If the rails look clean, and the sound you are hearing has the described
waveshape, then it is more likely that it is a buzz rather than a hum
(subtle difference to the experienced ear) and not a power supply issue at
all. It could be a grounding problem, but really, you need to know where the
offending noise is actually getting in to the amplification chain. With
items such as synths, where there are triangle and square wave generators
running all the time, a simple bad ground can allow all sorts of nasty
ground voltages to start appearing in amplification stages.

OK, I could take a photo from the CRO, but how could I post it here ?
I'm using Google. I could upload it to my site, and put a link, I
suppose.
Yes, a buzz it may be.

All grounding wires look intact.

If only I had another one of the same model, I could swap boards to
determine where the problem is.
Although, most are wired in place, it'd be a start, at least.

BTW: I replaced the caps next to one of the STK025's and it didn't
help, but I'm glad I did, because these caps should last another 30
years or so.

Try here for your data sheet
http://www.alldatasheet.com/view.jsp?sSearchword=STK025

Thanks, but I've been there b4 and all I find is other parts. EG: a 3
pin MOSFET.

Is there a sci.electronics.organ.repair or similar group?