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]

Originally Posted by 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.

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]

Originally Posted by mr_bassman

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]

Originally Posted by Twist

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.

Originally Posted by mr_bassman

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.

Originally Posted by mr_bassman

(Sorry, I´m swedish!)

Don't be sorry, I love your meatballs.

[mr_bassman]

Originally Posted by 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.

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.

Originally Posted by 52 Bill

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]

Originally Posted by Enzo

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?

Originally Posted by Enzo

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...

Originally Posted by Enzo

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

[Enzo]

Yes of course measure with power on. It would not be possible to check operation when it is not operating.

If you are getting 30V AC across those caps, I would say replace them for sure. I will assume the rectifier diodes are not shorted or fuses would be blowing.

Without good power supply voltage, all the other readings are meaningless.

Yes, that transistor. Look at it. Three legs on the bottom, the left is the base, the longer center one the collector and the one on the right you left unmarked is the emitter. But look at the top of the transistor - it has a metal tab with a hole through it for mounting.. THAT is the tab. You can verify with a meter if you like, but that tab is also the collector. it is internally connected to the center leg. I find it easier to probe and to explain by measuring at the tab instead of the leg, but either is fine. Ground your meter and measure at each point.

The schematic tells you there should be +30VDC more or less sitting on that tab, the collector. The only AC there should be the signal being amplified. With the unfilteres power you have, there could easily be that much signal there, but your +30v is also wiggling that much. SO we are back to fixing the powr supply before doing anything else.

[mr_bassman]

There is some AC in the transistor, but it´s very irregular. Jumping around 10-30V AC.

So I should replace both the ICs and the transformer on the underside? I have problems finding the specifications on the schematics.

[52 Bill]

What you are referring to as ICs are the main filter capacitors. IC is the brand name (Illinois Capacitor). And yes, if you read 30 volts ac on them, they need to be replaced. They are probably rated something like 2500 uf at 50 volts. Check the printing on the capacitor case, it should be right there. They are polarized and must be installed correctly or they will be damaged. The board may be marked, but for your own safety, note how the original caps are installed, so you can install the new caps correctly.

I would wait on replacing the transformer. Replace the filter caps first, then see if the transformer still hums.

[mr_bassman]

I am having big problems finding any axis polarized capacitors on 50v 1000uf without having to 100 of them. I read about a guy who replaced his with a 4700uF, and it fixed his buzzing. This was however a radio, but should have the same effect, right?

[52 Bill]

If the original caps were 1000uf, you could probably replace them with 2200uf or 4700uf as long as the voltage rating is 50 volts, and as long as they will fit in the space that is available. If the value is too large, you may find that the fuse will blow when you turn the amp on.

[mr_bassman]

So this "ripple" I read about is not that important? I have found a type MV-AX that seems fine.

http://www.elfa.se/elfa-bin/dyndok.p...htm?_67_230_76

This won´t set my house on fire, will it?

[52 Bill]

Originally Posted by mr_bassman

So this "ripple" I read about is not that important? I have found a type MV-AX that seems fine.

Ripple refers to the amount of ac that is left on a dc voltage rail after filtering. Your problem is that you have too much ripple, as the caps are not filtering anymore.

I tried to follow your link to the Elfa website, but it keeps crashing my browser.

Originally Posted by mr_bassman

This won´t set my house on fire, will it?

Is the amp getting hot now? Anything can set your house on fire, but as long as you follow normal saftey precautions, I'm sure you'll be fine.

[mr_bassman]

Originally Posted by 52 Bill

Ripple refers to the amount of ac that is left on a dc voltage rail after filtering. Your problem is that you have too much ripple, as the caps are not filtering anymore.

This one has a maximum of 1,75 Arms ripple. They are really cheap so I´ll just give it a shot! I don´t know if the amp is getting overheated, that part was just a joke. I´ll se when I get these new caps in.

http://www.elfa.se/elfa-bin/dyndok.p...48.htm&lang=en
Try that link, that shoule be in english. If not, here is just a picture. It looks a lot smaller and is not axis polarized, but it should work.
http://www.elfa.se/images/lowres/l16835.jpg

[Robert M. Martinelli]

Hi Bassman,
the case of your final transistors should be of the TO-220 type. This case has the center pin shorted with the upper lug, where the mounting screw is. The screw is insulated from the case, but the lug is in contact with the center pin of the transistor, which is the collector. The pin arrangement should be, from left to right, B (base) C (collector) E (emitter). Connect the neg of your multimeter to ground with an alligator clip, or be sure to have it contacting the ground, set the meter to read DCV, then measure the voltage with the positive of the meter on the center pin ( or the tab/lug, not touching the screw ) of the final transistors, and you' ll see a positive voltage on one of them and a negative voltage on the other. these are the voltages supplied by the Power supply to the final stage of the amp, they should be symmetrical ( e.g. +38/-38 VDC or +40/-40 VDC ). Switch the meter to ACV and check the same points for AC voltage, still keeping the negative of the meter to GND; as Enzo stated, there should be no AC voltage on the transistors' center pins or tabs/lugs(collectors), if there is any, you have faulty filtering caps on the power supply, they' re no longer able to filter ripple voltage and this ripple voltage is amplified as hum. Replace caps with ones having at least the same specs ( capacitance and working voltage ). You can also try to put in caps with a higher capacitance to better filter the noise, this action also improves the amp' s response to transients ( think about "slap bass" techniques ) but be aware that doing so you will possibly "mistreat" the rectifier section, as the initial current peak through the rectifier will be higher.....this would require the rectifier section to be upgraded as well, but first it' s better to keep our feet on the ground and think about fixing the amp...

Hope this helps

Best regards

Bob