Peavey cs800 hissing/static after warm

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My very old style Peavey cs800 started outputting some static last week in sound check for a show about 45 minutes into running it. I was running both channels. The static was there in both channels and whether there was an input connected or not, and whether the volume on the amp on either or both channels was up or completely off. I wasn't using the biamp or filter cans. I swapped it out for a backup because the static was too loud, but it still worked as an amplifier. Just now, I plugged it back in and it is running fine--no static. I'm handy with a soldering iron and voltmeter but don't have any other test equipment. Any idea of the most likely cause? This amp is HEAVY and has those massive blue capacitors in the power supply. I believe it's a 1980s version given the stated age of ones on that look like it. Thanks!

I checked the RCA68271 transistors. What I guess is the date code on all of them says HD 8332 so I'm guessing this was built sometime in 1983 or 84. SN on the amp is 4A-01735544
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This is not the model with an air duct in the front is it? Then it is very old.
Could be electrolytic capacitors, could be oxide on internal connectors, including the wipers on any pots. Rare cases can be a resistor transistor or IC, but usually those effect only one channel or the other. I'd start off by spraying out the pots while wiggling the knob. Remove & replace all connectors to scrap the oxide off.
Then I'd look at the date on the capacitors. Old ones have YYWW where YY is 19year and WW is week 01 to 53. New ones sometimes have a one digit year code, since you are supposed to replace all electronic devices in under 10 years these days.
Can be the protection relay contacts, although again that should affect one channel or the other.
Warning these have voltages over 24 in them. Wear no jewelry on hands or neck, 1 v can burn your finger to charcoal. Touch metal only after you've proved the voltage is under 1 with a meter to speaker ground. Don't use 2 hands measuring especially with the power on, 24 v across your heart can stop it. Wear safety glasses unsoldering and when powering up after repair, My PV-1.3k bounced steel transistor tops off the ceiling.
A guess at production date is the date code on the output transistors, usually the YYWW one. Less likely to be replaced due to field shorts are the op amps.
Peavey schematics are on here, use an external search engine since the native one finds all instances of the first word you type.
I think these are worth fixing since they can go 30 years between overhaul. Although people are losing patience with carrying a transformer, they would rather buy some Class D trash that blows up every 10 years. Personally if I replace one e-cap, I replace them all. 2 at time with a function test in between so I know whether I made a mistake on the last 2. If so, then I know right where the problem will be.
You might be able to find the "bad one" with a heat gun or circuit cool, although I find replacing one e-capacitor on a >20 year old product just means it is broken all the time as they go out one by one. You may find some of the e-capacitors have been done already if you haven't owned it it's whole life. To trace the exact source of the noise, first it has to happen, then you have to look at points with a scope, sound probe, or analog VOM on AC scale, to see where the noise starts. The sound comes in the back input jacks, goes through little parts, then bigger ones, then out the big metal transistors on the heat sink and out the back speaker jacks.
Film capacitors, disk capacitors, resistors usually last forever. Resistors near the output can blow due to output transistor short events. Semiconductors usually go due to an output shorted, a dirty heat sink, a blocked fan etc.
Complete rebuilds usually include new pots and a fan. Although power amp input pots aren't nearly the dead duck that master volume pots on mixers are. I'd buy a fan from Peavey, although ball bearing fans are available out there at some distributors. I put sticky filter material outside the grills of my old amps to keep the heat sinks from growing dust bunnies.
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Both channels? Same noise or same TYPE noise? In other words in stereo mode and nothing plugged into input, do the left and right track audibly? Or is it similar noises but not exact?

It is possible your Peavey was more sensitive to noisy power mains than the other unit.

Peavey used those large cans with screw terminals for the main power supplies, and so far I have yet to have to replace one. And I have been an authorized Peavey service center for 30+ years. And generally when power supply caps fail, it results in hum, rather than noise.

Date codes also on power transformer. For that matter, what is your serial number. PV encoded the year into that for years.

Either use a rubber mallet, or the god-given mallet attached to your wrist, and whack the top of the unit hard. Or the side, or both. Does that affect the noise? ie does it stop the noise or does it start the noise?

The amp worked poorly at the last gig, but worked fine at home? One thing changed. You transported the heavy amp between those times. A bump in the road, or just loading the thing can put considerable impact into it. Thus my whack it with your fist test.
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