Peavey TNT 100 Crackling When Turning All Pots - All Pots Replaced. Is it Caps?

[jurroppi1]

I am working on a Peavey TNT 100 and have replaced all of the pots and still have the same issue (crackling noise when turning any pot on the amp and a bit of distortion especially on the low end of the register). I had a sneaking suspicion that replacing the pots wasn't going to fix the problem, but did it anyway. The amp was stored in a garage in MN for a few years, so I figured they needed love anyhow. My nephew was playing through it (his amp) and stepped on his cord and trashed the input jack (already replaced both of them and his guitar cable, which was UGLY).

My guess is in the preamp section there is a cap or two that might be the cause and was hoping there might be a common cause component someone here knew of in this amp.

I'd appreciate any suggestions on where to look first.

[Helmholtz]

See if there's any DCV across the pots, i.e. measured between pot ends.

[jurroppi1]

About 100-200 uV on the volume pot across the entire wafer, Bass and Treble pots have about 3.2 - 4.5mV max (fluctuating).

[jurroppi1]

As I am probing for voltages I did notice something odd... When I touch the DMM + probe to any leg of the preamp first stage (a 953A), I can hear the transformer ring a bit (kind of a static noise). When I touch the base or collector of the second transistor (4249) it goes kind of nuts (much more pronounced static noise).

[Enzo]

I was an authorized Peavey repair center for over 30 years, and one thing I learned was those little 2uf 35v electrolytics in the signal path did not age well. I suggest replacing all of them. I see at least 5 of them there. At least a couple of them connect to control, more evidence.

[jurroppi1]

I was thinking the same, electrolytic caps. I'll probably recap most of the amp.

I appreciate you jumping in here also Enzo, good to hear from you.

[jurroppi1]

All I have on hand are 2.2uF caps. I'm thinking that will be fine. Thoughts?

[Delta362]

Yes, 2.2uF will be fine as long as your voltage ratings are same or greater.

[Enzo]

2uf or 2,2uf, it is only a 10% difference. Never forget, these are just guitar amps, not precision lab equipment. Close is good enough. In these amps if I ran out of 2uf, I would just use 4.7uf instead, and never look back.

[jurroppi1]

So I replaced all of the electrolytic caps in the amp other than the two main filter caps. I only noted one cap that appeared to be off in value even remotely, that was the 50uF cap. I am using a Keithley 3330 LCZ to measure capacitance, DF and ESR, so I'm pretty confident it is giving me accurate info.

It appears that the pots I used were causing part of the problem since the shafts were metal (plastic on the originals) I believe it was coupling noise into the amp whenever I touched the pots. The other problem is that the panel mount portion of the pots was shorter than the originals, so I ended up having the front of the PCB resting right against the front of the chassis. The problem that appeared to create is that there is a trace running all along the front of the PCB, so I figured that also wasn't good. I isolated that from the chassis with a strip of electrical tape, which appeared to help, until I found a dead spot in the master volume pot (keep in mind this is a brand new pot along with the rest of them).

I decided to just go back to the original pots since I had already cleaned them good with de-oxit after having removed them in the first place.

I just can't get Peavey parts department to respond to me at all. I've left voicemails and a couple emails. I don't know what to do about that. I've heard Peavey can be a PITA to deal with at times - I personally never had issues with them in the past. I don't get it...

I've been in the electronics industry since 1997, so it's not like I'm a novice. I've had to go as far as sending my FCC GROL number to LOUD Technologies in the past because they were concerned I wasn't knowledgeable and would electrocute myself, but after discussing with them they understood that I knew a bit about the subject matter and sent me the schematic I was asking for.

I wonder if I freaked out the tech in the parts department when I sent the email from my work account with my signature/card, because I work in the medical device manufacturing industry for one of the top 3 - arguably #2 of the top 3.

Finally, it does appear to be working properly now, but I'm less than thrilled about having to re-use old components that will likely just fail again.

[Enzo]

Re: Peavey. They won't have any parts for this amp, after all it is 40-50 years old. As to being non-responsive, there are very few people working there.,. After they moved production overseas, and then COVID hit, there is no on left. Roger is pretty much the customer service department and the parts department. He posted a parts sticky suggesting people who need parts go through a dealer like Sweetwater or Full Compass. And believe me, they don't care about your email address.

Parts are not noisy because they are off value a little.

I respectfully disagree with the metal versus plastic pot shafts theory. Noiseless amps have been made with both types for decades.

If it is working now, hey, just enjoy the amp. It may have a problem next week, or it may run flawlessly for another 20 years.

[jurroppi1]

I was just stating that as soon as I touched the pot shafts (metal ones on the replacement parts) the crackling sound would show up, but it was intermittent - happening at 9 and 3 o'clock most repeatably. That did not happen when I reinstalled the other pots (original ones) that had plastic shafts. I believe they were coupling noise or it was creating a ground loop somehow. The knobs for the pots are somewhat conductive as well, which explains why it was doing that with the knobs on or off. I also noticed that the ringing in the transformer went away with the original pots reinstalled.

As for the off value of the one cap, it had higher than normal ESR (was leaky) and was well below the bottom end of tolerance for capacitance, which to me would mean that the electrolytic probably dried up in that particular pot, thus perhaps that was the culprit all along.

I'm not trying to dig into you here, just explaining what appears to have been going on and what my findings were. I do appreciate you chiming in and I appreciate you sharing your knowledge.

I know the amp is old, it is sad that it is so hard to find replacement parts for it. I found an almost exact match for them, except the panel mount mechanical dimensions were just a bit shy and the shafts are metal vs. plastic. Other than that the PCB mount footprint is exact.

[Enzo]

Just for future reference: OEMs like Pevey won't have 40 year old parts on hand, but a lot of us shops do. If you need an old pot, never hurts to ask on the forum. I don't know which TNT100 you have, the 1974 model or the 1979 model. The older one - the one with the little transformer on the board - used pots 71190053 (50k), and 71190054 (10k) I THINK the '79 model uses the same pots, though that one has the one dual pot too. ANyway I happen to have a pile of those in my gently used collection - free for shipping in small numbers. Plus a whole sheet of other new PV pots. My shop is not unique in this.