Old Peavey TNT 100 SS Series Has Low Distorted Output

[jcollins]

I've got an old Peavey TNT 100 SS Series Bass Combo on my bench right now.

The output is low and it has a nasty splattery distortion. I've called Peavey
three times now requesting a Schematic and for some reason it hasn't come
through yet.

There is some kind of transformer on the board. The last time I called, I talked to one of the repair guys. He said from my description, it wouldn't be bad output transistors. If that was the case, he said, the unit would blow fuses or hum badly. He said to check to make sure the transformer leads were making good contact to the board and that the solder joints were good. He also suggested I go ahead and solder the output transistors to their sockets, which I did.

It still sounds the same.

My query. I'll try Peavey once more for the schematic (in the past they've been very responsive). Does anyone have a copy they could send me?

Any ideas on what could be wrong with this sucker?

Thanks,

Jack

[Enzo]

I posted this schematic at www.ampix.org in the Enzo gallery. Yes, the version with the transformer. Ther are several versions of the TNT.

Look, this is a solid state amp with a distorted output. Do the standard things.

1. check the power supply. Are both positive and negative power rails up to voltage and clean? I don't care if the schematic says 39 and the amp has 36, or 40. What I care bout is are they free of ripple and more or less the same voltage as each other. Expect 35-40 volts. Without proper power, the amp won't work right.

2. preamp power. If the main power is OK, the positive also serves the preamp through some resistors and decoupling caps. You got the +25 through the preamp stages?

3. apply a clean test signal and scope the output. We know it is weak and distorted, but what does it look like. The distortion is a powerful clue. Missing half the wave? Missing the tops on both sides? something else?

4. What does the signal look like stage by stage?

5. There aer three windings on the phase splitter transformer. Are all intact? (continuity) Are any of the little wires broken off? It may be sitting there, but a lead could have broken off but still be soldered to the board.

Here is a trick. Look at the two output transistors. Measure resistance from emitter to base. There is a 0.68 ohm and a 15 ohm resistor and the transformer winding all in series between those points. So a reading of about 18-20 ohms means all those things are intact.

6. Check ALL the cement power resistors - open ones will kill the sound.

7. Pull the two power transistors and check them with a meter for junction drops - the diode test on your meter.

8. Divide and conquer - is the signal OK at the collector of the driver transistor? That is the tab on the TO220 type 430.

[jcollins]

Wow Enzo thanks for that. And for the schematic.

I tested the rails and the positive preamp power,
that all looked good. I scoped the output and found the bottom half of the wave has a funny spike that goes right through the bottom tip. Also the bottom half of the wave looks a bit flatter than the top.

Tracing signal through the amp. Things definitely get strange at the SPS953 transistor that feeds the TO220. Signal coming from the master looks OK except it has a downward angle from left to right. Signal at the collector is has noise and is very saw wave shaped. Voltages around the transistor are exactly what appears in the schematic though.

Previously I had pulled the splitter transformer and tested the continuity of the windings, none of them were open. None of the cement resistors read open.

The signal at the collector of the driver transistor looked horrible, only one half of the wave and it is very rounded. Lots of little spikes, between the
waves.

My inclination is to replace the SPS953 and the TO220.

What's the best source for those items? Or, should I be looking elsewhere's?

[Enzo]

REally, transistors are SOOOOO amenable to subs, most anything will work, and work well. But that 430 TO220 is just a TIP29C. They also list it as a TIP31C. I recommend the 31 for its higher current rating and better dissipation. And lacking those, what TO220 NPNs you got in your drawer?

These transistor amps pretty much can use any transistor, it is nothing like shopping for 12AX7s for tone in a tube amp.

I'd probably replace the TO220 and see, but the small transistor costs maybe 25 cents tops, so why not both of them, indeed? Parts are cheaper than labor.

SPS953/2N3391 shows factory alternates as MPSA18, MPS8097, 2N6520, 2N6539. And I stock both the MPS types, so I look no further. Those are low noise types. But really, any general purpose NPN signal transistor will work. The circuit is running off a 24v rail, so I'd select a 40v or higher transistor. But beyond that, what do you have in your drawer? I might use 2N5210 if the drawer were closer.

Send me an email and ask for the Peavey transistor cross reference It's about 20 pages, and "decodes" a lot of house numbers.

And as you know, PV will sell you most any part. But we can get transistors on our own, yes?

That driver does tend to be a weak link in these amps.

[jcollins]

Well Rat Shack had a TIP31 so I used one of those. I replaced the small transistor with a 2N4401.

After that the amp still has the same spattery low powered sound.

With a clean signal injected, I still get that nasty saw wave between the
small transistors collector and the base of the TO220 and the same weird
signal coming out the TO220.

The other clue I have is that while the signal between the master volume and
the base of the small transistor is fairly clean, it has a pronounced downward
left to right slope. I pulled the 2 uF Cap and the .22 Cap on either side of
the master volume to check for leakage. They both showed over 2M of resistance.

Aside from replacing all the components around those two transistors, I'm not sure how to approach this problem.

Sure do appreciate all the help so far, Enzo.

[Enzo]

That 2M of resistance doesn't tell you if the cap leaks or is dried out. I'd be willing to bet all those little 2uf/35v caps are dried out.

If something funny goes into the TO220 base, you can count on something funny coming out the collector. So what does the signal look like at the small xstr base? At the top of teh master? At the tone controls?

The schematic has DC voltages all over it, how do yours stack up?

1-2-3-4-5, I see five 2/35v caps. Any more? They would be my next suspects. But really, apply sine wave at front and where down the circuit does the waveform screw up?

[jcollins]

OK. I replaced all the electrolytics around the two transistors in question and
the .22 cap. I actually used 4.7 uF instead of 2 because that's what I had.

Now I've got a nice clean signal at the TO220's collector...but, now I'm seeing nothing on the secondary of the phase splitter.

Also, now I don't hear any signal coming through the speaker.

Not sure what's going on. I might have screwed something up. I checked for
loose wires. Measured voltages on the components on the secondary side of the transformer.

The only thing funny I see is that there is no voltage drop across the primary
part of that transformer. I get 32 V on each side.

I'm going to work on this tomorrow night some more but, if you have any ideas I'd be happy to hear them.