Hi.
I recently bought a 2nd-hand Roland HP-860 electronic piano (88-key,
analogue).
It has a louder hum than the piano notes.

I'd like to fix it myself. I have been learning electronics most of my
life & I'm in the 3rd year of Electronics Trades at TAFE NSW
(Australia).

I've removed the plug from the final stage PCB (Revo board), and the
amplifier is quiet. This makes me think that somewhere before the amp,
there is hum being made.
I could poke around with my CRO, but I wouldn't know what to look for.

This piano has heaps of boards, for which I've worked out most of the
features.

I could try replacing all the aluminum electrolytic caps, but that
could just be a waste of time.
It was made some where between the late 70's and early 80's.

I looked for date codes, and only found 1, I think.

This piano uses a wiring loom design, with some PCBs using plugs, but
most are wired up (soldered).

To see pics of it, search Google images for
"roland hp-860"
use quotes and thaisecondhand.com has the pictures.
NOTE: I have the 240VAC version.

Please help!
TIA