GK Backline 600 Frying Output Transistors

[PepeSilvia]

I worked on a GK Backline 600 for a guy, tore into it and the 6 output transistors (Q21-Q26) were all fried. I replaced them, set the bias, and checked the 60V supply to make sure was correct, which it was. Tested the amp, not rigorously, but enough to know that it was working.

Got it back to him, he said he hit the Ground Lift switch saw a blue flash, and the amp went dead. I got it back, same situation, all 6 output trannys are toast. I've got the replacements, but before I pop them in, I want to make sure it doesn't happen again.

First of all, I can't see how the Ground Lift switch would cause anything to change in the circuit enough to cause the transistors to blow. That's got to be a coincidence, right? What else could cause those suckers to go? The 0.1 Ohm resistors aren't within the tolerance of my meter (I think it measures ~0.5 Ohm just across the leads), any recommendation on how to measure those accurately?

206-0240-B1.pdf
206-0181-B-B1-B2.pdf

[Jazz P Bass]

Cannot open the schematics.

[woodyc]

...First of all, I can't see how the Ground Lift switch would cause anything to change in the circuit enough to cause the transistors to blow. That's got to be a coincidence, right?...
[ATTACH]17517[/ATTACH][ATTACH]17518[/ATTACH]

I saw that happen in a Traynor keyboard amp once. In that case the amp was manufactured with a tiny metal filing embedded in the thermal pad which was shorting an output transistor collector to the chassis. The amp worked fine like that for years, namely with the circuit ground floating, until somebody stuck a grounded cable in the line out -- and poof. Probably irrelevant, just thought it was interesting.

[J M Fahey]

Can't talk about the ground switch, it should damage nothing, but maybe this guy had some weird ground problem and it raised its ugly head when lifting, so in an indirect way it might have somethiong to do.
Anyway: rebuild your amp, test it first with bulb limiter without load, then add a speaker load without turning it off, inject some signal and scope the output.
It might be unstable, on the verge of oscillation.
*Something* burnt it the first time.
If all looks well, plug it straight into the power line, load it and apply a steady tone to it (400Hz, 1KHz , your choice) raising power step by step until clipping.
Let it cook for a couple hours at full power. (here you might use a resistor instead of a speaker, for sanity).
Check it regularly for undue heating, weird waveforms, something strange.
Work the ground switch on and off.
In a nutshell, reproduce, at the Lab, anything your friend can do on stage.
This amp does not have a proper short circuit protection, but a mute.
Acceptable in a home Hi Fi amp, not enough in the rough stage life.
jm2c

PS: check speakers and cables used by your frriend, maybe it had some problem there and a click or pop caused by the ground lifting was the straw that broke the camel´s back.
A camel which was already quite overloaded.
I *have* seen amps which exploded at turn off, of all times.
Incredible, huh?

[PepeSilvia]

Hmmm, they work for me, I tried putting an extra line break, did that do the trick?

[PepeSilvia]

I'll definitely put it through the ringer this time before getting it back to him. If I get a chance I'll head to his place and check out his setup also.

[PepeSilvia]

You know, you may be onto something here. I wonder if I didn't properly seat the piece that sits between the tranny and the heatsink to make sure it's not making contact...

[don6470]

Hi! i´m needing this "206-0240-B1.pdf" file, but can´t get to open it, if anyone can help i´d be veery greatfull!, thanksss!

[g1]

This should be it. Or close.