Peavey Bandit Novice repair

[MCM1910]

Good afternoon. I bought a peavey bandit. The original owner explained that it had an issue where it smoked. I paid next to nothing for it thinking it'd be a good opportunity for me to try out another repair.

So here is what I know. Amp listed as not working and stating that it produced smoke at one point. So first inspection I notice the power cord has had the ground prong removed and one of the prongs remaining is damaged. I opened it up expecting a lot of burnt out parts but from what I can tell there is one cap that is damaged and pretty clearly spewed some liquid out of it.

So my plan of attack here is to replace the power cord and the cap and see if that takes care of the problem. Does anyone else have any suggestions? I'm really excited to get this thing going as I've heard so many good things about these bandits!

[Enzo]

The light tannish goo on the caps in the picture is glue.  They smear it on the larger parts to keep them from vibrating and breaking their wire leads.

Start with the power transistors, are they shorted?  Does this unit blow fuses?  Does it produce DC on the output?  Does it even light up?

No circuit works right without proper power supply, so we always look into that first.

Get your official model name from the serial number plate, and call customer service at Peavey for the correct schematic.

[MCM1910]

The light tannish goo on the caps in the picture is glue.  They smear it on the larger parts to keep them from vibrating and breaking their wire leads.

Start with the power transistors, are they shorted?  Does this unit blow fuses?  Does it produce DC on the output?  Does it even light up?

No circuit works right without proper power supply, so we always look into that first.

Get your official model name from the serial number plate, and call customer service at Peavey for the correct schematic.

Thanks for the reply Enzo! The unit does power on but does not produce any sound. What I was referring to with the cap isn't the glue but the faint trail of orange where it appears the cap let out some liquid and it was hot on the board. You can see how it affecting the area around the leads of the components next to it.

Thanks again!

[DrGonz78]

Yeah it does look like something nasty sticky fell in the amp. Try using a q-tip and some 99% isopropyl alcohol to give it a good cleaning. That 220uf electrolytic cap needs to go on a diet his body broke right out of his outfit.  :lmao:

[MCM1910]

Updates. Got schematic from peavey. All the voltages in the power supply seem to check out. Did some more fiddling around and found that if I take preamp out to another amp it works but unfortunately plugging into the power amp there is no sound. That capacitor is a part of the power amp so I'm still thinking that is my most likely problem.

Is there any thing else I should rule out other possible causes?

[Enzo]

Post your schematic or tell us the model Bandit, there were many Bandits.

Could it be the Bandit (Supreme) Transtube, 1995 schematic?

I don't see that as killing your sound, even if bad.  C71?  If that were bad I'd expect maybe distortion or hum or noise, not silence.

I see the speaker feed goes through a thermal breaker, is yours open or unwired?

Feed a signal into the FX return for the power amp, now follow it through the front of the PA.  You see the signal at the op amp outputs?  Looks like U3B, then U5A, then either side of U4, then U5B?  If it doesn't get that far, C71 has no chance to affect it.

[MCM1910]

I'll try attaching schematic to this but I believe you are correct in your guess of the model. (Peavey is great about this by the way. They emailed it to me in 9 minutes!)

I will do some signal tracing and report back. I've got some friends flying in tonight so it may be a few days. Thanks so much for all your help!

[Enzo]

Look in the corner of the drawing, does it have the 1995 date?

[MCM1910]

Im sorry I'm having trouble posting PDFs but yes it is dated 1995.

[Enzo]

OK, then we are on the same page, I have that one here.

[MCM1910]

Well here is the new development. I was thinking about the possibility that there was a speaker problem. So I though I'd plug in a small 8ohm speaker I had laying around and plug into the ext speaker jack. I did and there was indeed a hum when I turned it on and then a puff of smoke from the speaker. What in the world just happened besides probably ruining that speaker and discovering the Sheffield in the amp may be done for too...

[Enzo]

Your amp is producing DC, that will indeed damage a speaker.  Work with NO speaker and NO load on the amp until we know it is no longer making DC.

[MCM1910]

Your amp is producing DC, that will indeed damage a speaker.  Work with NO speaker and NO load on the amp until we know it is no longer making DC.

Thank you Enzo. Can you give me any tips for how to trace down how it is producing DC? I guess I'm still wondering if c71 is involved. It is damaged but I guess I don't understand what would cause it to have problems. What I mean is did another problem create the DC and that damaged c71? Or is c71 the culprit?

[galaxiex]

C71 looks bad, did you replace it?

If C71 is shorted it could put DC on the speaker.

Disconnect the speaker and connect a multimeter set to DC volts to the amp output. (where the speaker would normally connect)

Switch on the amp. Is there a DC voltage reading on the speaker output? How much?

[MCM1910]

C71 looks bad, did you replace it?

If C71 is shorted it could put DC on the speaker.

Disconnect the speaker and connect a multimeter set to DC volts to the amp output. (where the speaker would normally connect)

Switch on the amp. Is there a DC voltage reading on the speaker output? How much?

I just did this and it was holding steady with like 0.4-0.5mv and then all the sudden it spiked at 5.4v. It seems to spike when I tap on the amp, so I'm going to guess that is that cap.