Peavey XR-696F Powered Mixer - Strange Noises

[misterc57]

This is Peavey PA head week issues for me.

I have a Peavey XR-696F (XR696F) Powered Mixer that is sending background noise to the speakers on both sides that sounds like static and slight wind noise. It is similar to the sound of playing a vinyl record when the needle first drops on the record.

This noise comes and goes (but is there a lot) and happens with nothing plugged into the amp and all gain/volume settings set to zero. Changing master volume has no effect on the noise level.

The amp will function properly if I play an instrument through a channel, but the noise is still present in the background. Like it has its own life regardless of what I am doing with the amp.

It reminds me of a problem I had a while back on a Peavey KB1 where channel two made wind noise (channel one was ok). If I recall correctly that turned out to be a drifting diode.

Looking for help to isolate the issue.

Thank you!

[The Dude]

The best way to find this is to get a schematic, follow the signal path with a scope, and find where the noise starts. If you don't have a scope, you may be able to see it on a DVM. I realize that is a bit vague and possibly not the "magic bullet" you were looking for, but this could be any number of things. The fact that both amps are exhibiting the same symptom may indicate a power supply problem, but it could also be a shared op amp. I'd start with looking for noise on any of your power supplies.

[Jazz P Bass]

Schematic: http://music-electronics-forum.com/a...pvy-xr696f.zip

You could try to isolate the problem to the preamp or power amp by inserting a signal in either or both Power Amp In jacks.
Going in that jack will cut off any preamp signal.

Alternatively you could use the Mixer Out jacks to another amplifier.
That will only be the preamp signal.

If all tests result in the same noise then it may very well be a power supply issue.

[misterc57]

Some tests

Plugged signal cables into both power amp input jacks; Noise cuts out for a moment and then comes back, also the EQ board for each of the two output channels "-30 DB LED" lights up every time there is noise. With no signal cables attached these LEDs do not light up when noise is present.

Next test; I ran the pre-amp signal Left and Right to a different 2 channel power amp. No noise!

So it seems like the noise in on the output board/circuit. I do not think there was a schematic for the output board in the supplied schematics, will check again.

Thank you!

[Jazz P Bass]

With the inputs into the Power Amp in jacks, making noise, does the Mixer Mute button have any effect?

[misterc57]

I am not finding a mixer mute button.

[misterc57]

So I am looking for the problem on the front board? I will study the schematic tomorrow.

I cleaned all the input and output jacks. The noise has lessened in frequency and has now become a high pitch popping sound where before it was a bunch of different sounds.

[Jazz P Bass]

Belay that statement.
I had Mackie on my mind.

What I was looking at was the mute circuit, never thinking to look for a switch