1978 Peavey Standard 260 H

[joshdfrazier]

Recently purchased this head from a guy on Craigslist for $50... Once I got home, I plugged it in, and then... nothing. There is a "thump" when you plug the cab into the amp, though. Checked the output and I am getting almost 40vdc from the speaker output. Cool.

Two resistors were completely fried, and one of the driver transistors was actually melted. I tested the four to-3 output transistors, and they checked out OK. The driver transistors seemed bad. at least one had a short between collector and emitter/base. I tested all the smaller to-92 transistors on the board, and wasn't able to come to any real conclusion, but am going to assume they are BAD. In any case, I ordered all new transistors, except for the output ones.

In the picture, the fried resistor to the left (680 ohm) reads -42v on one side, and 21 on the other. I assume this is a symptom of a dead transistor. The other fried resistor is a 100 ohm.

Anyway, just want to make sure I am on the right track here. No schematic available, only a few pictures online. I've e-mailed peavey for a manual.

[Enzo]

You need the 260H schematic, which Peavey ought to send right off upon request.  Not sure what you mean unavailable.

That 430 is certainly fried, and those resistors LOOK like they would be the 15 and 10 ohm resistors in the V- leg of the 430 in the voltage amp stage.  10 ohms by the transistor and 15 by the white cap.   I am not sure how you determined those burnt crispies were supposed to be 100 and 680 ohms.   What those pieces of charcoal measure is not a useful reading.

Very surprised no outputs are bad, did you check emitter to collector as well as the two base junctions on each?  All those 5 or 10 watt power resistors?  Check them for opens.

The voltage across the burnt resistor means the current no longer flows through it and the circuit is open.

[joshdfrazier]

http://forums.peavey.com/viewtopic.php?f=19&t=22056

Scroll down to the 7th post. Someone posted a picture of the poweramp board from a 260 H. The one to the left is most definitely 680.. at least I think! I am seeing blue, grey and brown. The other resistor is more difficult to see. Certainly did not try to measure! They are both burned completely in half.

i used the diode test function on my dmm to test the output transistors. I checked for shorts only, and found none.

[Enzo]

Maybe that was part of his problem, I don't believe it belongs there.  Looking towards the right, beside it, I see a diode, then two more 680 ohm resistors.  THOSE are wired to the small transistors above - which are the input differential pair.  THOSe are supposed to be 680.   I cannot see any other 680 on the print.

Peavey will be open in a couple hours, give them a call for the drawings.

MY concern about transistor testing is this, since I don't know you.  People learn that a transistor is like two diodes from the base pin.  So they test base to colletor and base to emitter, and if both are OK, then the transistor is OK...  But that leaves out the emitter to collector.  If that shorts, it leaves the two "diodes" in parallel, so they both still measure OK when they are not really OK.

and damage like this is usually including blown outputs.   Not saying you are wrong, it is just my concern until shown otherwise.

[Roly]

Very surprised no outputs are bad

Also "very surprised".

In fact I'm wracking my brains to think of even one example of finding blown resistors and drivers where the output transistors were still intact.  Not saying it can't happen, only that across many hundreds of amplifiers I don't think I've ever see it.


Were the output transistors disconnected/removed from the circuit for the test?

[joshdfrazier]

Ok, so Peavey responded with the schematic, which I've attached.

This is the youtube "tutorial" I used as a guide to test these output transistors:
http://www.youtube.com/watch?v=REW2lF0sb74

All of the TO3's passed the tests in the video. If there are more thorough tests to be conducted, let me know. I tested all four OUT of the circuit, at least three times a piece.

[Enzo]

OK, the video does it corectly, so I'll accept it.  it is just unusual.

[Roly]

Okay.

Can you identify the components that fried on the circuit for us please?

[joshdfrazier]

Enzo, you were right. The resistor that I assumed was a 680 is actually a 15. the other is 10.

Roly, I've put a red slash through everything that failed/burned. Any idea as to what could have triggered this failure?

I replaced all of the diodes with in4007, except for the P-600D. Didn't have any on hand, and they tested fine.

I tested the 10w .33ohm power resistors in circuit, and they tested fine. Should I test them out of the circuit? Seems like if they burned through, there would be some obvious damage to the board.

Thanks for the replies!

[Enzo]

.33 ohm resistors will look like a piece of wire to your meter, so they are either open or "shorted" which means OK.   Do not assume bad parts have visual failure.  One of those resistors can burn open in an instant and look no different.  If they test like a wire, they are OK, if they check as a resistor, then they are not OK.  There is no external circuit that can make a wire look open.

I am not a fan of shotgun repairs.  That is to say just replacing loads of parts.  There are two diodes on your print with dashed lines around them.  They are not plain diodes, they are dual diodes and have twice the voltage drop of a regular diode.  They are used to establish a voltage difference.  if you replace them with 1N4007, then the voltage references will be wrong.

[joshdfrazier]

Enzo, I didn't replace the 13886 diodes. I only replaced the diodes because, well, they're old. I've seen diodes fail in old equipment like this, and they typically short which causes a slew of other problems. Just playing it safe.

[Roly]

Just playing it safe.

{Actually you are only muddying your own pond.  It's fine to replace any tired components after you have repaired the fault, but I'd harden up on what Enzo said about "shotgunning" or replacing parts on spec or in the hope of getting lucky - it is very unlikely that you will, and it is fairly likely that you will introduce complications or new faults in the process.

The Doc doesn't write you a scrip for antibiotics the moment you walk through the door and we similarly need to do some diagnosis based on tests first to identify the fault, then repair that.  It's not only the quickest and most reliable way (and reliability is important with band gear), it's the only way to be really sure that you have correctly identified and repaired the fault.}


Looking at your "deadstuff" I have gone from "very surprised" to mildly astonished - some of it makes sense, but some of it doesn't.

I'd guess that the initial failure was the 431C PNP driver going short B-E and rapidly taking the pre-driver 430C followed by the 3638 protection transistor then the 10 and 15 ohm resistors with it, but the cause of the 3642 upper protection transistor and 761's in the input differential pair dying are still obscure to me ATM.  That the output transistors are still intact is a minor miracle.

This is the sort of damage I would associate with some metal part sloshing around inside randomly shorting things.

In particular I would double check the upper NPN driver, 430C, out of circuit.


How to proceed?  Having replaced all the dead components you can find I would power the amp up via your limiting lamp, with the speaker disconnected, and see if the output voltage is now close to zero.

If so try a low level signal test, of not you have something else yet to find.

[joshdfrazier]

Okay, thanks for the advice. I replaced the dead components, and the amp works great now. Sounds great, too. Reminds me alot of a Twin.

When the amp is powered on, i get about 50mV of DC voltage form the output. This reading diminishes the longer the amp has been on. When I turn the amp off, I get about 8vdc, but only for a moment. Is this normal? I'm assuming that is the "thump" that comes from powering the amp on and off.

Sorry for the stupid questions! I don't mess with much solid state gear. 

[Roly]

 :dbtu:

When the amp is powered on, i get about 50mV of DC voltage form the output. This reading diminishes the longer the amp has been on. When I turn the amp off, I get about 8vdc, but only for a moment. Is this normal? I'm assuming that is the "thump" that comes from powering the amp on and off.

Yeah, pretty much.  50mV ain't bad, and it's falling as the semicons warm up.  Most solid state amps seem to produce a bit of a thump after switch off as the power supply discharges, the exception being amps fitted with speaker protection where the protection relay opens first.