Peavey TNT-100 humming

[gmessler]

Hello, I have a Peavey TNT-100 that I purchased in 1982, yep and oldie, I just pulled it up from the crawlspace where it's been resting for the past 12 or so years. When I put it away it was working fine.

Now when I turn it on it has a loud nasty hum with nothing plugged in to it. Gain knobs make no difference in the sound level either.

I dug around and found two different schematics for it. One has IC's on the schematic the other does not. My unit has IC's on the board.

I disconnected the speaker to try to troubleshoot and found that one of the output transistors gets hot as an iron and the other one stays cool. I metered the outputs of the rectifier diodes and I get ~16v on two and ~21v on the other two. I figure this difference is because of the problem.

I could use some help on this guys. I've had electronics training, WAY back when this unit was new. Problem is I've forgotten a lot. I believe with some help I can get this animal working again. I don't have a scope but have a meter and plenty of other tools.

None of the caps appear to be blown.

Any recommendations on where to start? I sure could use some advice here. I have an account on Mouser so getting parts shouldn't be a problem.

Thanks

Greg

[Chuck H]

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.

[52 Bill]

Welcome to the place. Whenever someone posts that they have a symptom of a loud hum that is not changed by any of the controls, it usually means one of two things. Either the power amp has a problem and it is putting a dc voltage across the speaker terminals or there is a loss of power supply filtration which is causing an ac voltage hum to appear on the dc voltage buss.

The fact that you have one output transistor getting hot, would lead me to suspect a problem in the power amp. As Chuck suggested, you should run some tests before powering up the amp again, and for certain do not connect the speaker to the amp until you know that there is no dc voltage on the output. You might look up and build yourself a light bulb limiter.

I would start by testing the two output transistors and the driver transistors in the power amp with a multimeter.

[Chuck H]

Good call on NOT plugging in a speaker yet. I always forget to suggest that for SS amps since I don't do SS very much. I did once try to burn up a V30 and G12H in my 2x12 though while learning this important piece of info.

[Leo_Gnardo]

Common problem in old Peaveys is the filter cap leads come unstuck - as in broken loose - from the PC board. And as Chuck said, in an amp that age, one or more could be shot anyway so it's a slam dunk replace 'em. Any low voltage supplies? plus n minus 15, like that, a bad cap, open dropping resistor, bad regulator or zener, will get ya hummin' too. And as bill says, a meter check of the output transistors is also worth a shot. For all I used to laugh at the TNT 100, it's not a bad amp at all. Of course, when it's working.

[gmessler]

Thanks guys!!!

I'll start with ordering up the replacing the caps. Then while putting them in I'll meter the output transistors.

Now for the waiting game. Mouser is pretty quick though.

Kind of thinking I should just go ahead and order up a pair of output transistors while I'm at it. $10 worth of insurance.

Thanks again guys.

[Enzo]

Which board do you have? Did you buy it new then or used? Does your board have a large number on it: 99002005 TNT? (Number is solder covered digits as part of the board.) Or is there a small transformer mounted right on the circuit board?

[The Dude]

It's not really all that difficult to tell if it's caps or outputs. Meter the speaker leads and see if the hum is DC or AC voltage. If it's DC, you likely have amplifier problems. If it's AC, you have power supply problems. Why guess?

[gmessler]

Which board do you have? Did you buy it new then or used? Does your board have a large number on it: 99002005 TNT? (Number is solder covered digits as part of the board.) Or is there a small transformer mounted right on the circuit board?

Hi Enzo, I bought it new in 1982. The board is a 99002005 TNT and there is no transformer on the board. There's also another number on the board 022279

It's not really all that difficult to tell if it's caps or outputs. Meter the speaker leads and see if the hum is DC or AC voltage. If it's DC, you likely have amplifier problems. If it's AC, you have power supply problems. Why guess?

Dude, I'll hook it back up and meter the output. See if it's AC or DC. (I think there's a joke in there someplace haha) Just don't want to spend a whole lot of time with it ON because of the output heating up so much.


Had my cap order all set on Mouser and accidentally closed my browser. Lost the whole order. RATS!

[gmessler]

I'm also curious if I should pull the IC's and hit them with a shot of corrosion X and re-seat them.

[52 Bill]

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.

Test the output transistors for shorts and/or test to see if there is ac or dc on the speaker terminals. Once you have the amp up and running, you can start doing maintenance updates.

[gmessler]

Hi Guys,

My package arrived from Mouser today. WOO HOO!!
I purchased all new caps and two MJ15015G power transistors ( I think the correct replacement).

First up was to pull the power transistors and meter them.
Here's what I came up with

transistor A B new
EB 2.6k 2.6M infinite
BE 2.4k .74M 2.1M
EC 1.5o 3.6M infinite
CE 1.6o infinite 7.3M
CB 2.6k infinite infinite
BC 2.4k 1.4M 1.8M

Looks to me like they're both dead. Next question though, what caused this? bad caps?
Should I be checking the other transistors on the board as well?

I have a bad feeling I'm going to be placing another Mouser order.

Let me know your thoughts.

[J M Fahey]

Hi Guys,

My package arrived from Mouser today. WOO HOO!!
I purchased all new caps and two MJ15015G power transistors ( I think the correct replacement).

First up was to pull the power transistors and meter them.
Here's what I came up with

transistor A B new
EB 2.6k 2.6M infinite
BE 2.4k .74M 2.1M
EC 1.5o 3.6M infinite
CE 1.6o infinite 7.3M
CB 2.6k infinite infinite
BC 2.4k 1.4M 1.8M

Looks to me like they're both dead. Next question though, what caused this? bad caps?
Should I be checking the other transistors on the board as well?

I have a bad feeling I'm going to be placing another Mouser order.

Let me know your thoughts.

Don't use the resistance scale but the diode check one.
And when you state, say, BE or EB, also state which gets the red probe and which the black one.
Such as:
"red first, black second, EB infinite/over range/open ; BE 0.65 (V) or 650 (mV)" and so on.
It's 3 pair measurements per transistor.

[Leo_Gnardo]

First up was to pull the power transistors and meter them. Here's what I came up with

transistor A B new
EB 2.6k 2.6M infinite
BE 2.4k .74M 2.1M
EC 1.5o 3.6M infinite
CE 1.6o infinite 7.3M
CB 2.6k infinite infinite
BC 2.4k 1.4M 1.8M

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.

[gmessler]

Don't use the resistance scale but the diode check one.
And when you state, say, BE or EB, also state which gets the red probe and which the black one.
Such as:
"red first, black second, EB infinite/over range/open ; BE 0.65 (V) or 650 (mV)" and so on.
It's 3 pair measurements per transistor.

Sorry, my bad, Forgot to mention in that table the first letter gets the positive lead.

Re-did the test using the diode function and here's what I got.

in EB the E gets the positive lead and so on.

transistor A B new
EB infinite 1.2v infinite
BE .46v .42v .5v
EC infinite .001v infinite
CE infinite .001v infinite
CB infinite 1.2v infinite
BC .45v .42v .5v

Just a note.....A and B may or may not be the same as in the first table.

Should I test the other transistors as well?

I used the following link to get my replacement transistor MJ15015G. And read the values off of each cap to get replacements.

Peavey Semiconductor Cross Reference List