Solving the case of the missing output transistors

[Ptron]

I'm working on a Peavey MarkIII Musician. It had a weird distortion on the output. I don't know what this kind of output configuration is called but the "opposing" output transistors are not complimentary. They are all NPN with the output coming from the emmiters of the positive half and the collectors of the negative half, each half having three parallel transistors for a total of 6. I opened it up to find that two of the output transistors had been removed...from the same side! so that the positive side of the output only had one transistor while the negative side still had all three. So I might just take one from the negative side and put it in the positve side. Perhaps someone can tell my if and why this is a bad idea. I don't see any issues other than a little loss of power and maybe a little higher output impedance, but then again I don't know much about how such things are determined in solid state amps.

Of course this assumes there is nothing else wrong with the amp.

[Enzo]

That is called Quasi-complementary, and is quite common really.

To function - yes, at lower power - you only need one output device per side, and yes again, two and two would be far better than 1 and 3. Or better yet, replace the missing ones and have all six. What numbers are they? Please do not mix types. In other words if there are two SJ6357, don't fill the third hole with a 62792. Stay away from NTE, it won't mix with the other parts, and it costs several times more than real parts.

WHat does it say on the rear panel of the amp? 260C? 400 series? 400BG? If nothing else, on the main power amp board, the board type will be in letters in the foil. You need to get the proper schematic for it. peavey will send it if you email parts@peavey and ask.

Are you sure all three output transistors are in parallel on each group? ALL THREE basses tied together? Or is the end one actually wired as a driver? Its emitter is wired to the bases of the other two? I ask because if the two outputs were removed and the driver is left, the amp might still function but would not drive a clear signal into a load.

These amps are easily restored to full operation and will work forever.

[Ptron]

Thanks Enzo, very informative. I don't know about the rear panel#, I don't have the amp with me right now, but yes, it's a driver and then three parallel output transistors. All sj6357. My first search for a replacement didn't turn up much which is why I thought about saying screw it and just leaving a pair out. I searched again and came across your reply on another board saying that sj6357 was a house# and it's actually a mj15003. Found that one in about 2 seconds. Thanks again! Yeah Peavey's great with the schematics aren't they? This was the second time I've requested one in a late night email and been sent one early next morning.

[Enzo]

I more or less expected it was 6357s you had. I buy MJ15003 and others of that family from Allied. Mouser sells On semi, but doesn't have the price. Like over $2 differene each!

[Ptron]

It seems things are not so simple...

First off, the amp is a 400GH. Also, it was the side attached to the negative rail that was missing two transistors, not the positve half as I said earlier. AND before reading this whole thing, the positive rail is 50v while the negative is only -43v. Could that imbalance cause the weird distortion this thing has?

The distortion on the output is a little hard to describe. It's not a consistant fuzz or raspiness or anything. When playing guitar through it it's kind of a putt putt on top of the otherwise clean notes, and If I play quietly enough, it's not there. When I run a signal generator on a sine wave (into the PA in so as to eliminate the preamp as a culprit), the distortion varies from pulsing to constant as I sweep the frequency. I switched a transistor over to balance the output and...it didn't change the sound one bit.

Before I took it apart, I had once heard the amp suddenly start buzzing loudly for a few seconds then stop. Sitting on my bench after surgery, It started buzzing again. While I was digging around for a chopstick to start poking at things, something started to let it's smoke out. Instead of being smart and reaching for the power switch I tried to see where the smoke was coming from, but it stopped buzzing and smoking just as suddenly as it had started. The thermal breaker could have tripped but I didn't realize it was there yet, so I didn't check. The smoke looked to be coming from a couple of the cement PA emitter resistors, from the side that had previeously been missing the transistors. In fact all the emitter resistors on this side were really hot. The transistors were relatively cool. I took the resistors off the board to measure them and sure enough the 1ohm on the driver and the .33 from the first OT were brown on the bottom. They all still measured ok though. I took every 6357 out and checked them with a meter and they all read ok.

I put it back together and it still works and it still has that distortion. I tried using only two drivers and two OTs and then switching different ones in and out. I've checked every voltage in the PA that the schematic gives and other than the main rail imbalance I mentioned before, they're all ok except that the unregulated ones are all about 5% low. BTW the low negative voltage exists even when the output stages are disconnected.

