Peavey Monitor Amp Head 260C on my bench.

[bigsbybender]

I'm far from overjoyed from all the solid-state work I've taken in, but work is work...... 

Today's trivial piece is a peavey 260C Monitor Head. It's a neat piece from a bygone era of PA. This amp plugs into the (monitor out) of the main PA head and powers up to 2 monitor cabinets.  It really only puts out ~130w so it's great for a bar, not for a music fest.

Anyway, I've got a customer who wanted 3 of his gone through and refurbished.  THe first two required pot and contact cleaning and little else. This one came to me doing nothing but putting out aLoud Square Wave which sounds terrible, I'd say the sound is at full amp volume capability.
I started tracing the amp from the inputs with my "Listening amp", The signal is clean and clear. Then I got smart and swapped the input (pre-amp/EQ board) into a known good amp. No problems there.  So the issue is on the power board section, so I started tracing there, The signal was fine until I got to a TL074 IC. The signal was good throughout until I got to pins 1,2,3. So I replaced the part since it only costs 60 cents, but that didn't solve it.

Now I'm stumped. I haven't tried to trace output end toward the input (the opposite direction that I have been going). I can't easily get to the pins of the output transistors, as I'd have to lift the PCB from it's grounds to do so.

If anyone would like to help, I'd greatly appreciate it.       

I have the schem posted below.  Peavey emailed it to me, so I don't know about the public posting of it, but I'll take it down if they object.

j.

[PRR]

> Peavey emailed it to me, so I don't know about the public posting

It is an ancient and brutal circuit. "Competitor copy-catting" would be a bad idea.

> Loud Square Wave

What Frequency?? (If 120Hz, you got a clue already...)

Does it do it No-Load??

But.... while it has the heart of an old-fashioned quasi-comp power-amp, this model has been dressed-up with a really wacky overload-control scheme. Sorting this out on the repair bench is Not Worth It.

Blind stab: replace C9 10u 25V Tant (use a low-leakage Aluminum).

Can you remove U2 from a socket? (Don't use the chisel; this is a very scarce part.)

[bigsbybender]

Thanks for the reply, I really appreciate it.

Sorting this out on the repair bench is Not Worth It.

You're telling me.....     
This customer is really attached to his 30+ year old Peavey Stage setup.

Does it do it No-Load??

Yes

What Frequency?? (If 120Hz, you got a clue already...)

  My meter read inconclusive but it sure sounds like it. (I don't own a proper scope)

Can you remove U2 from a socket? (Don't use the chisel; this is a very scarce part.)

  Yes it's socket loaded and can come out.  I haven't pulled it yet.  I pulled U1 and just bought a new TL072 since it was only 54 cents.

Blind stab: replace C9 10u 25V Tant (use a low-leakage Aluminum).

That should be easy (and cheap) enough to try. Looks reverse-biased  + to Ground.

[bigsbybender]

Here is what I've done so far to no avail....

1. Replaced the Tantulum cap with Electrolytic.

2. Replaced U1 with a new TL072

3. Removed U2 from socket.

When I had both Chips out, the loud waveform was significantly less audible. 50-60dB instead of >100dB.

When I replaced U1 with a new one, The noise returned with a vengeance. I was blaming it initially because that was the first place in the signal path that I heard the noise using the listening amp.
As I'm gathering it, U1 must be amplifying the problem, not causing it.

Hmmm. 

[bigsbybender]

IT works finally.

I ended up pulling out caps to see which were passing DC.  I found a few.  Replaced 7 e-lytics and the TL072 Chip and it's singing again.

According to the customer it hasn't worked in years. If I paid myself for every hour I spent on this unit, he'd have to take out a mortgage.....