Anyone know anything about transistors?

[Liam]

Only joking, but...

Not posted for about 5 years (on the old ampage), so hi everyone I've not been in touch with - how are you all? (Moved towns, stopped playing so much, any of the "old school" still around?)

Got a Fender Stage 160 DSP on the bench (don't ask). Never knew transistor amps were so complex. Came in with a blown fuse. I replaced the fuse, switched on, and the driver transistor for half the main power transistors caught fire and then exploded. Its opposite number smoked but didn't explode. At the same time both speakers moved out to one end of their travel with quite a bit of (mains?) hum. Biasing resistors for the driver transistor and the grid resistors (I guess that's base resistors) for half the power transistors also turned brown and smoked a bit.

My assistant, a German Shepherd cross with a keen interest in electronics, was quite impressed. I don't let him help with valve amps, but with lower voltages he was allowed to witness the demise of the Fender.

So far I have replaced all of the decoupling caps for these transistors (both sides), both transistors, and the filter caps for the circuit they are in. When I turn the amp on it no longer catches fire, but the speakers still push out to full travel with a load of mains hum.

I assume there's some DC getting into the speaker circuit (are these amps Class B or something, it's diode city!) so it's likely to be a cap problem especially given the high level of low frequency hum. So what do I do next. (And don't say "pull the power tubes and check B+ for ripple because I'm cursing the invention of semiconductors at the moment")

Any help greatly appreciated. Can email a schematic if that's any use.

TIA

Liam

[Amp Kat]

Hey Liam and welcome back. !I was KB on the old forum and still am but I work for Big Kat Music so now it's Amp Kat. Even though Enzo is much better at these and taught me what I know about SS I'll take a shot at it and tell you what I do. If you can power the amp without it blowing a fuse that's a good thing but if you start to smell something burning or feel it getting hot that's bad. Check your positive and negative voltage rails on all the power transistors drivers and op-amps if any. If there is a bad transistor somewhere it will drag that voltage down usually the negative side. Check your emitter resistors that should be arout .3 to .5 and make sure they are not open. Check your output transistors for shorts. If all is well you may have to get a scope and follow a injected signal thru the preamp send and make sure it's good up to that point. If it's not there it's a preamp problem. If it's there then move on to the drivers and check there. My guess is you have a bad transistor dragging the supply down.

[R ski]

Being a while since I worked on solid state, but the beauty of them, is circuit testing ability, easier when the power is off. The key, is use a multimeter on diode test mode. Most bi polar transistors show a reading much like a diode when forward biased. Should the transistor be a darlington, check the net for reference. Same goes for a fet devices.

All transistor amps have current limit resistors, that are power type, read the resistor, and confirm continuity. Coupling capacitors will show leakage as well.
Check the rails, once you are sure of the first step. When energized, check for possible AC on the rails, that would indicate a shorted diode at the power supply rectifier section, that would possibly cause havoc when power up and smoke appears

Good luck

[Euthymia]

Sounds like it's got railitis, which for me, is the Ebola Virus of the SS amp (nearly impossible to cure, and best not to even touch the patient).

But that's my own pessimism.

Please to unplug that amp from the speakers before the DC that is probably on the output sizzles their voice coils.

Do you have a lightbulb limiter?

As far as troubleshooting transistor output sections, pray to the Enzo-god and hope he listens.

[Enzo]

DOn't try to make it more complex than it is. It blows fuses, so something is shorted. It is far more likely a shorted output transistor than anything.

Slapping a new fuse in it and throwing the switch is not the best route to success. Bring it up slow on a variac or at least use a bulb limiter. Also, disconnect any speaker or load. SOlid state amps do not need a load, and if there is DC On the output, the load will do nothing but draw current from it.

Speakers that move one way and stay is classic DC on the output. Pull the wires off the speaker, let a voltmeter or scope watch the DC.

it is not a cap problem, the poor power supply trying to provide a steady DC current at its max is dragged down and showing a ton of ripple. Tha caps are fine. SOmetimes the two main filters sticking up will crack free of their solder, and do check that when the board is out, but all that happens then is a nasty hum, but no DC.

It would not be a main rectifier problem, since that would just blow fuses, but not make DC on the output.


With shorted outputs, powering it up again has now burnt your
'drivers."


There are six output xstrs. Three each polarity. More or less in parallel, one shorted will make them all read that way. Measure resistance at them. Three legs, none should be shorted to another. ANy that are, pull them out NOW. TIP 142/147 can be had from Mouser or Allied or other places.

