Peavey Musician Mark III series 400g troubles

[ricksdaname18]

Hey guys new to the forum and new to electronics troubleshooting for that matter. I do know how to use a meter however and hope with a little guidance I can repair the said amp. I am supposed to be receiving a schematic from peavey for my model soon.

When I first started it was blowing fuses when switched on. Called PV tech support and he told me to unplug the two transistor bank connectors (with red, white, blue wires) from the power supply board and try another fuse. Didnt blow a fuse this time but smoked a small resistor on the power board. Called him back and he said I might need to unplug the autoformer which could be mimicking a load. I have not yet proceeded with this step.

For some background on the unit, at one time in the past it was discovered that some of the transistors on the back of the unit were bad, which seems to be a common issue, and so replaced with a supposed acceptable part. The first 2 (from left to right on back of unit) SJ6357's were replaced with NTE284's and the third with a ECG284. I have done enough researching and reading of posts to know that I should go back with the OEM parts. All the rest are original to the unit (1979). No direct shorts on any of them but different ohm readings among the originals and understandably among the replacements. Should I be concerned with this?

On SJ6357 transistors 4,5,6 I get a reading around 28-37 Mohms from b-c (I get 0.55 if I reverse leads); 7-9from e-c; 5-6 from b-e. However on transistors 7 and 8 I get an open line from b-c although the rest of the values are in line with the others. Does this affect me or am I spliting hairs? Just for fun I dont get a reading from e-c on either one of the NTE transistors and I only get 5.8 from b-c on the ECG.

So as of right now I need some transistors to at least replace the aftermarkets and a new resistor that I know of. Im gonna have to find out about that resistor. The schematic is going to help with this.

[Enzo]

400G or 400B/G?

Those schematics ought to be on this board somewhere, do a search.

Forget the overpriced and imprecise subtitutional NTE stuff. There are eighy SJ6357s, all of which are MJ15003 under the skin. Easily obyained in the general parts market, or order them from Peavey parts.

Q2,Q12 are your predrivers, a 5331 and 5332. (Those are PV house numbers, get them from PV.) Check them. Since something burnt with the outputs removed, thoswe are suspect, as are the resistors that serve them. Those are a couple of 47 ohmers and a co0uple 4.7 ohm ones. R26 and R61. They burn open when transistors fail.

Do indeed disconnect the autoformer, and also, work without a speaker or load until we are sure there is no DC coming out of the amp.

Between the bases of Q2, Q12 should be a pair of diodes. One is a dual drop diode. Just make sure neither is open.

[ricksdaname18]

On the back it just says 400g but on the boards inside it says 400 b/g so I guess its the latter. Looking for the schematic.

[ricksdaname18]

did checks

R26 turned out to be the resistor that burnt. R61 is ok. Checked diodes cr9 and cr14 and appear ok. checked between base-collector and base-emitter on 5331 and 5332. Got .5 v on all except 5331 where I got .08 between base and emitter.

Rechecked all outout transistors on diode setting on my meter. Checked between base-collector and base-emitter and all seem ok. Checked white cement resistors and they check out.

[Enzo]

0.08v sounds like a bad 5331 to me. And that would explain the burnt R26 too.

Transistors check as if they were two diodes radiating from the base. But don;t forget to test E-C as well. If emitter and collector are shorted together, the two "diode" junctions will still check OK, except now they are in parallel. Your meter won't know that. So always check for voltage drop from base to E and base to C, but also always check for E-C shorts. In output transistors, E-C shorts are way more common than base shorts.

In my experience, those are easily repaired amps. The outputs are most common failure. Note that in each set of four outputs, the end one is really wired as a driver. SO there are six outputs and two drivers on that board. Hence calling the 5331/5332 predrivers.

If one of those two bias diodes open, than can cause the outputs and drivers to fail. But if the failure started in the power transistors, it rarely burns back any farther than the 5331/5332 pair, and of course all the related resistors.

[Jazz P Bass]

Peavey 400BG Schematic

On the back it just says 400g but on the boards inside it says 400 b/g so I guess its the latter. Looking for the schematic.

Here is the schematic.
In my opinion the best way to check the output transistors is to pull them, if a cursory check shows an anomaly.
And carefully inspect the "isolation pads" that are installed under the transistors.
If any are cracked or show signs of punctures, replace them.

[ricksdaname18]

Thanks for the advice

Thanks for all the help. Ive ordered the parts and hope to get the thing up in the next week or so. Let you know how it turns out.

