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Peavey powered mixer - distortion . . . eventually

Started by Peter Blair, April 25, 2023, 08:20:16 PM

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Peter Blair

Hello all,
Well, last time I was here I got some great advice. I'm hoping for 2 for 2.
So, I have a XR500C Peavey powered mixer that I've been using for several years in an acoustic duo. The sound has always been excellent, in spite of the age of the unit.
My problem: Everything sounds great at sound-check. One and-a-half, to two hours into the gig, I start getting distortion. Sometimes it's worse than others. Had a local guy look at it. He found nothing after running it for several hours. I suspect it was not being run at the sound level that we perform at. He said he just listened to music from his I-pad in it. In my mind, it won't start distorting unless you're really pushing some air. The distortion happens across all channels, suggesting (to me) it's not the pre-amp section. Upon the distortion starting, I have tried omitting everything in the input signal chain except for a single guitar. Same distortion. Try another guitar, same results. Just to be sure, change out cord, same results still. Upon inspection nothing visual is jumping out at me. Everything looks clean. Nothing looks like it's experienced any overheating. No apparent physical damage. I'm getting quite frustrated. Every gig I go to, I just hope it's not going to happen again. I'd just go out and buy new, but to be honest, I have a hard time finding anything that really measures up to this relic Peavey.
Anybody care to weigh in?
Thanks and appreciation to any suggestion,
Pete
p.s.
All I have for diagnostic equipment is a Borbede (BD-168A) multimeter. In light of this I would be willing to try replacing a few components if you think they might be the likely suspects.

Loudthud

Is this one of those 'Guitar Amp Head' style PA's with a 260 series power amp across the back ?

What speakers do you usually use with the PA ? Does the back of the amp get warm ?

An intermittent type failure can make it difficult to track down the exact cause.

Peter Blair

#2
Thank you for responding, "Loudthud".
Yes to the first question, but I think it's a 300 series amp. I also made an additional discovery upon further visual inspection last night. During original factory assembly, it appears that a trimmed off piece of transistor wire accidentally found it's way to the back of one of the power transistors, between the transistor and the aluminum bar on the edge of the pc board, (which bolts up to the heat sink). I could see that the transistor was not making good contact with the aluminum. I de-soldered the trans' and it was plain (from the footprint of the thermal grease), that the contact patch was only across maybe 1/4 to 1/3 of full contact. Seems like this might have been causing the transistor to heat up and maybe distort? So my follow-up question would be, once the transistor has seen enough heat to make it distort, is it junk? This is just my hypothetical so far. I haven't had opportunity to test the amp yet.

I can't tell you the model name of the speakers, but they are Peavey also. One 12" and a horn. I use two of them, most of the time. I believe these are the ones that were marketed with this amp.

Again, thanks in advance for any suggestions.
Pete

Tassieviking

I have come across many instances where thermal grease has dried up and is no longer performing well, most of these instances were DC drives for very large motors.(200kw+)
I used to remove all components on the heat-sink and clean the old thermal paste away and apply a very thin coat of new paste before re-assembling the units.
I have also seen this in many computers on the CPU, clean and reassemble.

Unless there was a fault I would not replace any components, just fresh thermal paste applied very thin and fully covering the transistor and heatsink.

Make sure you use good quality insulating thermal paste, there are thermal paste that conducts electricity, they should not be used in amplifiers.
Cheers
Mick
There are no stupid questions.
There are only stupid mistakes.

phatt

I'd first be looking at the load you are driving.
I assume the use of 2 speakers,, maybe check the total load is not below 4 Ohm.
If 2 speakers are used try running one speaker and see if it distorts.
An 8 Ohm load (1 speaker the power devices will not run as hot and far less chance of distortion due to running hot.

One other possible cause is the bias is drifting when hot leading to Xover distortion.
i don't know the XR circuit but it may have a small trim pot inside for adjustment and as the unit ages that part might be effected by heat.
Talk to the teck guy again and ask him to check,, it may just need a clean of the trim pot.
Phil.

Peter Blair

Thanks Phil,
I didn't notice any trim pots inside, but I wasn't looking for one either. I'll take another look.
Pete