Peavey XR600C PA amp problem

S

Sam Lloyd

Guest
Just wondering if anyone has ever seen this particular problem before:
When I exceed a certain output level, the volume begins to modulate from
normal to almost zero in a smooth, cyclic manner. This occurs at a rate
of around 0.5/S- almost like a very slow tremolo circuit. I haven't
cracked it open yet, but I'm guessing it has something to do with a
negative feedback circuit gone bad, or maybe just a power supply issue.
Whatever it is it only happens when the output level reaches a certain
(fairly low) amplitude. Below that it works fine.

Any guesses? Bad PS cap?

Thanks,
Sam
 
If there is a compressor/limiter circuit, it may be kicking in even if
turned off.

"Sam Lloyd" <slloyd@usit.net> wrote in message
news:3F6DCF79.1050204@usit.net...
Just wondering if anyone has ever seen this particular problem before:
When I exceed a certain output level, the volume begins to modulate from
normal to almost zero in a smooth, cyclic manner. This occurs at a rate
of around 0.5/S- almost like a very slow tremolo circuit. I haven't
cracked it open yet, but I'm guessing it has something to do with a
negative feedback circuit gone bad, or maybe just a power supply issue.
Whatever it is it only happens when the output level reaches a certain
(fairly low) amplitude. Below that it works fine.

Any guesses? Bad PS cap?

Thanks,
Sam
Click to expand...
 
That's one of the possibilities I was thinking of when I mentioned neg
FB, but it doesn't have any compressor/limiter built in, at least not an
adjustable type. I checked the PS rails, and the voltage drops as the
output increases until the point where the "modulating" starts. I found
that if I increase the volume VERY slowly I can sometimes avoid the
modulation and achieve high output levels.

I scoped the rails and they vary along with the modulation. Tried
substituting some different power caps, but no difference. I also found
that there's about .65 VDC offset at the spkr output, FWIW- don't see
any pot for adjusting the DC offset.

I've emailed Peavey, so hopefully they'll send a schem tomorrow, and
help illuminate the mystery.

Sam


taGroover wrote:
If there is a compressor/limiter circuit, it may be kicking in even if
turned off.

"Sam Lloyd" <slloyd@usit.net> wrote in message
news:3F6DCF79.1050204@usit.net...

Just wondering if anyone has ever seen this particular problem before:
When I exceed a certain output level, the volume begins to modulate from
normal to almost zero in a smooth, cyclic manner. This occurs at a rate
of around 0.5/S- almost like a very slow tremolo circuit. I haven't
cracked it open yet, but I'm guessing it has something to do with a
negative feedback circuit gone bad, or maybe just a power supply issue.
Whatever it is it only happens when the output level reaches a certain
(fairly low) amplitude. Below that it works fine.

Any guesses? Bad PS cap?

Thanks,
Sam
Click to expand...
 
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