Make sure none of the 5w 0.47 ohm resistors are burnt open. I don't care what they look like, measure them.

When you refer to drivers smoking, exactly which transistors do you mean? Q10,11?

Q10,11 are not really drivers, the outputs are darlingtons and act as their own drivers essentially. That whole totem pole of Q10,11 and a bunch of resistors should be checked. Any open resistors, and are both +/-16v present?

Make sure neither C55,56 are shorted.

Yes it is class B but not because of any diodes.

[Liam]

When you refer to drivers smoking, exactly which transistors do you mean? Q10,11?

The ones that burnt were Q12, and Q13. Replaced them with a BC327,337 pair as they were the closest thing I could find. D21 stopped being a diode as well, and conducted nicely in both directions. I'll check Q10,11. Are they the phase inverter?

Yes it is class B but not because of any diodes.

Trying to get my head round the power amp circuit still. Had kind of assumed that D21-D24 were biasing the output transistors so they only had current when there was a signal. I'll have to read up on this stuff.

Thanks for your help everyone. I really like the idea of running an amp with no load - how cool will that be. Got no variac, so I'd better find out what a light bulb limiter is I guess (yeah, I know, I've probably burnt more components than I should have over the years.)

[Liam]

Ok, done some work.

5 of the 6 output xstrs are short circuit collector to emitter. Some are shorted base to emitter, some aren't. 5 of the 6 0.47R 5W resistors are open, or near enough. Q10 was OK, Q11 was open collector to emitter. Pulled the whole lot and checked some voltages. The rest of the resistors in the Q10,11 stack were fine. I had already replaced the electrolytics (C55,56)because I spend too much time around old tube amps.

Positive 45V supply is at 49.9. Negative 45 volt supply is at -46.6. Positive seems high. 16 volt supplies are present and at plus and minus 16.6.

I also noticed that R155, which is part of the smoothing network for the 5 volt supply, seems to run quite hot. Don't know if that matters.

Think Enzo was right, and at least one of the replacements Q12 and Q13 are now shorted. So....

My plan of action is to replace all transistors in that section of the amp. While I am waitng for them I will power up and chase a signal through the preamp with a 'scope.

Thanks for the help so far everyone. Amp Kat, now all I've got to do is remember you're really KB, your guess was about right. Enzo - I can see why people were speaking so highly of you, so far you nailed the problems straight away.

If anyone thinks of anything else I should be doing please let me know. What I'd love to know is what caused this little house of cards to collapse in the first place.

Any thoughts gratefully received.

Liam

[Enzo]

Q12,13 are limiters. The output current is sampled across the 0.47 ohm ballast resistors. When the voltage there gets high enough - ie the current is high enough - it is sufficient to turn on Q12 or 13, which then shunts the base drive of the output via D21 or 22.

Since those burnt, check all the 1.5k resistors, and that group of parts that includes D23. Make sure those BC series replacements you used have their emitters and bases in the right holes.

Those limiter circuits are not necessary for amp function, so you can remove the D21,22 to disable them while testing. D21-24 have nothing to do with bias. They are part of the protection circuit.

If five of six outputs are toast, replace them all. DOn't get cheap for one transistor. Same with the one good 0.47 ohm resistor. Replace it with the rest.

There is no phase inverter in a SS amp. The same exact signal goes to both rows of outputs. There is a DC offset for each row, but the phase is the same.

Q11 was open collector to emitter.

Transistors are supposed to be open C to E. If all those resistors aer OK, chances are Q10,11 are OK too. The whole point of that string of resistors is to provide the DC offset for the outputs. There should be DC of about +1v at Q14 base and -1v at Q15 base ( and at the bases of their friends next door.)

DOn't worry about a couple volts difference in the power rails. Most likely it will settle down once all the rest of the circuit is fixed.

If Q12,13 are now blown, I'd be betting the leads are not in the same order as the original parts. Those are wired E-B-C, looking at the flat face.

What originally happened was an output transistor failed. When one fails it takes one from the other side with it. If a failure went to its base, it usually takes out the row of them.

[Liam]

Q12,13 are limiters. The output current is sampled across the 0.47 ohm ballast resistors. When the voltage there gets high enough - ie the current is high enough - it is sufficient to turn on Q12 or 13, which then shunts the base drive of the output via D21 or 22.

See what you mean.

Since those burnt, check all the 1.5k resistors, and that group of parts that includes D23. Make sure those BC series replacements you used have their emitters and bases in the right holes.

