Guitar amplifier emits a continuous beep, which pitch vary depending on "bass" and "gain" knobs. Finding what's wrong in the circuit

Question

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).

Labelled with chips

• LF351 Op Amp
• LF353 Dual Op Amp
• CD4069 Hex Inverter
• CD4066 Quad Bilateral Switch

Answer 1

Is there an easy fix to it?

Fault finding on this circuit should be easy; you have a pre-amp circuit board and, it appears you have a power amp circuit board set deeper into the picture and in the middle. Disconnect the signal input (from the pre-amp) to the power amp and see if the noise goes away. If it does then the problem is in the pre-amp.

Quite possibly it's an electrolytic capacitor that has gone bad/dry and, whilst it should normally smooth a supply rail in the pre-amp it no longer does thus, positive feedback via the power rails is a strong candidate.

Answer 2

Andy aka suggested I raise this to an answer, although it's only a guess. There have been many novel approaches to getting a good distortion sound in solid state amplifiers, and I strongly suspect that the 4069 hex inverter is being used outside its "manufacturer intended" role (hex inverter) as the distortion/drive circuit. CMOS chips can turn up in all sorts of weird configurations like this.

I would have a look and see if it is part of the analogue circuit (I may be wrong, it may just be some kind of control for the analogue switch 4066) and if it is, I would try swapping it out. CMOS is static sensitive and it may have been damaged.

Take static precautions when handling a replacement IC.

Answer 3

It is also possible that one of the components has become microphonic . To Quote the Wikipedia article

Guitar amplifiers that incorporate the electronic chassis into the same cabinet as the speaker are susceptible to microphonics.

I would suggest that you use a piece of soft plastic (insulating) to go round the boards and gently tap each component. If any are microphonic you will hear a noise in the speaker.