So, any ideas? I would probably start poking around with my o-scope to see if I could find the source of the distortion but mine's dead right now. I can email the schematic if anyone wants to see it.

[Ptron]

I pulled up the "driver board" to have a look underneath and yikes! This thing obviously had a catastrophic failure at some point. I can't belive it looks ok from the top. From the excess leads and sloppy soldering I can tell that most of the transistors and a handful of other components in the PA driver circuit have been replaced. There are a couple traces missing and replaced with wire and there's even a severely charred spot by one of the bridge diodes. There's some residue that leads me to believe something was spilled inside the amp. I can't believe this amp works at all.

I guess I better make sure this thing was put back together correctly first before I continue looking for toasted components. I got my o-scope working at least. It just had a bad power cord.

[Enzo]

Low rail voltage, hums, distortion, these are symptoms of a bad filter cap.

First check the solder to the two large can caps. WHen hum comes and goes, I look there first. These caps are very reliable, though they can fail. 5000 @ 55v is no longer available. If you need to replace them, use 4700/63v which is a standard value. Don't worry about the extra few ufs, if you order a 5000/55 from PV you will get a 4700/63.

Scope the power rails. That low -43 might well be low because it is lumpy. It will show up as distortion on the higher portions of the output waveform.

Burnt amps often look uglier than they really are.

If it were mine, I would verify all the power resistors on both th epower board and the driver board.

Note also there are two sets of LV rails. The three leg regulator ones go off board for the preamp. The zener ones stay on the power amp card for its circuit. Make sure neither zener is crumbling - 15 solid volts - and make sure it is clean.

Run a sine through it up to clip and back off a hair so it is full out and should be clean. No load. What does it look like at the output. Can you see distortion on the waveform? Now add the load. Does it change? The load might reduce the amplitude a little, but if the load adds distortion, then somthing in the output is not doing its job.

[Steve Conner]

It sure does sound like the filter cap on that main rail is bad, or has just got disconnected from the circuit.

BTW, as far as I know, the missing power transistors will make no difference to the sound. But since the remaining transistors have to work harder, 4 ohm loads that the amp used to be able to drive safely may now make it explode

Most amps with paralleled pairs of output trannies take the signal for the current limiter circuit from only one pair. If yours is one of these, then you'll either have protection that triggers at one third of the current it should, or no protection at all, depending on exactly which ones went missing. So they definitely should be replaced ASAP.

I'll guess that the remaining trannie on the positive side must be in the socket that has the protection, or it would have died. The premature current limiting could explain the distortion you're hearing.

[Ptron]

Thanks guys. I've had all of 5 minutes to work on this amp since I last posted. All I've done is scope the rails and a couple other places. I'd considered the possibility of filter cap in being responsible for the low voltage and distortion but let it go because I expected the output to buzz loudly all the time if that cap was bad. The negative rail is whacked so apparently this is not the case.

On the scope the negative supply looks like the top of a roosters head. This also appears on the base of driver sj6357. Perhaps this is why, other than the two times when it went nuts for few seconds, the output is quite quiet?

I'll order new caps and transitors today and go from there. Enzo, do you happen to know off the top of your head if the snap in caps fit? Those are the only 4700/63 I'm seeing in stock. Maybe I'll have to go to PV for those.

BTW, as far as I know, the missing power transistors will make no difference to the sound. But since the remaining transistors have to work harder, 4 ohm loads that the amp used to be able to drive safely may now make it explode

I think I get this. The driver is really a current supply so if two transistors are missing it all goes through the reamaining one, correct?

Most amps with paralleled pairs of output trannies take the signal for the current limiter circuit from only one pair. If yours is one of these, then you'll either have protection that triggers at one third of the current it should, or no protection at all, depending on exactly which ones went missing. So they definitely should be replaced ASAP.

This one doesn't have appear to have any current limiting that I reconize, other than the emitter resistors acting as fuses.

[Enzo]

Q4,5 are the limiters, though they do not sample off the emitters of the outputs. RElatively unusual. They sample off R39,40, which carries current from all the output xstrs. The outputs here are simple parallel with emitter ballasts added. The ballast - emiter resistors - are not there as fuses, they are there to insure current sharing between the transistors. So you can operate the output xstrs in any of the three holes on each side.

