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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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Peavey XR600C PA amp problem
Just wondering if anyone has ever seen this particular problem befo
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 |
#2
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Peavey XR600C PA amp problem
If there is a compressor/limiter circuit, it may be kicking in even if
turned off. "Sam Lloyd" wrote in message ... Just wondering if anyone has ever seen this particular problem befo 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 |
#3
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Peavey XR600C PA amp problem
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" wrote in message ... Just wondering if anyone has ever seen this particular problem befo 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 |
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