Peavey TNT100 no output

[Tone Meister]

Early Peavey TNT100 bass amp with no output on the speaker output jack. I did measure ~800 mADC on the output jack before disassembling the amp. Removed both RCA 62792 output transistors from circuit and both test good. Removed EP430 driver transistor from circuit and it tests good as well. No visible signs of trauma and every resistor I've tested tests good, so far.

Tried to apply some of the techniques found in some previous threads here for the same amp, but the schematic I have does not include any component designations (Q4, Q5, etc). That leaves me flying in the blind even more than I usually do.

The amp I have has the transformer soldered directly to the board with a single EP430 driver transistor and no other ICs (regulators?) on the single PCB board. Right now the amp is still partially disassembled in case it's recommended to shotgun all the small signal transistors (except Q1, Q2, and Q3), which I have seen recommended in previous threads.

I have attached the schematic I received from Peavey.

[J M Fahey]

Nothing points yet to bad transistoirs so no point in shotgunning them.
Your symptom is

no output on the speaker output jack

so try to solve that first.
Yes, later *maybe* you find some bad parts.

So a logical sequence would be:

1) check that the speaker actually works
2) that itŽs actually connected to the speaker out rail at the power amp.
Besides your eyes, use the multimeter.
3) that it's also connected to ground.

4) that the power amp gets DC voltage (I guess around +/-35VDC)
5) that the amp amplifies (or not) signal present at its input (touch the net 10K-.22uF at its input with the tip of a screwdriver and check it hums or pops.

6) if it passes sound, go backwards with your finger-signal-injector , such as all transistor bases , then the net 22K-22K present at the input, then a bare plug connected in either jack.

7) somewhere along the path, youŽll lose signal, only then weŽll ask for some voltage measurements and eventually plugging your soldering iron.

Replacing parts is the easy job, the main one is *finding* the problem cause.

Shotgunning does not do that and may very well miss the problem or worse, introduce new ones.

[Enzo]

We don't shotgun all the parts as an initial strategy, we might do that as an expedience if we were having trouble narrowing down a noise or something.


No sound? First things first. Does it thump or pop out the speaker when first turned on? And sitting there running, is there ANY little background sound coming out? A slight hum or hiss maybe?

ANy system needs power, so look for it. You have two output transistors, the case of one should have the +36v, and the emitter resistor of the other should have the -36v. Just close is fine, these are not exact. The tab on the driver transistor should have about +30 on it according to the drawing. In fact the drawing has a bunch of voltages on it, how do yours stack up?

Does the master volume control affect any background sound? Turn the volume way up, and inject a signal at the control. Often, just touching the thing with a screwdriver is enough to make hum. Does that come out the speaker?

Likewise, play some music into the regular input, and use a scope, a signal tracer, or just an AC voltmeter to see if it gets as far as the volume control.

Instead of hauling those poor transistors out of there, if you want to solder something, I'd recommend all those little 2uf 35v electrolytics as likely needing to be changed out.

[Tone Meister]

10-4 Juan. Should have mentioned a few things in my first post. In reference to your points above:

1. Plugged in known good speaker and known good speaker cable to output jack. No sound
2. Yes
3. Yes
4. Yes
5. Yes - When 1K signal is fed to input jack, it can be traced to the Base of the EP430 driver, subject to the MASTER VOLUME control
6.
7.

[Tone Meister]

We don't shotgun all the parts as an initial strategy, we might do that as an expedience if we were having trouble narrowing down a noise or something ...

... Instead of hauling those poor transistors out of there, if you want to solder something, I'd recommend all those little 2uf 35v electrolytics as likely needing to be changed out.

Enzo, I'm not looking for something solder, but I was getting some kulgy readings with those chips in the circuit so I lifted them and now know for sure they are good. Yes, the +36 and -36 voltages are present at the output connections. I can feed a 1K signal at the input and can pick it up at the Base of the EP430 driver. The MASTER VOLUME does affect the signal there, of course.

The driver transistor does NOT have the +30 on it, but I measured 34.5 VDC at the anode of that 50uF 50v electrolytic cap.

[Tone Meister]

I'm getting no continuity on the front winding of that transformer between the anode of the 50/50 cap and the collector of the EP430 chip.

[g1]

So you have a lack of continuity between the + end of that cap and the collector of the 430 transistor.
Is it an open coil or a bad connection?

[Tone Meister]

I carefully touched up the connections on the board , BUT I also went above the board and measured the legs coming right out of the transformer. Definitely an open winding.

[g1]

I recently had the same fault with an oddball Yamaha TA-30 amp. The weird think is, it still passed signal, the output was weak and distorted.
Hopefully, the transformer is still available from Peavey. In my case, the part was extinct. With nothing to lose, I unwound the tape around the transformer windings and found the winding wire broken from it's terminal. A bit of solder and re-tape and done. Very lucky.

[Tone Meister]

Yeah, the transformer is still available from Peavey, but they want $41.26 for it. I just might try to repair this one before ordering one at that price.

Can I get away with using 4.7uF/35v to replace those 2uF/35v Enzo was talking about?

[Tone Meister]

Found a broken lead into the front winding of the transformer and repaired it with a short piece of hookup wire. With 122 VAC wall voltage out of the variac, I now have ~24 VDC at the Base of the driver transistor. Should I take that and run with it or try to find out where the missing +/- 6 volts have gone?

[g1]

Get those voltages sorted out first. Should be 1V at emitter, 1.6V on the base. Check the emitter resistors & connections of that transistor and the one driving it.

[Tone Meister]

1.7V at base and 1.1V at the emitter. Tightened all the circuit board grounds and now have 29.8V on the collector.

[Enzo]

Great, looks like that is solved.

If you have 4.7uf in stock versus ordering 2uf caps, sure, why not. They will work. They MIGHT add a little bottom end response.

[Tone Meister]

I'm still monitoring the voltages at the collector of that transistor to make sure it isn't fluctuating too much. I was gonna replace all those 2uF caps, but someone has already replaced them with 2.2uF 50v caps that test fine on the ESR meter. If those voltages stay steady enough, I think I will leave well enough alone and screw it all back together.