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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).

Front of the amp with diagnosis on each knob Circuit board of the amp

Labelled with chips enter image description here

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    \$\begingroup\$ That sounds like it could be some form of positive feedback in the amplifier (like what happens if you hold a microphone up to a speaker). Do you have any schematics of the amplifier? It looks fairly easy to reverse engineer. \$\endgroup\$
    – Hearth
    Commented Oct 3, 2022 at 13:16
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    \$\begingroup\$ Very approximately what frequency is the "beep"? \$\endgroup\$
    – jonathanjo
    Commented Oct 3, 2022 at 13:18
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    \$\begingroup\$ It is clear to me that you have some unwanted feedback in the amplification section. I would start by cleaning all pots with Deoxit or a similar electronics cleaner. Just make sure it is suitable for carbon pots otherwise you might damage them. It is very likely that you will have to replace all electrolytics as they seem original and this type of capacitor degrades over time. If you feel comfortable with soldering, retouch all solder joints too and do not forget to use plenty of flux. \$\endgroup\$ Commented Oct 3, 2022 at 13:21
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    \$\begingroup\$ @C.Crt That's why I mentioned reverse engineering. Trace out the circuit, identify all the ICs you can, and make a schematic yourself. This doesn't look like a very complicated circuit. \$\endgroup\$
    – Hearth
    Commented Oct 3, 2022 at 13:42
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    \$\begingroup\$ I fix alotta guitar amps. Without a schematic this will be difficult for sure. What I'd suggest is the CHOPSTICK TEST. 9 times out of 10, with an old PCB amp, it turns out to be a wonky solder joint on some component. With the amp powered on, poke EVERYTHING with a wooden stick (i.e. a chopstick!) and see if you can identify some component that magically makes the beep stop when you touch it. Obviously if you find one, resolder it. \$\endgroup\$
    – Kyle B
    Commented Oct 3, 2022 at 22:21

3 Answers 3

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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.

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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.

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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.

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