Yes, 3 of the 1.5k had turned brown, but measured OK on a meter. Changed them when I changed Q12,13 and they are still intact. I had figured that C and E were reversed on the BC series, pretty sure I got them the right way around.

Transistors are supposed to be open C to E.

Ah yes, my bad, that means that on that transistor's opposite something must be shorted to base.

Can't find any details of the pinouts for Q10,11 (2SD1875, 2SB1236). Do you know what an "ATV" package is? I'm going to replace them with ZTX653 and ZTX753 unless someone (Enzo!) tells me it's a really bad idea.

Can the 0.47 resistors can be 7 watt without causing problems? Is one TIP142/147 much the same as another? I have found them in a variety of packages, with a variety of power dissipation figures, and at a variety of prices. I'm currently favouring Multicomp in a TO247 package.

There should be DC of about +1v at Q14 base and -1v at Q15 base ( and at the bases of their friends next door.)

Can I check that without the power transistors in circuit? I'd rather know that the DC off sets are about right before I blow any more TIPs.

Thanks so much for your help.

Liam

[Enzo]

I am not familiar with TIP142 and TIP147 in anything but a TO218. 12A 100v, 125w

By the way, that is 2SD1857, not 1875. I don't know much about ATV package, little square thing, nor do I know your proposed subs. These small power transistors are mounted in the heat sink for thermal tracking. I'd not wander too far from the original. Pinout is ECB.


Yes, of course you could use higher wattage ballast resistors. All the circuit sees is the resistance. But will 7 watt resistors fit physically? If you are going for better reliability or something, I'd say don't waste your time. Thy will only open when the transistors short.

[Liam]

I am not familiar with TIP142 and TIP147 in anything but a TO218. 12A 100v, 125w

Trust me to find too much choice! Even found a 90 watt version. Pretty sure what I've ordered will fit, just a slightly thicker package around the heatsink.

By the way, that is 2SD1857, not 1875. I don't know much about ATV package, little square thing, nor do I know your proposed subs. These small power transistors are mounted in the heat sink for thermal tracking. I'd not wander too far from the original. Pinout is ECB.

You're quite right. Have managed to track down the correct items now.

But will 7 watt resistors fit physically? If you are going for better reliability or something, I'd say don't waste your time.

Can't get 5 watt ceramic housed resistors easily here in the UK, nor could I find any 4 watt resistors that were short enough. I'll have to think again if what I've ordered doesn't fit.

[Steve Conner]

The old "Can't get it in the UK" excuse! :P

http://www.rapidonline.com/productin...moduleno=66473
http://www.rapidonline.com/productin...moduleno=66474

You'd think we were a third world country. If Rapid don't have the value you want, try RS or Farnell.

[Liam]

I'd tried RS, Maplin, CPC and Farnell, but hadn't considered Rapid. Luckily the 7 watt versions from Farnell arrived just now, looks like they will fit well. Unluckily at a much higher price than the link you sent me... I'll remember that for next time! Thanks Steve.

Liam

[Liam]

Well I FINALLY got hold of a pair of 2SD1857/2SB1236. I've changed them, along with any other resistors and diodes that didn't read sensible values in that part of the circuit.

Now here's a weird thing. The centre point of the resistor chain R87 to R94 is floating at about 9V. Top and bottom of the 16V supply rail seem to be OK, the "strange" voltage drop is across R93, which has got about 23 volts across it (-15 to +8). (I left the power transistors out while I was checking this, kind of glad I did, but haven't got a load connected either.)

The diodes seem OK in that part of the circuit. I assume I'm looking at a faulty Q11 or Q13? Should I just pull all the semiconductors out and check it biases OK without them?

Any assistance gratefully received.

[Enzo]

I'd be more inclined to suspect the op amp. U7. What voltage is at pin 1? Q10,11 just establish bias levels for the outputs - they maintain a particular voltage difference between the bases of the output xstrs. But it is U7 that yanks the whole thing up and down with the signal.

If the voltage is higher - more positive - at U7 pin 1 than at the right end of R85, then I'd say the IC is driving this offset. I might hazzard a replacement. But first check the input to it. Is there a large DC offset at pin 2 of U7? If so, the IC could just be bad, or mute JFET Q22 could be leaking. Look above U7 and a little left on the drawing for Q22.

I think you might get away with no output devices in this one, but most SS amps need them as part of the feedback loop. Left out, the input stage of the power amp tries to "correct" for the missing output reference.