Peavey TNT-100 humming

[gmessler]

The C-E measurement of 1.5-1.6 ohms is the giveaway: practically a short. I bet that's the one that was heating up.

Juan's suggestion of mentioning red probe black probe is good, but I do have meters where red is the -, that is negative probe, with respect to black.

With the output transistors removed, it's a good idea to measure the drive transistors, make sure there's nothing funny going on there.

Good luck with your TNT. Hope all else goes smoothly.

Thanks Leo, Wish I'd have marked them before removing them. Don't know which is which to see if it was the one that was hot.

My meter is a Fluke 87, black marked as Com. I'm assuming neg.

I tried to measure the drive transistors while still on board. Got strange readings. Think I'm going to have to pull them.
Kicking myself for not ordering those up also. For just a couple bucks I'd have them here now if there is a problem.


Thanks everyone for your advice!!

[Leo_Gnardo]

Wish I'd have marked them before removing them. Don't know which is which to see if it was the one that was hot.

You can pretty well count on it.

My meter is a Fluke 87, black marked as Com. I'm assuming neg.

You can always check with another meter.

I tried to measure the drive transistors while still on board. Got strange readings. Think I'm going to have to pull them. Kicking myself for not ordering those up also. For just a couple bucks I'd have them here now if there is a problem.

Not unusual to measure unexpected resistances/junctions in-circuit. There's probably a couple of current-limiting transistors too that should be checked. Some diodes & resistors besides. Any of these could be in the line of fire when output transistors go bad. What a hassle. That's why I avoid solid snake amps as much as possible.

Yes I'm lazy. "I stand on the shoulders of giants, and they're getting tired of it."

[gmessler]

Cleaning the sockets is a good idea, but before you get ahead of yourself, fix the original problem. Changing caps or cleaning things before the amp is fixed may add to the problems making the repair harder if not impossible.

Will do Mr. Bill.

This is complicated enough for me. Don't need to add another issue to the mix.

[gmessler]

Even though no caps appear to be bad they could well be somewhat dried up inside. Just replace all the electrolytic caps in the amp. What else could have changed with no influence but time? I wouldn't fire it up again until this is done. A power supply failure could take out other, harder to find components.

Okay, replaced all the caps. Funny thing the 2200's were a lot smaller than the originals. Same voltage and capacitance. Is that normal?

Also I metered CR9, 10 and 11 . They all seem fine.

R51-53 all measure .6 ohms each. Is that because of the meters range?

Metering Q4,5,6,and 7 I get strange results on board. Should I bother pulling them and metering them?

[Enzo]

Caps are a lot smaller these days than they were back then.

When I find a shorted transistor I immediately bend one of its legs over flat as an indicator it's a bad one.

[TomCarlos]

Greg.... Sounds like you are making progress. I too have a TNT100. I have a 1979 schematic... See attached. Not sure if this schematic your amp- but here it is anyway.

As for testing transistors, I found the "Peak Atlas DCA Semiconductor Component Analyser Tester." This is a slick tool, handheld gizmo for around $100. You simple attach wires to 3 leads of a transistor- does matter which go to which. The device figures it out for you. You then get a small display showing the type "PNP" or "NPN", which wire is what leg (E,B,C), and the gain of the device. Their is another model to this analyzer that has a USB output and you get a video interface to a PC- very nice.

[gmessler]

Hey Enzo, I was reading another thread on a Peavey TNT-100 repair and you had suggested using a 100w light bulb for current limiting for testing purposes.

I think I'll give that a try instead of pulling the drive transistors or current limiters.

[gmessler]

Greg.... Sounds like you are making progress. I too have a TNT100. I have a 1979 schematic... See attached. Not sure if this schematic your amp- but here it is anyway.

As for testing transistors, I found the "Peak Atlas DCA Semiconductor Component Analyser Tester." This is a slick tool, handheld gizmo for around $100. You simple attach wires to 3 leads of a transistor- does matter which go to which. The device figures it out for you. You then get a small display showing the type "PNP" or "NPN", which wire is what leg (E,B,C), and the gain of the device. Their is another model to this analyzer that has a USB output and you get a video interface to a PC- very nice.

