Peavey TNT 100 popping.....?

[StratTone]

Just got a Peavey TNT 100 on the bench yesterday. I wrote Peavey for the schematic since I couldn't find it.

Anyway. They guy at the shop said the customer brought the amp in for repair because it was breaking up bad. He ordered him a speaker and put it in and the store owner said the poping is much better but still there.

Fastforward to today. I took the amp home and plugged it in and the amp plays fine but when you hit an open A it will fart/pop and the speaker slightly reacts irracticly.

Took the chassis out and looked for normal problems with solder joints. Tested the transister, diodes, and power supply caps and all were good. Put it back on the bench till I get a schemic and I can hook my function generator up to it. Got any ideas of where to look?? Thanks for the help.

[Enzo]

Sure. connect a bench speaker, and apply a steady signal to the input. Now while it is running, pick the thing up about 3 inches and drop it. Or if you are to squeamish, whack it with a rubber mallet on the ends and the top of the sides. Does this mechanical shock affect the amp? Looking for a loose connection that opens at that A because it resonates in the cab.

You can apply a signal to it and run it without a schematic. What does it do when maxed and the speaker is not in the cab with it? To me, deciding if it is vibrational or electrical is key to the process. Of vobrational, then we look for a loose conncetion, if electrical, we look at the signal stage by stage.

The main filter caps have to make smooth DC, so they? If one is going soft, you might not hear a lot of hum, but it would break up easier than it should. The two largest caps will be those, don't need a schematic for that either.


And the speaker connects with a two pin Molex connector, make sure the female pin has not spread, making the connection loose. The female on the board is usually OK, the female on the wires gets loose.

Do you have the 1974 version with the small 298 transformer and only transistors on the board? Or do you have the 1979 version with Shift and Pyramid controls and op amps in the preamp?

If the older version, CAREFULLY look at the wires from the little transformer to the pc board. ANy broken? GENTLY tug on each, since you might not see if it is broken off up inside the thing. The thing has mounting tabs soldered to the board in addition to its wires. If those have broken loose, then all the more reason to suspect the wires.

[StratTone]

Thanks for the tips man. I'm going to give it at try tonight. I must have the older version. No opamp chips on the board. The only thing I know for sure is the pots seem to have been sprayed with nonlubricated spray at one point because they are all stiff. Makes sense that a certain note would bring out the pop since the vibration may be causing the connection to short and go open some. I gave it a once over and tested the filter caps and transister and all looked fine but I really need to give it a closer look.

[StratTone]

Ok, I isolated the chassis from the cab to see if the vibrations were causing a short and the problem still exsists. I got my schematic yesterday for it so I can now start doing some trouble shooting. I touched up any solder joints that could use it but to be honest all looked good. I'm thinking I have like one component that is causing the problem. Any ideas where to look? Seems to mostly only pop with volume, bass, and a good hard note played. I'm thinking a cap under load may be the problem.

[Enzo]

OK, you have the older one, so there is that small 298 transformer on the board then? Make the transformer checks I mentioned above.

Caps are possible. The little 2uf/35v ecaps of which there are several are suspect. Scope the power rails and see if they glitch under the symptom.

This is only a three stage amp, and ther eare only 6 small transistors. The first four are really two pairs. Scope at each stage to see where the noise occurs.

There are DC voltages on the schematic through the stages, are your DC voltages more or less similar? No signal while making DC readings please. If a DC voltage is way off, it could knock out headroom, and you are hearing clipping.

[StratTone]

I did a good check of all the transisters on my lunch break and all looked good. I also checked the leads off of the transformer on the board and they seem snug and tight. I'm going to hook my function generator up to it when I get home and see if the 2/35s leak any when a signal is applied. It is kind of a stumper since the amp seems to be working fine but clipping. The only thing I know for sure that seems odd is the pots are tight when turned from someone using nonlubricated cleaner on it. I put my meter on it and turned the knobs to check for opens but other than it being tight they looked ok on the meter.

[Enzo]

A meter test of the transistors won't tell you if they are noisy or what they will do under load and at working voltage.

Stuff a sine wave into the front, and scope the signal stage by stage. You ought to see where it craps out. I would be prepared to replace those small ecaps.