Peavey KB4 very noisy

[tmullin1017]

I recently was given a used Peavey KB4 keyboard amplifier that is extremely noisy when running and nothing is plugged in to it. The wall outlet that it is plugged into is properly grounded and does not cause noise in other gear or cause ground loops when multiple outlets in the room are used with my home studio rig.

Looking for any suggestions or ideas as to the possible causes and whether the anticipated cost to have a service center repair it would exceed the value. They go for $400 new. This one has been serviced once already a few years ago while still under warranty but I was told it has always been somewhat noisy. Presently the noise is outrageous, sounds like ocean waves crashing against the shore.

I have posted a short audio sample of the noise here, recorded with an Audio-Technica AT4033b large diaphragm condenser mic in an otherwise relatively quiet environment:

http://www.thresholdmbtribute.com/KB4noise.mp3

[52 Bill]

That sort of noise is common when an opamp or transistor goes bad. Contact Peavey and ask them for a copy of the schematic for your amp and then look into which parts are suspect.

Do any of the controls change the noise? If you can isolate the noise to a specific part of the amp, it will be easier to troubleshoot.

[tmullin1017]

Various controls don't seem to change noise

I tried twiddling many of the knobs but it didn't seem to make the noise better or worse.

[52 Bill]

How about the Master volume?
Is the noise present in the Headphone output as well?

I haven't been inside of one of these, is the power amp discrete components or an output chip?

[Enzo]

What Bill was going for there is a report of whether ANY of the controls have ANY effect on the noise. Not expecting some setting to cure it, but if we can turn the volume of the noise up and down, that means the volume control is after the source of the noise. Same with EQ, if we can alter the tone of the noise with a tone control, that means the noise is happeneing before those controls. And the other way around, if no controls have ANY effect, then the noise is in the power amp.

Power amp is discrete transistor, though there are op amps in its front end.