Strange AC30 Problem

[strato17]

Hey Gents,

I have an early 90's AC30TBX, one of the korg ones, made in GBR. As of late, when playing the amp, I will hear the volume slowly fade out randomly. After a while, maybe a few minutes, it will slowly fade back in and continue to work. I cant quite figure this one out. It only happens maybe once during a few hours of play. Does anyone know where to even begin with something like this?

[Structo]

Those kind of problems can be difficult to trace.

Isn't that amp PCB ?

First of all rule out tubes by installing known working tubes.
If you don't have enough to retube it, substitute one tube at a time and turn on the amp as you go.

I think what I would do is inject a signal into each stage to try and find the offending area.

It could be a bad ground connection or even a bad tube for that matter.

I seem to recall that those amps get really hot and as they heat up stuff expands.

I would pull the chassis and look the circuit boards over carefully to see if any thing is loose or burnt looking.

[jestaudio]

i had a similar probem on a Vox, turned out to be effects loop, be worth checking if you have one

[pdf64]

A slow fade out then back in sounds like a intermittant connection in the heater circuit.
Try reseating the tubes / heater fuses, and check pcb for dry joints.
Pete.

[strato17]

Hey everyone, I will give your tips a try. Thanks!

For what it is worth, when the sound fades, the tone/overdrive characteristics do not change whatsoever. It sounds like someone is literally just pulling down a fader on a mixing console. Sounds completely different, the the sound you get as the caps bleed out when you turn off an amp and continue to play.

[pdf64]

That's consistent with an intermittant heater.