Yes, I have an sx-2500. It's original as far as I can tell. I didn't even clean the pots.
With volume set to minimum I can still hear sound and hum (60 or 120hz, can't tell the difference, not just yet)
On power up it has a massive thump and that's why it's not in use.
With all the above...
I've used these for cleaning contacts and you can find them in Europe
kontakt chemie 60 - cleaner
kontakt chemie 61 - lubricant
Read data sheet for extra peace of mind, but I've used both with great succes.
Another thing to check is the relay; try tapping it with a pencil to see if you've got a...