[ricksdaname18]

Got new parts

Have the new parts. When I install them, is there any precaution I can take to prevent something else from burning up when I cut it on for the first time? I believe I read about using incandescent light bulbs in series with the power from the wall if I dont have a variac.

[Enzo]

Sure. First NO SPEAKER UNTIL WE KNOW THE AMP IS STABLE.

And yes, that light bulb limiter is a good thing to use. If it glows brightly, ther is a good chance you saved further damage. If it glows dimly, chances are things are OK or close to OK, and you can remove the bulb and try powering up direct from the mains.


And yes too, ther is always the possibility some unforseen circumstance will allow the amp to blow itself back up.

[ricksdaname18]

Put it back together and used light in series. Tried with autoformer unplugged. Light bright at first then dim. Tried with autoformer plugged in and light stayed bright. Only left connected for a few seconds. Ohmed autoformer. 0.7 ohms from black to white; 0.5 from black to green; 0.1 from white to green. Bad autoformer? If so can you use amp without?

[Enzo]

That there was the definitive test. Amp works without the autoformer, and blows fuses with it. Yes the amp will work without it. That board and the extremely similar 400BH were used in many amp models without it. You cannot measure transformer resistance with an ohm meter and get anywhere, at least not on the speaker windings. The function test tells the tale. Go measure the speaker winding resistance of most any amp - almost zero ohms.

ANy jack switching connetion stuff would be rendered useless, so a little jack rewiring might be needed.

Before connecting a speaker though, even without the autoformer, power up and check if DC voltage is sitting on the speaker output.

[ricksdaname18]

48vdc

I now have 48 vdc to ground on the white wire on the speaker jack.

[Enzo]

The your autoformer may in fact be OK. Transformers do not like DC across them. By the way, is that +48v or -48v? I will assume +48v for discussion. If in fact it is -48, then teh same approach holds, just you;d be looking at the opposite side of the circuit.

But the amp does not blow fuses with that 48v there, at least when there is no speaker load attached. We can do this, don't give up.

Look at the schematic. Q2, Q12 are the predrivers. Normally they would be sitting with roughly +1v and -1v on their bases. I am going to bet you find about 48v on each. But here is the clue, normally the two are about 1.5-2v apart, let's just call it 2 volts. SO even if they are now sitting at 48v (which the outputs will dutifully follow), se if in fact the voltage at Q2 base is about 2v more positive than the base of Q12. SO something like +49v at base of Q2 and +47v at base of Q12. Go ahead, I;ll wait. That 2v difference is maintained by the bias diodes, CR9 CR14. Whatever voltage is fed to them, they keep those predriver bases 2v apart. And that is how the amp works, we grab that point in the circuit, and move it up and down with the music, and the outputs will follow.

WHat does that grabbing? Q9. Q9 is called the voltage amp. All the stuff to the right of that are really amplifying current for the speakers. The more Q9 turns on, the more towards the negative rail it pulls the bias string. And the more it turns off, the more it lets the bias string climb up towards the positive rail. So if I find the bias string pulled up almost to the +V rail, then I start to think that Q9 is not conducting.

Q9 can be bad - open - or maybe Q10 has shorted, holding Q9 off forever. Darn rare for Q10 to burn out without Q9 also being shorted. In the other hand, Q9 pulls the bias string down towards the -V rail, but if it loses its connection to that rail, it has nothing to pull against. SO we want to check R59 R60 for opens. Check the emitter of Q9 - if you see about -50v there, then those resistors are probably OK. yes, other things like Q8 could be bad, and unable to drive Q9 to turn on, but since the amp had been blown, I tend to suspect the things closer to the blown parts.

This took longer to describe that it should to check.

Other possibilities, a bad TL074 IC, Q8, who knows what-all else.

MY spider sense is tingling now that some failure sent DC from the amp output, the autoformer objected, and that cause the damage to the output stage

[ricksdaname18]

checked out

Hey there finally got a chance to check stuff. It is a positive 48 volts at the white wire on the speaker jack. Checked the bases of Q2 and Q12 and there is around 48 volts. There is about .7 volts difference. Got 51 volts at the emitter of Q9. Pulled Q8, Q9, Q10 and all seem ok. Is there a way to check that IC?

[Enzo]

The emitter of Q9 - do you have 51 volts or NEGATIVE 51 volts? -51v would be normal. 51v, meaning +51v, would indicate an open R59, R60. Or a missing V- rail.

And the fastest way I find to check the IC is to stuff another one into the socket. If yo uhave no TL074s in your drawer, there may be one in the preamp you can borrow.