Peavey Lm3886t power amp blew up

[Den.]

I bought an inoperable 2001 Studio Pro 112 and quickly found the damaged LM 3886t. I've removed the 3886 and I am ready to order a replacement. Is there anything/anywhere else I should look for related problems?

[Enzo]

Probably not.

[phatt]

Probably not.

Hi Enzo,,, Can you simplify the technical aspects of that rather complex answer?
:lmao:

love your style :tu:
Phil.

[Enzo]

Pro, as in professional

Bably, as in I like to babble

Not, as in what my wife says to most of my ideas.

[Roly]

Yep.  :tu:  I also noted the economy of that technically correct answer - could only be beaten by "yes" or "no".   :lmao:


I'm reminded of the definition of "expert" - "ex" as in 'has been', and "spurt" as in 'drip under pressure'.

"Why are you just standing there doing nothing?" the factory manager demanded.

"I'm not doing nothing, I'm thinking about it."

[Den.]

Here's the latest. The LM3886T arrived the other day and I soldered it in. I screwed the amp back together and tested it. I started with the clean channel. It worked fine at low volume while I tuned my guitar. Then it started to sound fizzy. And cutting in and out. The more I played the fizzier it got. Same in the lead channel. Hardly any volume too. No amount of knob twiddling had any effect on the sound. Then, after about 5 minutes, it quit...nothing.

I opened the amp up again and the LM3886T looked fine.

I started looking around and spotted this honey colored stuff underneath these two caps. It's dry which made me think that it had been there before and I just didn't notice it.

I checked around all of the other caps and other components didn't find any sign of the honey stuff.

I'm prepared to replace those two caps if that looks like the problem.

[Enzo]

That is glue.  They solder the caps onto the board, then to help keep them from vibrating and shaking, they glue them down.  VERY common.

Don't try to think in terms of the other repair, just repair this as if you just now are seeing it for the first time.   I doubt this has anything to do with your 3886 repair anyway, unless you missed a solder joint.

You have FX loop jacks, so plug the guitar or some other signal source into the FX return.  Get sound?  That checks the power amp.

Plug a spare cord from FX send to FX return, any help?

Turn the reverb up midway and rock the amp back and forth to crash the reverb springs.  Does that noise come out the speaker?

Plug the guitar into the regular input jack, and set the controls as normal.  Connect a cord from FX send to the input of some other amp and speaker.  Get sound there?

Obviously there are other things, any point in the signal path could have failed.  YOu have mute transistors that could be stuck on or failed, and so on.

[Den.]

Thanks for your reply.

You have FX loop jacks, so plug the guitar or some other signal source into the FX return.  Get sound?  That checks the power amp.

No sound.

Plug a spare cord from FX send to FX return, any help?

No. Still no sound.

Turn the reverb up midway and rock the amp back and forth to crash the reverb springs.  Does that noise come out the speaker?

No reverb crash.

Plug the guitar into the regular input jack, and set the controls as normal.  Connect a cord from FX send to the input of some other amp and speaker.  Get sound there?

Yes. All of the controls and channels and mode switches work except for the T. Dynamics knob.

Obviously there are other things, any point in the signal path could have failed.  You have mute transistors that could be stuck on or failed, and so on.

[DrGonz78]

So it looks like the preamp is working fine in various ways with exception of the T. Dynamics knob. So, there is something there that needs to be looked as far as connections within traces between that location of the PCB board. A schematic may prove to be helpful... Edit: Enzo is like a human Peavey schematic!! So I am sure he'll be the best to help with these amps in general.

Disconnect the speaker and take voltage reading on the terminals, any DC on the output? This will help to know whether there is something wrong on the output. I would do this just to make sure before doing anything else. That way we know that the output is relatively okay.

[Den.]

Here is the schematic:

[Enzo]

Thanks for posting that.

Does the headphones jack work with phones?   That jack does have a cutout for the speakers, so if that jack goes funny or cracks free of its solder, it will stop the speaker.

The T-dynamics control is part of the power amp, not the preamp, so it makes sense it would have no effect.

[Den.]

Does the headphones jack work with phones?   

The schematic is for both the Studio Pro and the Envoy and points out the different components to install and omit. The Studio Pro does not have the headphone jack.

[Enzo]

Sorry, I looked at the schematic without focusing on those differences.  Couldn't hurt to check the jumper in place of that jack for good solder.

But looking I see that the headphones jack itself remains, it just becomes an extension speaker jack instead.  And if I read my schematics right, that extension speaker jack looks like it cuts out the amp's interior speaker.  SO that leads me right back to where I was with headphones.  Does some other speaker work when plugged into the extension speaker jack?  Any chance the extension speaker jack has a bad cutout contact or solder?

I was looking for simple ways to isolate the problem.

Let me start fresh.  Let us assume nothing.  Take a wire off the speaker, and touch a 9v battery to the speaker terminals a moment.  A working speaker will pop each time you touch the battery to it.  SO is the speaker working?

Connect the speaker back up, turn the power on, then off.  Does the amp make any sort of pop or thump out the speaker when it is switched either way?  ANy sort of sound at those times?  With the amp on, is there ANY sort of background sound coming out the speaker?  ANy little hum or hiss or anything?  Just want to know if any sign of life comes out.

I am going to assume the +/-15v rails are OK, but are both +35 and -35 OK?

What gear can you trace a signal with?  Scope?  SIgnal tracer? AC volt meter?   We need to send a signal into the FX return and follow it through the stages.

[Den.]

Well, good news and bad news and more good news. The first good news is that we've figured out what was wrong. If you go back to my original post where I am describing the amp, "It worked fine at low volume while I tuned my guitar. Then it started to sound fizzy." That was the bad news. It turns out what I was describing was the sound of a Peavey speaker failing. Tonight the speaker failed your 9v battery test then plugging the amp in to an extension speaker cabinet brought it to life. The rest of the good news is that I have a couple of extra speakers laying around that will match up to this amp just fine. Thanks for your help guys.  Den