Peavey KB100 popping and thumping with humidity

[macjedimatt]

I have a Peavey KB100 keyboard amp that works great most of the time. However, for the past couple years, when things get humid, it pops and thumps periodically until it warms up. I have taken it to a musicians repair shop at the suggestion of my musical director but they can't find anything wrong with it. I'm wondering if I should just go ahead and re-cap it. Does this sound like something that could be caused by bad caps? It is at least 20 years old so I suppose it is possible that even high quality caps could be wearing out. Any other thoughts?

[Sparks88]

Re: Peavey KB100 popping and thumping with humidity

humidity introduces moisture, I would check for dry joints especially in power supply section and output stages, even if the solder looks good I would redo them anyway using original leaded solder.

[budm]

Re: Peavey KB100 popping and thumping with humidity

I would also inspect the board real close for any contamination and clean the board real well.

[stj]

Re: Peavey KB100 popping and thumping with humidity

they forgot to ask for foto's

[macjedimatt]

Re: Peavey KB100 popping and thumping with humidity

Well, it is still at the church. I hope to get it home this weekend and take a look at it myself. The shop that looked at it claims to have already re-soldered a bunch of it. We'll see about that.

I was able to get the schematic and the Peavey parts department has sent me a list of all the capacitors with part numbers and pricing. I doubt I will have to replace all of them. There are about 20 100pF and 0.1uF caps in there. I'm thinking that maybe the power filter caps (2200uF/50V) could be having a problem. But without actually looking at it, this is a big guess.

Being a meteorologist (not on TV!), I understand that humidity does have effects on capacitors and electronics in general. In fact, some instruments depend on this. One thing to note is that it seems to go away after it has been on for an hour or two. My theory is the heat is effectively drying it out. So it could be board damage as well.

Will definitely have to put this under the magnifying glass.

[SteveNielsen]

Re: Peavey KB100 popping and thumping with humidity

I agree solder joints are certainly suspect. Not just those on the board either. Bad solder joints can hide really well on ground points soldered directly to the chassis.

One very elusive pop, crackle, thump problem I had in a guitar amp wound up being the reverb tank drive transformer that had an intermittent winding short so check that puppy too. You may not be able to measure an intermittent short in it but heating it up when cold and having the noise with a hair dryer could expose it.

Since this is a 90s amp you have it's probably a good idea to replace the electrolytics, or a least check them. I wouldn't worry about the small signal caps. If they are ceramic they could be microphonic but that would present itself differently, with ringing or oscillation at certain frequencies, not pops and thumps.

[stj]

Re: Peavey KB100 popping and thumping with humidity

when i think of churches - i think of cold damp buildings.

something you probably dont know - flux obsorbs moisture.
make sure it's not got loads of flux all over it.

[ben7]

Re: Peavey KB100 popping and thumping with humidity

I agree with STJ.

Is this a tube amp?

Also, the extra moisture may be causing a resistor or capacitor to break-down.

Yes, you should do a preventative re-cap of the electrolytics.

[budm]

Re: Peavey KB100 popping and thumping with humidity

It is solid state amp.
Diagram

[macjedimatt]

Re: Peavey KB100 popping and thumping with humidity

budm, That is the "newer" version of it. I have the 1984 which is slightly different. Hoping to get it apart and post some pictures Tuesday night.

[Marko7]

Re: Peavey KB100 popping and thumping with humidity

Carefully resolder all low-level audio stages, clean the P.C. board in the preamp area, dry it COMPLETELY and then try spraying some clear non-conductive sealent over both sides of the board.
The goal is to make any connections which are in close proximity with each other [in the preamp or output driver I.C. area] from having moisture [beads] conducting spurious D.C. currents around, like between pin 3 (+ In) & 4 (-Vcc) on a dual (or single) op amp I.C. That should help.

[kc8adu]

Re: Peavey KB100 popping and thumping with humidity

any evil brown glue?

[Agent24]

Re: Peavey KB100 popping and thumping with humidity

I fixed a friend's subwoofer\amp that was thumping when the cable was moved, at first I thought it was the plug\cable, but it turned out to be broken PCB traces for the jack.

[macjedimatt]

Re: Peavey KB100 popping and thumping with humidity

I've finally got it and taken it apart. I've gone ahead and pulled all the electrolytic caps. I'm going to start with replacing them and I plan on inspecting the solder joints while I put them in.

Some observations:
- The COMP. light is always very faintly lit.
- The power light isn't particularly bright.
- It thumps when turned on the first time.
- The PCB is a very cool design.

Pictures attached. Any other questions or recommendations welcome.

[goontron]

Re: Peavey KB100 popping and thumping with humidity

number 5 looks like it could be a wallpaper, what camera are you using?

[macjedimatt]

Re: Peavey KB100 popping and thumping with humidity

number 5 looks like it could be a wallpaper, what camera are you using?

A Canon EOS 20D. That one was using a fixed 50mm lens.

[Agent24]

Re: Peavey KB100 popping and thumping with humidity

Some observations:
- The COMP. light is always very faintly lit.

Could be indicative of a fault, like a leaky component. What is that LED connected to? I can't see it on the schematic.

- The power light isn't particularly bright.

The LED might just be old and worn, R198 might have increased in resistance, or there's a poor connection somewhere.

- It thumps when turned on the first time..

That might not be a fault. Some amps do not have de-thump circuitry (especially old ones) and all amps thump at power up to some extent. If it's not very loud it might be normal.

- The PCB is a very cool design.

That would have been 'taped out' by hand.

[macjedimatt]

Re: Peavey KB100 popping and thumping with humidity

Could be indicative of a fault, like a leaky component. What is that LED connected to? I can't see it on the schematic.

The attached PDF is the closest I could fine to my version of the amp. The COMP LED is connected to a 4558 amp on the lower left of the schematic.

The LED might just be old and worn, R198 might have increased in resistance, or there's a poor connection somewhere.

Right. I think on this one it would be R111. I'll check it out tonight.

Thanks!

[Agent24]

Re: Peavey KB100 popping and thumping with humidity

That one makes more sense. R111 would be it.

What does the 'COMP' LED actually indicate?

[macjedimatt]

Re: Peavey KB100 popping and thumping with humidity

I think the COMP led is "Compression" it lights up whenever it thumps on power on, or when it pops/thumps due to whatever is causing it to do that. I can't find a manual for this particular version. The one in the Peavey archive is for the newer version that has the XLR jack on channel 1, and no COMP light.