, not ebay or amazon parts.

Known issue with these PSU’s - replace ALL caps before they leak and destroy the circuit board - will also fix your relay problem which is a delay circuit governed by one of the smaller caps

It seems the power supply issue is resolved. The more difficult issue is with the crackly sound board problem.
All the various sounds work but they are all distorted and crackly, especially when volume is increased. I’m wondering where to start checking. All the caps look good. Same distortion comes through headphones so the problem is in the sound board somewhere.
Thanks for your advice on this!