Peavey KB100 popping and thumping with humidity

budm · 07-18-2014, 08:21 PM

Re: Peavey KB100 popping and thumping with humidity

Glad you found the and narrow it down to that section where U5 and U6 are. What is the designator of the cap? C??

macjedimatt · 07-18-2014, 08:06 PM

Re: Peavey KB100 popping and thumping with humidity

Finally got it all fixed! Last night I discovered that one of the small NP caps in the EQ circuit was the culprit. It had a bad solder joint. I found it by poking the board with a marker while listening to it through headphones.

Thank you for all of the suggestions!

budm · 07-16-2014, 07:19 PM

Re: Peavey KB100 popping and thumping with humidity

At least you are getting close.

macjedimatt · 07-16-2014, 07:09 PM

Re: Peavey KB100 popping and thumping with humidity

Originally posted by budm

So now we have to start tracing backward from U6B output pin 7 back to U5A (since you use J2 and it still thump). So you replaced all those lytics caps C18, 19, 24, 39 in the signal path?

Yes. I'm going to have to get back to this tomorrow. 4AM comes awful fast. Thanks for the help so far!

budm · 07-16-2014, 07:07 PM

Re: Peavey KB100 popping and thumping with humidity

Might as well resolder all the connections and use good flux.
So now we have to start tracing backward from U6B output pin 7 back to U5A (since you use J2 and it still thump). So you replaced all those lytics caps C18, 19, 24, 39 in the signal path?
Check the DC offset at the pin 1, 7 of U5, 6 also.
BTW, the thump is really loud? and only when you it on, otherwise the amp sounds fine?

macjedimatt · 07-16-2014, 07:06 PM

Re: Peavey KB100 popping and thumping with humidity

Ok, just discovered that flexing the pan that the board is attached to is causing it... So it probably has a bad joint or damaged trace somewhere that I missed.

macjedimatt · 07-16-2014, 06:57 PM

Re: Peavey KB100 popping and thumping with humidity

Originally posted by budm

Insert the blank phone jack into the Power amp in J4 (this will disconnect the line out signal from feeding into the power amp input section), then connect a Powered speaker line input to the J5 line out and see you do hear a thump from that line out, and see if still hear the thump from the internal speaker.
It sounds to me the thump is from the preamp section.

Confirmed. Signal patched in, headphones on the pre out, and a blank jack plugged into the pwr amp in. Thumped in the headphones. I think my neighbors like this test better :-) Now the question is, what the heck would cause this?

macjedimatt · 07-16-2014, 06:51 PM

Re: Peavey KB100 popping and thumping with humidity

Just put the meter on the 15v lines. -15 was -15.2 and pretty rock steady. +15 was 14.8 and also steady. Not sure if my meter just wasn't picking up the spikes though.

budm · 07-16-2014, 06:50 PM

Re: Peavey KB100 popping and thumping with humidity

Insert the blank phone jack into the Power amp in J4 (this will disconnect the line out signal from feeding into the power amp input section), then connect a Powered speaker line input to the J5 line out and see you do hear a thump from that line out, and see if still hear the thump from the internal speaker.
It sounds to me the thump is from the preamp section.

macjedimatt · 07-16-2014, 06:32 PM

Re: Peavey KB100 popping and thumping with humidity

I did some more testing tonight:

Going straight into the PWR AMP IN = Clean with a couple faint clicks that could have been line noise.
Headphones on the PRE OUT = Heard thump in headphones and speaker
PRE EQ Patch in only = thumped, seemed to do it a few times also when messing with the EQ knobs.
Bypassing the reverb coil = still thumped.

I've attached an annotated schematic with what I have tested so far. At this point, I feel like the main power amp is probably fine. I'm not sure if it is something in the EQ or maybe the +/-15V lines of the power supply. I have a DMM with a peak hold. Should I put that on the +/-15V lines and look for spikes? Could the transformer be going bad?

Attached Files

ben7 · 07-15-2014, 06:47 PM

Re: Peavey KB100 popping and thumping with humidity

I wonder if there is a bad transistor or IC somewhere.

macjedimatt · 07-15-2014, 06:33 PM

Re: Peavey KB100 popping and thumping with humidity

So I have finally replaced all the capacitors and it doesn't seem to have gotten any better. It thumped a few times in the first minute or so of turning on. I hooked some tunes up to it and caught it in action on video: http://youtu.be/zjHkZr2dD54
No combination of turning/pushing/pulling on the knobs seems to replicate it reliably. Also does it with no source connected.

budm · 06-26-2014, 08:40 AM

Re: Peavey KB100 popping and thumping with humidity

It is the compression indicator to show that the compression circuit (U1A/B, U7B, U2(87478 is an OTA (Operational transconductance amplifier): https://cdn.badcaps-static.com/pdfs/...323fc6188c.pdf, ) has kick in if the signal feeding the power amp section is too high.

macjedimatt · 06-26-2014, 07:55 AM

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.

Agent24 · 06-26-2014, 04:51 AM

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 · 06-26-2014, 04:34 AM

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!

Attached Files

Peavey KB100 (standard pots) Service Schematics.pdf (1.84 MB, 22 views)

Agent24 · 06-25-2014, 10:18 PM

Re: Peavey KB100 popping and thumping with humidity

Originally posted by macjedimatt

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.

Originally posted by macjedimatt

- 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.

Originally posted by macjedimatt

- 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.

Originally posted by macjedimatt

- The PCB is a very cool design.

That would have been 'taped out' by hand.

macjedimatt · 06-25-2014, 07:45 PM

Re: Peavey KB100 popping and thumping with humidity

Originally posted by goontron

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.

goontron · 06-25-2014, 07:31 PM

Re: Peavey KB100 popping and thumping with humidity

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

macjedimatt · 06-25-2014, 07:24 PM

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.

Attached Files