Noisy Crate PA head

[Chulio]

I have a Crate PA6FX PA head that makes a very little distortion sound.
I bought it on Ebay years ago and it worked good for about a year.
The noise is almost like the speaker is buzzing but it is in the amp not the speaker.
It makes no difference if I have the effects on or bypassed.
Any ideas?

Thanks

[Enzo]

Isolate the problem - preamp or power amp? Plug a signal into the power amp in jack - still distort? If so, then the power amp has the problem, if not then the preamp does. And you can send the main out to another power amp and speaker adn see if it is clean.

First place I would look though would be the power supply. Scope the rails, any exces ripple on one or both? Ripple might not come out as hum, but it can stil modulate the signal.

[Chulio]

Thanks Enzo

"Plug a signal into the power amp in jack"


This unit has only main outs
effects loop in & out pre
effects loop in & out post
Aux in
Tape out


How can I isolate the preamp from the power amp?

[Enzo]

Sorry, most mixers like that have such a jack.

Closest we can come then is the post efx return. You can still send hte main out to another system for a listen.