Thanks Tom. This forum has been a big help! That's the Schematic that I have. It's pretty handy having the circuit board in the .pdf.

I took a look at that tool, Pretty cool. Money's a little tight so probably won't be able to get it. Plus I don't do much for electronic work anymore so it would be just a "toy". I do play around with the Arduino stuff though.

Keeping my fingers crossed that I can fix this thing. Kind of weirdly sentimental about it.

[TomCarlos]

I used to have an old Heathkit Transistor Checker - very clumsy device. I then bought a Heathkit gizmo that tried to simulate the functionality of a curve tracer. Again, clumsy device. The Atlas DCA is cool. And if you have another $75, you get the DCA Pro version that has the PC interface. It gives you lots of visuals on the device. This device also claims to work

If you need a schematic for a Current Limiter, here is one that I built. There are YouTube videos that show you how this thing works. Turn on, light shines, then dims (assuming your device under test is ok). Just make sure you buy extra incandescent bulbs before they are extinct.

[gmessler]

I used to have an old Heathkit Transistor Checker - very clumsy device. I then bought a Heathkit gizmo that tried to simulate the functionality of a curve tracer. Again, clumsy device. The Atlas DCA is cool. And if you have another $75, you get the DCA Pro version that has the PC interface. It gives you lots of visuals on the device. This device also claims to work

If you need a schematic for a Current Limiter, here is one that I built. There are YouTube videos that show you how this thing works. Turn on, light shines, then dims (assuming your device under test is ok). Just make sure you buy extra incandescent bulbs before they are extinct.

Back in the late 70's I actually had an old Heathkit Oscilloscope. Don't remember whatever happened to it. I remember it was even old at that time. I think I just dated myself. Haha. I was just thinking about the whole incandescent thing. I remembered a month or so ago Menards had a jumbo sale on them.
They were VERY cheap. Hope they still have them.

[gmessler]

Well, all I could muster up was a 75W bulb.

I think I still have an issue because it lights up about half bright.

Back to the drawing board!

Guess I'll start by pulling the drives and current limiting trans'.

[gmessler]

Howdy!

Well, I dug up a 100w bulb. It goes on about half bright.

I metered the output and it looks like I'm getting DC. What would that be from?

[Leo_Gnardo]

I metered the output and it looks like I'm getting DC.

How much DC?

[gmessler]

IDIOT ALERT

Hi Leo,

Well....This is really weird.... NOT!!! IDIOT ALERT!!!

Here's how the story goes. When the 100w bulb lit up half way I figured it's not drawing too much current so I hooked the speaker back up.
It was still making that strange hum/buzz only quieter so I shut off and disconnected it. My guess is because the bulb was limiting the current so it couldn't run crazy and cause damage.

So I metered the output, Didn't pay too much attention to the values and decided to post. Bad idea because now I look even more like an idiot! HAHA

When I went back to get a value for you I pressed down on the probes and because I didn't have the heat sink screwed to the case the circuit board did a little jump and the lamp went out. That's when it hit me. The board was probably shorting against the case. Thankfully it didn't cause too much problem.

I set the board up on an angle so as not to short anything out. The lamp was still out so I re-connected the speaker. Nothing coming out so I turned up the volume and got that familiar (amp working) buzz crackle. So I plugged in my bass to see if it would work and everything works fine!!

I'm guessing that the original problem was probably the caps that caused one of the power transistors to fail. Changing them both out has fixed the problem. But doing a boneheaded thing like leaving the solder side of the circuit board against the aluminum case made me believe there was still a problem.

I'd like to hand out a great big THANK YOU!!! to everyone who helped me with this!!

THANKS GUYS!

[gmessler]

Well, I spoke too soon.

I put things back together and got nothing. Figured it was the speaker connection. It was. But now I get sound but it's real distorted. I'm thinking that the pots all need a good cleaning. Whenever I move any of them I get a lot of popping and crackling.
After playing around for a little while (2-3 minutes) I smelled hot electronics and felt the outputs. They were cooking hot.

I'll get a little contact cleaner going on this thing and see if that helps.

UGH!