I have an old transistor (EDIT: not transistor, but OP, from comments) guitar amp, a Laney Session 45 Reverb. I couldn't find any schematics online unfortunately, it doesn't seem to be a very popular model. I got it second-hand and it worked fine for a while, but suddenly, each time I power it, it emits a continuous, loud beep. The characteristics of the beep are the following:
- Loudness varies with volume knobs, from silent to very loud.
- Reverb, treble, frequency controls from the EQ don't seem to alter the beep
- I tried taking away the amovible spring reverb and the behavior is the exact same
- Overdrive channel switch and overdrive volume seem to affect the beep (but it's hard to tell since it's a continuous beep)
- I think guitar signal still goes through the amp, but I'm not sure because the beep is so loud I can't hear it
- when I turn the bass or the gain knobs, the beep varies wildly in pitch. It goes from "eeeeeeee" to "eeewooowiieewooeiiwiee", like a synthesizer. It can go fairly low to very high. Low bass or low gain makes the beep low, high bass or high gain make it high. The gain knob seems to affect the pitch a bit less than the bass knob.
I am new to electronics so I didn't completely take it apart yet, I just exposed the circuit boards (there are two) and blew air to clean them. They are covered in a slightly greasy or sticky substance, and there are a few cables that have been obviously re-soldered. My coworker had a look at the boards and said that the greasiness probably came from a tropicalizing spray that aged poorly; and that the problem seems to come from a broken component, not a circuit short. I took a multimeter and tested the continuity of connections that seemed artsanal or re-made. Apparently, no short.
Basically, my amp turned into a synthesizer. Where does that beep come from? Is it symptomatic of a broken/dead component? Is there an easy fix to it?
EDIT: additional pictures. They show both boards (the back one is hard to photograph).