Solid State Amp Repairs

[earth tones]

Is it worth it to repair an old Fender Ultimate Chorus Amp?? The problem I am having is a buzzing oscillation that rides on top of the guitar tone, which sounds normal otherwise. This buzzing oscillates at varying frequencies depending on which note I fret on the guitar. The buzzing is greater in amplitude when fretting bass notes, which seems logical. The clean channel on this amp is very useful, and if a hundred dollar repair would fix it, I would be happy for right now. At least until I can afford to purchase another amp. Any advice on whether to repair, or even how to probe and troubleshoot at home.
Thank you. Brian

[rolandk]

If you like the tone of the amp when it was working properly then I'd say its worth having a shop look at it. There will be a bench fee (~$50.00) for them to diagnose the problem and you can have them fix it or not.
From your description it sounds like a fairly minor repair. Might just be a broken ground connection somewhere.

[vvv]

Have you checked the speaker?

[earth tones]

Yes, I just connected a speaker that is definitely functional and the same noise is produced. So. some component or connection is failing, I would assume.

[kingnimrod]

here's a question:

can fooling around with the insides of a little solid state amp kill you as is warned about other amps?

I have messed around with the guts of some little practice amps with abandon...was that stupid?

[rolandk]

Yes, theres 120V ac in the power supply section.

[space_ryerson]

I would be very careful. Obviously, unplug it before you work on it. Second, drain the filter caps. I've done this before, but there is probably someone better here at this on this board to describe how to do it.

[Milkmansound]

ok - solid state amps do have the potential to kill, because they run off of the same 120V that everything else does. Plus, transistors require quite a bit of current to operate to the transformer will pass a bit more amperage than you may be expecting.

To drain the filter caps just short them (the neg legs) to ground using a long screwdriver - or even better, an alligator clip with a resistor which will drain them a bit slower. Sometimes the snap is fun though, and it leaves a little black burn mark in the metal that makes you glad that it did not discharge into you!

Once you get that out of the way, the amp should be fairly straightforward. I am not sure what will be causing the problem you describe - but a good way to check it out is to run the amp, inject a sine wave into it, and use an oscilliscope to check where the distortion starts. Chances are there is going to be a little transistor in the preamp stage that is either leaky or out of tolerance or just behaving badly in general.

If you have no scope you can just go hog-wild and replace all of the little transistors that you can find, and any op amps. That'll get it. If it is still there, then chances are it could be something external to the amp that is causing this problem.

Really the best thing to do is get it to a tech who knows what he is doing, or has a schematic. The thing many people forget about us techs is that we usually see several pieces of equipment that have the exact same problem. So maybe the one near you has seen this before and can tag it within an hour. Thats a good thing.