Peavey XR600B Buzz

[tdlunsfo]

I have a fun one here if anyone is interested in doing some puzzle solving this morning. I'll try to be brief with the facts:

1. This is the 400B/G model
2. Main filters have been replaced as well as the smoothing caps for the 16v rails.
3. Amp has been repaired in the past as it has two new output transistors.
4. I couldn't get many watts out of her but after removing the compression OTA, I get all I want. So this is obviously bad and appears hard to get so it will stay out and the compression disabled.
5. I am unable to find any failed parts. All the transistors, resistors, diodes and zeners check out. The remaining caps test good for ESR.
6. I have disconnected the preamp and ran signal straight into the molex connector with the same result.
7. I have disconnected the finals board and I get the same result.
8. I have replaced the TL074 with the same result.

It sure is an interesting waveform. My conclusion is I have injured drivers and/or VAS from the previous failure/repair. Thoughts? Anything I'm missing?

Schematic = Free download Peavey 400BG Power Module Schematic (audioservicemanuals.com)

[Enzo]

OK< so WITHOUT a load, does the output still look that way? Or does it change?

Zeners may check out as diodes, but do they check out as zeners?

SO are both the main rails and the two 15v rails up to voltage and clean? Is any of this waveform riding the rails, so to speak?

Does collector of Q9 look like the waveform?

Lift CR10 and CR15 to disable the limiters, any change?

You know, that 87478 could even be OK, that waveform mess might just be triggering it.

[tdlunsfo]

The signal cleans up without the load. Sorry.. didn't think to check this. As for the zeners I must've gotten this confused with the B/H schematic as this power supply doesn't have them. Regardless I have my 15v rails and they measure approx 15.3 and when I plug in the load I do see the trash on the rails. The trash is also on the collector of Q9. Lifting the limiters made no difference.

[g1]

That kind of waveform I usually associate with a bad op amp. You replaced TL074, but how about U3 ?

[tdlunsfo]

Not sure why, but U3 isn't populated on this board.

[g1]

Hmm. The BH schematic does not have U3.

[Enzo]

The 400B/G is actually either a 400B or a 400G. B/G for bass and guitar. They used the same board for several models. U3 circuit is not used to your variant. And you have U2 removed. You would have the B version.

Right above Q12 is tiny inductor L1 parallel R44 4.7 ohm. In many of the boards that inductor is replaced with a second 4.7ohm resistor. And if ypou look at 400BH, that is now in the schematic. So don't freak out if your inductor is a resistor.

Associated with Q2 and Q12 are two 47 ohm resistors, make sure they are healthy. Power down and disconnect the two three-wire cables to the power transistor board, now no parallel resistances to mess with readings. After that restore the connections.

When I see clean signal with no load and garbage when loaded, to me that usually means the output stage cannot handle current demands. Make sure all the 10 watt resistor on the power boards are OK.

There are little DC voltages on the schematic, do yours approximately match?

[tdlunsfo]

I had previous checked all the big resistors on the output board and they are healthy. I've had the same experience as you stated with the clean signal becoming dirty when loaded but the thing that makes me question if the outputs are the issue is I can disconnect the output board and I get the same dirty signal even with very little load applied (as not to load down the drivers without any finals connected). I'll check those 47 ohm resistors and the schematic voltages and report back.

[tdlunsfo]

I have updates. I failed to mentioned this earlier but some of the previous damage was a decent burn spot under R61. R61 has been replaced with the correct 47ohm, L1 is now what measures to be a 7ohm carbon comp resistor and R44 was replaced with the correct 4.7ohm resistor. C26 is open. It doesn't appear that Q12 was replaced all though looking under the board you can see the heatsink got hot.

[Enzo]

question if the outputs are the issue

The outputs cannot source the current. That doesn't mean they are bad. They COULD be bad, but they also could have driver circuits unable to perform.

Whenever a resistor burns, whatever semiconductor it is wired to is automatically suspect.

[tdlunsfo]

Unfortunately I don't have any SJE5331/5332's. I guess I could try the MJE15032/33G's. Seems overkill but for testing purposes I don't see any issue. If this fixes it, then it looks like CE has the right transistors. Unfortunately if it turns out to be the finals.. it will be beyond the owners budget to repair.

[Enzo]

Just watch out for the leg order, TO220 and TO126 differ.

Anything is possible but in my experience, for the most part the output transistors either work or they don't. And usually one or two fail, not the whole set.

And just for science, you should know that the 5332 guys are PRE-drivers, nor drivers.

[tdlunsfo]

Welp, I replaced Q2, 12, and 9 and same results. So back to the drawing board. I guess I could pull the other transistors and check for leakage but if they check out I might have to pull my parts and send this on its way.

[Enzo]

There are little DC voltages on the schematic, do yours approximately match?

I asked you this in post #7, did you do it?

[tdlunsfo]

I saw it. I haven't gotten to them as my attention got diverted to a Vibrochamp...but I have printed off the schematic and have them highlighted. I'll try to look at them in the morning. Enzo, it doesn't say on my schematic, but do you happen to know how Peavey wants the conditions in order to properly take these measurements? I assume these are no signal applied? Then again, like they say at the old power plant, never ass-u-me.