Roland E-20 works for only a few minutes

Hello friends,

I picked up a lovely Roland E-20 synth from the 80s. It works great…at first. After some minutes it stops making music, though the speaker makes random little crackle sounds, unrelated to pressing keys or anything. (At first, I thought it was a plastic shelf slowly buckling under a heavy weight). Power off-and-on and it works again, for a while.

I peeked inside, and the ICs all date to 1988, and the capacitors look fine–no swelling or leaks or anything. Anyone got ideas on where to look for trouble?

Thanks! -m