TNT-100 Repair Problem 2

[mr_bassman]

I found this baby, a Peavey TNT-100, lying on a pile of junk near our rehearsal room, and decided to try it out and see if it worked. To my surprise, it sure did. The only problem seems to be that it puts out a very loud hum no matter what volume I play on.

Now, I´ve only connected a standard hi-fi speaker totally unshielded. However, this should not produce this extreme hum.

As you can see on these pictures, it seems to be burnt or something like that:

http://img526.imageshack.us/my.php?i...sm10066th7.jpg
http://img150.imageshack.us/img150/9800/ssm10067fv0.jpg
http://img150.imageshack.us/img150/5805/ssm10068nl8.jpg
http://img518.imageshack.us/img518/145/ssm10069kh1.jpg

I´ve read that the power transistors sometimes can be the problem behind humming. What do you think?

[drewl]

One or more of the filter caps is either not making a good connection to the board or is bad.
Common problem.

[mr_bassman]

I recently discovered that the sound also comes from the part on the underside of the amp (http://img518.imageshack.us/img518/145/ssm10069kh1.jpg).

[52 Bill]

I'd agree with drewl, that the hum is probably a problem with your filter caps. These are the two large grey cylinders located on the right side of your photos. Either they have come loose or they have dried out and gone bad.

The entire amp is covered with soot, but I don't see any real burn marks on the inside of the chassis. I can't be sure from just looking at it, but the power transformer has been rewired, and may have been replaced at one time (the part on the underside of the amp). Maybe that's where the fire was.

[Twist]

If your familiar with a volt meter try reading the voltage across each of the mail filter caps. They are the two large caps that are side by side with "IC" printed on the side. I believe they should be around +40 and -40 DC volts. If there is a difference between these you'll hear hum in the output and they will need replacing. Also unplug the speaker and see if you have any DC voltage on the speaker jack. A bad filter cap can also produce an AC voltage on the speaker jack too. Check it out and let us know.

[mr_bassman]

If your familiar with a volt meter try reading the voltage across each of the mail filter caps. They are the two large caps that are side by side with "IC" printed on the side. I believe they should be around +40 and -40 DC volts. If there is a difference between these you'll hear hum in the output and they will need replacing. Also unplug the speaker and see if you have any DC voltage on the speaker jack. A bad filter cap can also produce an AC voltage on the speaker jack too. Check it out and let us know.

the ICs were both close to 37 (DC V I think). The speaker output had 8.9 volts in it, no matter what volume. What this means, I don´t know.

YouTube clip of the hum; http://www.youtube.com/watch?v=KfhJG4xE_U0

[52 Bill]

the ICs were both close to 37 (DC V I think). The speaker output had 8.9 volts in it, no matter what volume. What this means, I don´t know.

It means that you have a problem with the power amp. If you know how, test each of the two output transistors and check the drivers as well.

I just watched the video, and I didn't hear a hum that I would expect to hear with 9 volts on the speaker output. Are you talking about the high pitched metallic buzzing?

[Twist]

Was it 8.9 volts AC or DC? Did you measure it with the speaker unplugged?

[mr_bassman]

I´ve located both of the output transistors, but I don´t how to test them. What are the drivers? (Sorry, I´m swedish!)

Yes, the somewhat high pitched buzz.

[mr_bassman]

Was it 8.9 volts AC or DC? Did you measure it with the speaker unplugged?

8.9 AC. It was measured without any speaker connected.

[52 Bill]

I don't think that you are reading the voltage on the speaker output correctly. If there was 9 volts across the speaker, you wouldn't hear the guitar signal. Maybe it's 9 millivolts that you're reading there.

Yes, the somewhat high pitched buzz.

This buzz sounds mechanical to me. Try tapping on the power transformer. Does the buzz change? It may have come loose from the chassis or the assembly bolts may have loosened up. Try tightening up all of the screws, etc. Worst case, is it will need to be replaced.

