Peavey PV-4C

[Bill_Nephew]

Recently, I managed to buy an old rackmount Peavy Pv-4c from my school for next to nothing ($5 haha) as a result of them redoing their PA system. They already plugged it in to show me what was wrong with it. As soon as it started up, there was a bit of smoke coming out of the left front vent. After opening it up to see what was going on, it appears that R119, CR112 (a 1n4148 silicon diode), and Q103 were the parts emitting the smoke. I also went on to check the output transistor board, and found that two of the .33ohm 5 watt resistors were reading open. Before I replace these parts, is there anything else I should check for?

[soundguruman]

undoubtedly, the output transistors are shorted.
you have to tear the thing down and check all the semiconductors. Don't rely on just what "looks" burned or is burning.
You are looking at he symptoms, not the real root problem.

[Bill_Nephew]

I just got done testing Q100-103, the output transistors for Channel A. Two of them are reading shorted, and the other two are reading open. The testing was done out of circuit on a digital multimeter using the diode test function.

[soundguruman]

It's good to use an analog meter, because you can see small leakage in the junctions that a digital meter will not detect.
Look also for reverse leakage, not just short circuits. But the ohm meter test is good enough to get the amp working again.
I replace the ones that have a hint of reverse leakage, also. Most techs ignore that.

[Bill_Nephew]

I am going to replace the output transistors, the parts mentioned in the first post, and the final driver stage transistors for each channel on the drive board. Some new thermal paste will be applied to improve the reliability of this thing. Just so I am not mistaken, reverse leakage is current that is flowing back in a direction where it's not supposed to conduct, correct? It won't hurt to replace the smaller input driver transistors in the TO-92 packages. I am in the process of out circuit testing all of the diodes on the driver board, I will get back with the results from that once I finish.

[soundguruman]

Yes, the reverse leakage flowing back in the wrong direction.
These leaky transistors may run hotter, or eventually fail, the leakage is a symptom of possible future failure.
I usually do rebuild a blown amp, going back more stages than it actually blows.
Like if it blows 3 stages back, I will rebuild it 6 stages back. The transistors that were stressed, and did not blow, I replace, just to gain increased future reliability.
Many techs don't do any of that, they just make it work again. I like zero returns on repairs, and expect that an amp I repair will last for many years.
It's kind of like I don't want it to fail, for a very, very long time.
And that costs about $20 more in parts, but to me it's more worth the time. I can guarantee it for 6 months, and expect it to last 6 years.

[Bill_Nephew]

I understand that completely. why repair an amp to last only 2 years when you can make it last for quite a while longer just for a bit more $? On the bright side, all of the output transistors for channel B tested perfectly. I took them out anyways so I can redo the thermal compound because of how dry and cakey the old Peavey stuff is looking. Some arctic silver ought to help it quite a bit. I also tested all of the semiconductors for channel B and found no defects. It definitely concludes that the problem is only in Channel A. I will replace all of the semiconductors possible for that channel without a doubt.

[soundguruman]

the arctic silver is bitchen' but its also conductive

[Bill_Nephew]

what would you recommend for thermal compound? There is a sheet of mica in between each output transistor and the heatsink, so I don't think conductivity should be a major worry if the transistor's case is already isolated from the heatsink. I used the same stuff on my Crate TD-70 when I replaced the output transistors and the rectifier diodes that shorted.

[soundguruman]

That's fine as long as its applied right the silver is much better, but do it carefully
I have been using Z9 or type 44 but I would rather put silver on everything.
The difference is huge for heat dissipation, like you said.
It's not as much an issue in that amp, I think the output was shorted, not overheated.
It was probably shorted accidentally when the speakers were hooked somehow.

[Bill_Nephew]

Actually, judging from some new findings, the input was shorted somehow. I tested all four 1n5393 diodes on the input board and all of them read shorted! Also, one of the traces was burnt up leading to the connector that leads to the driver board. I think I found the root of the problem. It looks like this is going to be quite a rebuild. A lot of the diodes on the driver board read either shorted or open also.

[Jazz P Bass]

Lightning strike, perhaps?

[soundguruman]

your kidding, oh crap.
Maybe somebody plugged a speaker output from another amp into the input of the PV. Dat would xplain it. Power Boost.
It never ceases to amaze me what people will do to their equipment. At times it boggles the brain.
But now u see why I check all of the semiconductors. You might never expect that the input diodes would be blown,
and I would call that pretty rare.....but yet....

[Bill_Nephew]

Haha that would explain it, since these diodes are only meant to handle 1.5A before failing. I just got done ordering the parts off of mouser to replace all of the diodes and transistors. Before I ordered, I also checked every resistor to make sure I wasn't getting an open reading on them just to be sure. I am quite surprised by the bozo that thought it was a good idea. This is the first time I have actually had to rebuild an entire amp like this. On the bright side, once everything is back up and running, it will be running for quite a while.

[Bill_Nephew]

After replacing the two output transistors that shorted along with all of the small to-92 package transistors along with bad diodes, I got the amp to start up without any smoke coming out the front. However, channel A is very quiet, and channel B clips too early. Should I look to the TO-220 transistors on the driver board next?