A crappy filter won'r always make constant hum. If you have the sawtooth waveform of ripple, either the cap is soft, or there is excess current being drawn from the supply. I always have a current meter in my test outlet on the bench, so I can see how much draw from the mains the unit is causing. If the amp is sitting there drawing 150 watts at idle, something is drawing a ton on the power amp somewhere.

You do realize you can ditch the preamp and just run this amp panel by itself, I hope. I usually set the head cab on end next to me with the rear panel flopped out on the end of the tranny leads. Sometimes I remove the four whole screws and pull the tranny out onto the bench and set the cab aside. A couple clip leads can apply signal to the pins at the corner of the driver board.

I think the snap-ins will fit the holes, if not, there is plenty real estate. Stick one leg in an existing hole and drill a new one for the other. Sand off any conformal coating if there is any in the way. The ones from PV are g DC voltages around the amp. Several are on the drawing. Note the neg side voltages are across resistors, NOT respecting ground.

[Steve Conner]

Oh, the "Top of a rooster's head" thing is a sure sign that the filter cap on that rail isn't working. That doesn't necessarily lead to hum at idle, because solid-state amps have such huge amounts of negative feedback, they may be able to reject all that ripple. You may not hear anything wrong until you start trying to pump power to the speaker, and the rail physically craps out 120 times a second and can't supply it.

Yes, if two transistors are missing, the driver stage will try and force the remaining one to pass three times the current it should. (Enzo explained that the protection in this amp works on the total output current.) I wouldn't expect it to last long.

[Ptron]

Damn you guys are good. I probably won't get a chance to work on this amp until after Christmas but parts won't be here until then anyway.

Q4,5 are the limiters, though they do not sample off the emitters of the outputs. RElatively unusual. They sample off R39,40, which carries current from all the output xstrs.

Oh, ok. I just assumed that was some sort of feedback going on there, which of course it is, but yeah, that makes sense.

Speaking of sawtooth wave, "top of a roosters head is not really as good of description as "compound saw blade with round teeth". It's not what I would have expected.

[Ptron]

Got the caps and transistors today. The new caps cleaned up the rails nicely and the odd pulsing distortion is gone. Thanks guys! Both rails are 50.8v with 75-100mV ripple which I assume is not that bad for an unregulated supply. However...

It did seem to be breaking up really early so I tried the test Enzo suggested earlier. Plugged in a signal gen sine wave, cranked it up to almost clipping, plugged in a power soak, and presto, the positive half of the sine wave disappeared. I guess I have more work to do. Anyway, I'm too tired to figure it out right now.

One thing to note: R47 and R49 get VERY hot. These are the 5W, 1k and 2k respectively, resistors that go from the positive and negative rails to ground. They measure ok and the rails are ok voltage-wise so this must be normal right? (I don't really get why they're different values or why they're even there but I suppose that's another matter.)

Oh, yeah. I didn't install the new transistors yet because I forgot to buy the gaskets and I was too tired to realize I could just use heatsink compound since the cases are not isolated from the the heatsink anyway.

[Enzo]

Um. you better insulate those transistors. The case is the collector. In tghe upper tier, that means the heatsink would be connected directly to the +52VDC rail. The lower tier collectors are wired right to the output bus. So unless those two sets of xstrs are on separate heatsinks AND they are insulated from chassis, you must have insulating mica washers.

Look for an open Q8. It is a driver, but same as it's output neighbors, so swap places Q8 and Q10 or something like that. MAke sure R34 is not open.

Lift CR4,7, to kill limiters Q4,5.

[Ptron]

Um. you better insulate those transistors. The case is the collector.

ok so I'm an idiot. Don't know what I was thinking but I was really tired.

I only had a few minutes to work on the amp last night. R34 and all the ballast resistors measure ok. I pulled CR4 and CR7. The mains fuse blew when I powered on. I didn't see any shorts so I replaced Q8, and Q12 for good measure, with the new MJ15003s. This time I used a variac turned all the way down. Again the fuse blew as soon as I powered up (apparently my variac has about 8 volts on it even when it's all the way off) so I think I might have a hard short that wasn't there before. I'll get the bulb limiter out and try to isolate it by disconnecting the OT board and such when I get a chance. Got a gig so it won't be tonight.