Peavey KB 2

[catstrat]

I've got a Peavey KB 2 amp on my bench and it displays parasitic oscillation on the waveform. After 7 minutes it shutsdown. Has anyone on here had experiance with these? Is this just a design flaw? I heard some people complaining about the same problem. Thanks

[J M Fahey]

it displays parasitic oscillation on the waveform.

Waveform where?
Scope pictures?
Does said oscillation respond to any control?

After 7 minutes it shutsdown.

Does some amp part overheat?
Does power turn off ... it just becomes mute? ....

Has anyone on here had experiance with these?

Yes, not necessarily with that particular Peavey model but certainly with same series or type.

Is this just a design flaw?

As in: this amp has been oscillating and cutting off since it left the assembly line?
Even worse: most of the tens of thousands of similar Peavey amps present the same symptom?
Short answer: no.

I heard some people complaining about the same problem.

And what did they say?
Did anybody solve it?


To make a long story short: if you expect somebody answering :
"hey !!! same thing happens all the time !!!! there's a well known solution, replace C142 by a 100pF ceramic and it will stop!!! "
or similar answer, well, things do not work that way.
Not impossible, but so unlikely that you can't count on it happening.

So you'll have to trobleshoot, on which we'll gladly help you, if possible.

[catstrat]

KB 2

Hello. Waveform pics are probably not going to get posted. But i know what Oscillation looks like. It does reach a point where it shuts down completely. NO indicater light, nothing. The oscillation get worse as it heats up. I don't know how many people are complaining about this issue, but a friend of mine tried using one exact amp they had at church and it shut down just like this one. Online you can read where some people are grumbling about it to. BUt other people are praising these amps. I just tested the pre amp waveform and it's definitely happening in the O/P stage and not the Preamp. The O/P heatsink does get hot but, i don't know what is normal for this amp. I was pushing it pretty hard and that was with a load. The first time i tested it was with no load at all and i didn't see any oscillation. I'll see what i can do but if it's going to get too involved since i'm a tube guy. THis is the amp my wife uses in her band so that's why i'm trying to solve it. I'll call Peavey and see if thay can't send a complete output board. After all, this is supposed to be under warranty. I've got a good working relation with Peavey so lets see what thay can do. The reason for my post was to see if anyone had any experiances with a KB 2 doing the same thing. Thanks everyone. Still open for suggestions.

[Enzo]

PV will honor the warranty by sending you to the nearest authorized repair center. Peavey or any other company will not generally send out whole board sets to anyone. As an authorized PV repair center myself, I know i am expected to repair the amp, not just replace the innards.

Your amp overheats, possible from oscillating, and shuts down. Someone else had one that shut down. We do not know if his also was oscillating first.


Can you tell us which power amp board is in that model? Should be labeled on the solder side. Never mind, I looked it up. Has its own board.

There are two power amps, one woofer amp and one tweeter amp. Are both oscillating or just the woofer?

Both power amps ar basic IC types, a LM1875 for tweeter and LM3886 for woofer. I would check for an open R4 on the woofer amp. I would also start by verifying that all power supplies are right. +/-20v for power amps and +/-15v for op amps.


But I also would recommend not trying to fix an amp while it is under warranty unless you are an authorized station, you might ruin your warranty for some serious issue that might arise later.

[catstrat]

KB 2

Hi Enzo. You gave me something to think about. I don't know if it's a bad componant causing the oscillation or the oscillation is causing the componant to go bad. Testing the amp without a load both tweeter and woofer showed no oscillations. I know with a load the woofer oscillates but i didn't think to check the tweeter side. You're right about the warranty. I did have a Trace elliot bass amp through here a couple of years ago,i guess Peavey bought them or something, it was under warranty and they gave me two options. They could either send me the board or i could send it to them. I'll call them in the morning.Thanks

[Enzo]

Well, are you a warranty station or not?

With a load on the amp, does it just sit there oscillating? Or does it only oscillate on peaks? To ME a parasitic is an oscillation that rides in on a waveform, usually on a peak. That is not the same as the amp simply unstable and self oscillating. So will it oscillate without signal? All the more reason to look for an open R4.

That resistor is part of your stability network, but other than that, the LM3886 is pretty much the whole woofer power amp, and a ne IC is the usual fix.



They may have offered a board swap in that circumstance, but as a rule that is not how service centers operate.

People tend to concentrate on components, but an open ground connection somewhere can be just as deadly to the operation.

[catstrat]

KB2

Hi Enzo, nope, not a Peavey Warranty station but Orange amps and Ceriatone. I've had to talk to Gene Ford up there on occasion and on the instance with the Trace Elliott, i think he was the one that suggested me just replacing the board. I turned down a chance to become a Peavey fixit station with a friend of mine. I'm retiring shortly and i just don't want the work right now. Anyway the amp oscillates with a load and a signal. I saw no evidence of oscillating at idle. It oscillates only on the peaks. The more i think about, i'll just see if i can send them the chassis and let them deal with it. After all, that's their job. I was doing this as a favor to my wife. Since i haven't done anything except test it i don't think i've voided the warranty. Thanks.