Peavey Bass 400 has DC on output, need help trouble shooting

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jcsul
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Peavey Bass 400 has DC on output, need help trouble shooting

Post by jcsul » Thu Nov 05, 2020 6:21 pm

Hey everyone, I've got a sick little Bass 400 on the bench that's got about 8v DC on the output. I'm hoping you guys can give me some input on how to go about troubleshooting this issue. I'll post some pictures and go over what I've done so far.

If you look at the attached images, you'll see one of the power amp board with some voltage readings I've taken where the same 8v at the output is more or less also present. I didn't add these into the picture, but the 8v is also present on the Emitters and Bases of the four left (from the orientation of the photo) output transistors and I believe just the Collectors of the right four output transistors. You'll also see two pictures with my oscilloscope reading the output of the amp at the speaker jack with a 4x12 cab connected as a load. God rest its soul and damn my stupidity for not checking for DC at the outputs as my first troubleshooting step, but as a hobbiest that's the price I pay for learning proper practices. I picked up the amp for a friend with the issue being it was intermittently cutting out, so my go-to wasn't to check the output as I was assuming it was probably a power supply issue or just cold solder joints on an input or output jack.

So far I've changed out every transistor on the preamp and power amp board aside from the output transistors, as well as every electrolytic cap on the preamp board. I've got replacements for the electrolytic caps on the power amp board, and I'm planning on replacing them this weekend or possibly even tonight if I have time. I've also taken measurements with the preamp board disconnected, and that doesn't seem to impact any of the measurements. At this point I know the 8v DC on the output is an issue, I'm just not sure where I should start poking around to identify where the root of the issue is.

I've got a decent multimeter, a scope, and a sig gen so I should have all the tools I'll need to troubleshoot the issue. I just lack the knowledge to figure where or what to look for next. Any input would be super appreciated.
Attachments
Oscope with controls at 0.jpeg
Oscope with controls at 0.jpeg (2.7 MiB) Viewed 974 times
Oscope with controlls at 1.jpeg
Oscope with controlls at 1.jpeg (2.78 MiB) Viewed 974 times
Peavey Bass 400 Board with Voltages.jpg
Peavey Bass 400 Board with Voltages.jpg (227.09 KiB) Viewed 974 times

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Enzo
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Re: Peavey Bass 400 has DC on output, need help trouble shooting

Post by Enzo » Thu Nov 05, 2020 10:28 pm

So far I've changed out every transistor on the preamp and power amp board aside from the output transistors,
Oh, don't do that. Probably nothing wrong wwith most of them or even all.

The Bass400 had a couple versions, if your schematic is for the A version, get the B version from customer service at Peavey. The circuits are almost identical, and the B version has test voltages around the circuit.

jcsul
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Re: Peavey Bass 400 has DC on output, need help trouble shooting

Post by jcsul » Fri Nov 06, 2020 12:23 am

Thanks for the response Enzo. You actually helped me out with this same amp about five years ago when the preamp power supply went belly up. I've got the schematics for both A and B models on hand thanks to the champs at Peavey customer service. I'll test through those tomorrow. I've got all of the original transistors still, I made sure not to clip their leads when removing them. All the originals still tested fine on my cheapo ebay component tester, so placing all the originals back in the board shouldn't be an issue; just a nice little time sink.

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Enzo
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Re: Peavey Bass 400 has DC on output, need help trouble shooting

Post by Enzo » Fri Nov 06, 2020 1:15 am

Well, your new transistors are probably fine also. I just dislike changing lots of parts, it just adds opportunities to make a mistake, and the more we solder, the more we might damage copper traces.

jcsul
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Re: Peavey Bass 400 has DC on output, need help trouble shooting

Post by jcsul » Fri Nov 06, 2020 1:27 pm

Mid-day update: While going over the B schematic and comparing voltages I noticed the 8v DC seen on the output disappear on the negative side of a 33uf capacitor on the power amp board. I decided to try to get a measurement of its capacitance in-circuit using my multimeter but couldn't get anything besides an open circuit. So I decided to pull it and test to make sure something up or downstream wasn't interfering, but it still read open circuit even when pulled from the board. After replacing it with a suitable substitute the 8V DC is gone from the output and anywhere else I had found it before replacing that cap. My business partner was headed over at that point and I haven't gotten to do any further testing as we have some projects to hammer through this afternoon. At least getting rid of the DC on the output is an improvement, even if it turns out to not be the only issue. Will resume testing later this evening.

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