(Sorry, I´m swedish!)

Don't be sorry, I love your meatballs.

[mr_bassman]

I don't think that you are reading the voltage on the speaker output correctly. If there was 9 volts across the speaker, you wouldn't hear the guitar signal. Maybe it's 9 millivolts that you're reading there.

I have the multimeter set on 200 V AC and it shows "08.9", which I read as 8.9 or maybe , gulp, 89 V AC. I can hear the guitar signal, especially if I turn it up.

This buzz sounds mechanical to me. Try tapping on the power transformer. Does the buzz change? It may have come loose from the chassis or the assembly bolts may have loosened up. Try tightening up all of the screws, etc. Worst case, is it will need to be replaced.

I found one of the screws way too untight and after I fixed that; the buzzing coming from the power transformer almost stopped, but I can still hear it if I put my ear to the chassi.

I have tried tapping and jerking it while plugged in to a speaker and did change the sound. Could there be a problem within the power transformer that can be fixed? If not; how do I know what to buy? It doesn´t say much on the transformer itself.

Here are the schematics:
http://jrem.dyndns.org/tnt_100_schematic.pdf

[Enzo]

Power transformers sometimes vibrate and buzz. It may or may not get loud enough to be annoying. But that mechanical noise is a totally separate issue from anything coming out the speaker. it happens at similar frequencies because that is the frequency the system operates on.

Go back to those main filter caps - the gray IC ones on the end. Set your meter to AC volts and connect it to the ends of each cap. How many AC volts are across each cap. I don't want to find more than 1 or 2, preferably less. If one measures several volts or more it is either bad or unsoldered.

Those voltages are the power rails for the output stage. If they measure the same as each other and the AC there is low - the AC there is called ripple - then we proceed into the circuit.

The schematic you linked to is not for your amp. Notice the layout of the parts is all different. Notice how your board has only small transistors while the schematic has a bunch of integrated circuit chips.

You have the original TNT, not the 1979 version. You can contact parts at Peavey and they will send it to you. I will be happy to post it here this evening when I get to work. And someone else here may have it handy.

The output transistors are driven by that transformer on the board next to the IC caps. There are no driver transistors. The primary of that transformer is driven by that TO220 transistor to the left of the transformer. Measure AC volts on the tab of that transistor. SHould not be any. If there is some there, then that is the hum being amplified into the speaker.

When we get the right schematic up we can then refer to specific parts.

[mr_bassman]

Go back to those main filter caps - the gray IC ones on the end. Set your meter to AC volts and connect it to the ends of each cap. How many AC volts are across each cap. I don't want to find more than 1 or 2, preferably less. If one measures several volts or more it is either bad or unsoldered.

If this is the case, my ICs sure are busted. One read 37 V AC and the other 30.7 V AC. Was this supposed to be measured with the power on?

The schematic you linked to is not for your amp. Notice the layout of the parts is all different. Notice how your board has only small transistors while the schematic has a bunch of integrated circuit chips.

You have the original TNT, not the 1979 version. You can contact parts at Peavey and they will send it to you. I will be happy to post it here this evening when I get to work. And someone else here may have it handy.

Man, am I glad to hear that! I´ve spent quite some time trying to figuring out how to read the schematics...

The output transistors are driven by that transformer on the board next to the IC caps. There are no driver transistors. The primary of that transformer is driven by that TO220 transistor to the left of the transformer. Measure AC volts on the tab of that transistor. SHould not be any. If there is some there, then that is the hum being amplified into the speaker.

I measured what I think is the TO220 transistor like this:
http://img206.imageshack.us/my.php?i...0067fv0sc7.jpg

It reads 29.5 AC V when I measure the left and middle pins. I´m not sure if this is what you mean by measuring the tab of the transistor.

[mr_bassman]

Correct schematics found!

http://www.peavey.com/media/pdf/manuals/tnt100.pdf