Peavey TNT-100 Repair Problems

[jrem]

There needs to be a path for current through the transistor or it won't work right.

right, current, not voltage, forgot that.

I hate to put in new transistors in Q6 and Q7 just to cook them (even though I bought a few of each just in case!) . . . can I do something to soak the current off of Q3 to see if it's still alive? Like a 1 meg resistor from the collector to ground, then look at the signal off the collector?

[Enzo]

What did you get for Q6,7? the 430 is a TIP29C or TIP31C. That is NPN. Make sure to get the C version. A and B are lower voltage parts. TIP29C is a 1 amp part, while the TIP31C is a 3 amp part. Stick with the TIP31C. The 6 amp TIP41C is another option. Or even the common 2SC3298. I'd use that since I stock a lot of them

The 431 is the PNP. It is a TIP30C or TIP32C. Again, I'd stay with the TIP32C of those two, again%o short than open.

REmember too that if there is NO speaker connected, 39v of DC can appear on the output without burning things up. Connect a load to that and stuff starts to cook.

Instead of blowing up lots of parts, try this. Take a 100 watt light bulb and a socket for it - the cheap white porcelain type is fine - run a couple wires to it adn put clips on the ends of the wires. be mindful of exposed wires and screw terminals etc. Now remove the main fuse from the amp, and clip this bulb across the fuse holder. The bulb now serves as fuse.

This is the poor man's current limiter. Now when power is applied, if there is a short or something in teh amp that would have drawn a lot of juice, the bulb will light up bright, but the amp will see little voltage. if however there is not much excess current, the bulb will glow dimly. The bright bulb is a sign to turn it off quickly, and it also is a sign you are saving the amp circuitry.

Of course the official way is to use a variac to slowly bring up the mains voltage applied to the amp, while monitoring the current the amp draws. If the current starts to rise sharplu and the mains voltage is only up to like 15 volts, turn it off. Again, nothing burns up that way. My variac sits beside me here all day every day.

[Steve Conner]

Now you can see why we just suggested replacing all the silicon in the output stage

Enzo, the diode test function in a DMM uses a constant current source to put 1mA through the device, and has the meter read the voltage across it. So your 9V battery and resistor gizmo should work fine, for folks who don't have the diode test function. Having said that, you can pick up a DMM that does have it for under 10 bucks these days. I use it all the time for checking that I wired diodes, transistors and LEDs the right way round, and I could never be bothered hooking up a battery and resistor.

On the subject of diodes, I think you could improvise a replacement for the dual diode assembly by soldering two 1N4002s (or similar plain vanilla kind of diode) in series, and blobbing them onto the heatsink with heatsink goop.

As for whether you need to lift one end of a diode to test it: It depends where it is in the circuit. Sometimes other components can leak enough current round it to make it look like it works when it's actually blown open circuit, or to make it look shorted in the reverse direction. But usually a junction reads about 0.5 to 1V in the forward direction and "More than the meter can show" in reverse. If I see anything else, I usually pull the device out of circuit and recheck it to make sure.

The variac and light bulb limiter are both excellent tools.

[jrem]

What did you get for Q6,7?

TIP31C and TIP32C.

Instead of blowing up lots of parts, try this. Take a 100 watt light bulb <snip> My variac sits beside me here all day every day.

yes, I was discussing this with my bud Neal the other day (he frequents here and is the one who told me to post, he's a fan of yours, too!). Replacing the fuse is a new option I didn't think of, I like that idea, less mess to wire up than the G Weber concept. I think I could get my hands on a variac if necessary.

Thanks for the tips, I'll do the light bulb trick and see what things look like.

[Rcubed]

> but in the absence of R42, ther is no path for current through Q3, so who knows what voltage would appear there.

If R42 is out the Q3 voltage doesn't look wrong as such.

I'd wait until R42 is back in and Q6 and Q7 have been replaced.

*** make sure you put R49 and R50 back in. If you try to run the amp with these pulled out the bias current through the output stage can be quite high and it could create more problems. While they are out, it wouldn't hurt to check that the values of R49 and R50 are still OK.

[jrem]

Now you can see why we just suggested replacing all the silicon in the output stage

yeah, looks like I'm getting close to doing that, aren't I?

<snip> So your 9V battery and resistor gizmo should work fine, for folks who don't have the diode test function.

