I've been repairing an old (~1982) Fender solid-state guitar amp. While I had it apart I took the opportunity to replace some of the RC4558 op amps with NE5532's in DIP sockets I installed.
It was working great (much quieter). I left it on the bench for a couple of weeks and today when I came back and switched it on, I got a loud low-frequency "motorboat" from the speaker.
So far it looks like the buffer amp at the output of the tone control stack is oscillating. It seems to come and go, with varying amplitude—sometimes it seems fine, sometimes it's a fairly low-level sine wave, and other times a clipped almost-square wave. Update: this is the op amp towards the upper right corner of the schematic, labeled "1/2 IC2". I notice that there don't seem to be any bypass capacitors on the supply leads, so adding some might help.
The only possible variable I can think of is temperature: my bench is in an unheated room and the weather has turned colder. Hardly seems likely, though.
Any ideas what could be causing this? When I read about op amps oscillating, it's normally at a high frequency, right? The amp's other, high-gain "lead" channel seems OK.