Solid State Guitar Amp Forum | DIY Guitar Amplifiers

I have come into possesion of an Accoustic Control Corps 127 2x12 combo.
http://acoustic360.homeunix.net/products/main/127.html (http://acoustic360.homeunix.net/products/main/127.html)
just like that.


The only issue with it currently is that the built in graphic equalizer seems to be causing the most god awful loud hum to come out of the unit. With the low frequency sliders all the way down the hum is reduced substantially, likewise with them up  - the noise is rediculously loud - thus rendering the equalizer redundant.

This may be a dumb question but - any idea's on what the cause is / how i can go about rectifying.

My experience at repairing things like this is minimal - any help is appreciated

Likely fault scenarios are ground loop induced hum or faulty capacitors that have lead to increased amount of supply ripple. The latter fault does not usually cause a very loud hum (rather just a slight increase in it) but since the preamp circuit topologies of old Acoustic Control amps are horrible from the PSRR point of view I wouldn't consider this to be an impossible scenario either. Anyway, main point is that you have the hum injected to the signal and amplified. Since you can use the EQ to control the amplitude of the hum it tells the hum is injected either in - or before - the concerned stage. Locate the stage that injects most hum to the signal. This will help you to deduct what causes it in the first place: For example, is the supply ripple of the concerned stage exceptionally high. If yes, why? If not, and the stage still notably contributes the hum, then you can expect to find a ground loop, bad solder joint, faulty component etc. If the hum is indeed as loud as you describe it is there is a very big chance that the injection point is somewhere in the early preamp stages - not in the EQ circuit.

sounds like the same issue i was having.  i learned my lesson.  resolder that fader's/pot's connections, whichever it is and see what that does.  if that doesn't get you any where i happen to know (thru the internet, not personally) a guy that worked for acoustic control corp.  i bet he could help you.  better hurry though.  he's pretty old and could croak at any moment.   xP ;D