I set that up and tested the diodes, it looks like the dual diode is okay (battery v=8.92, diode forward 8.90, reverse 1.239). I also checked the TIP31C that I previously put in there, it checks the same as a new one, so maybe it's okay.

Having said that, you can pick up a DMM that does have it for under 10 bucks these days.

I have my eye on a new one that does it all, autoranging, $80. But I also have my eye on a Yamaha DX-7, so we'll see which one I get first!

The variac and light bulb limiter are both excellent tools.

got it, thanks.

If R42 is out the Q3 voltage doesn't look wrong as such.

I'd wait until R42 is back in and Q6 and Q7 have been replaced.

okay, will do

*** make sure you put R49 and R50 back in. If you try to run the amp with these pulled out the bias current through the output stage can be quite high and it could create more problems. While they are out, it wouldn't hurt to check that the values of R49 and R50 are still OK.

okay, they're being replaced with FP's anyways.

If I get to this tonight or tomorrow then I'll post, else it might be next weekend until I get to it. Either way I'll post sometime.

Thanks, guys.

[Enzo]

(battery v=8.92, diode forward 8.90, reverse 1.239).

Actually the 1.2 is forward, and the full 9v is reverse. No current flows in reverse, so ther will be no voltage drop. Forward, current flows but has to overcome two junction potentials in series. Thus there is twice .6 volts dropped in that direction.

[jrem]

Peavey TNT-100 Repair Problems

okay, I'm finally back at it after the holidays, and this amp is till on my bench. I hooked up a light bulb in series on the mains (alligator clips on the fuse holder).

All balast resistors are replaced.

With R42, R49, R50, Q4 and Q5 lifted the bulb is dim.

Same with just R42, Q4, and Q5 lifted.

Solder in R42 and the bulb is bright.

So my question is where is the current coming from?

Thanks . . . and happy new years!

[Enzo]

The current comes from both positive and negative sides of the power output stage being turned on at the same time. So positive and negative rails try to fight it out.

You need to go back and make sure none of the little 47 ohm resistors are open and that none of the transistors are shorted - Q7 for example. ANd out of circuit - remember that E-C in Q7 is the same as C-B on Q9 since they are wired together.

[jrem]

<snip> remember that E-C in Q7 is the same as C-B on Q9 since they are wired together.

oh, you mean Q7 & Q8, so if Q7 is shorted E-C then Q8 is always 'on'? Or if R50 is shorted open, then the base of Q8 is pulled up?

[Enzo]

Did we have a disagreement earlier on whuch xstrs was which? I was just pointing out that in-circuit tests can be confused when two transistors are wired together. In my drawing, Q7 driving Q9 are the bottom pair, and Q6 driving Q8 are the upper pair.

In any case, yes, the things you mention would be the concern.

[jrem]

no disagreements from me, sir.

I have Q7 & Q8 upper, Q6 driving Q9 lower.

I think I see, anyways, an EE slid by my space today and we had a long talk. His take was the power transistor is probably bad, which took out the ballasts, and then shorted out the driver. So maybe the power transistors look okay tested with a nine volt battery, but as soon as you (um, me) start putting current on them, they give up and short out, taking down the driver.

Best to just buy all new silicon and put new darlington pairs in there.

I'll report after I get the right parts. Thanks for your help.

[Enzo]

Careful how you say things. Two xstrs wired together may be darlington pair, but there are amps make with darlington pairs in a single TO3. DOn't confuse anyone with the term. Do no install darlington transistors. I don't think you are confused, but it was possible.

ANy time the ballast resistor for a power xstr is burnt open, it is a pretty good bet the xstr itself is bad. Certainly those predrivers, the 5331, are a weak link here - always suspect when the output stage blows But if the driver is OK, then the predriver usually is too

When the output works but collapses under load, that usually means the driver or predriver was presenting the voltage to the output bus. But they cannot provide the current the load wants. So fir sure check the 5331, but also the driver 6357 (or whatever you have there)

[TomCarlos]

Since this topic addresses the "Dual Diode" ... why is this device mounted on the output transistor heat sink? Does it draw that much current that it needs to be on a heat sink? I could never figure that out...

Tom

[52 Bill]

The dual diode is part of the bias network. It is mounted on the heatsink so that it can sense the temperature rise of the output transistors and then reduce the drive to them, to prevent the amp from overheating.