Fender stage 100 blowing outputs

[daz]

http://support.fender.com/schematics..._schematic.pdf

Theres the schematic. And the problem is i have checked every part in the amp and can't find what may be causing the outputs to blow a fuse and take several output transistors with it. But i think it must be a component that reads ok till it gets hot then shorts because i can play it for 3 or 4 minutes till it blows again. And yes, thats with the heat sink bolted down. Any idea what is the likeliest component?

[52 Bill]

During the 3-4 minutes that the amp works, how hot do the output transistors get?

I believe that that is one of the amps that Fender refers to as a "Flying Rail" design. The speaker drives from the center of the power supply. The speaker jacks are not directly grounded.

Use a light bulb limiter if you have one. Check to see if the amp is oscillating at high frequency, check the Zobel network at the output. Make certain that the heat sink bar is properly installed and is making good contact with the chassis and the output cases. Watch where you run the many wires around the chassis, if you cut cable ties that hold the speaker wires to the back chassis be sure to route the wires away from the pc board.

[daz]

Thanks, but It's not any of that. Already been there.

[daz]

heres a thought...what about R195 in the 45v power supply. It's only a 1/4 watt 47k and while it reads ok, how likely is it that may be shorting or close as it heats up? I stuck a 1 watter there just to be safe. Still wish i could be sure when i find the issue so i don't have to keep blowing outputs and fuses. I have the outputs removed now and chacked the 15 and 45v supplies and they're good. But like i said, this happens after a few minutes of play.

[km6xz]

Are you keeping a load on it while suspecting the amp to be unstable.....that can get pretty expensive.
As a general rule, no load on a solid state amp until proven stable.

Checking random parts on a pc board is really a good way to blow a lot time that could be used tracking the problem based on symptoms and measured observations. What does the amp do with no load and the output transistors in place? Waveform looking normal, any DC offset at the outputs? Are you monitoring current while running the amp, if so what current from the mains do you see?
What DC level do you see at test point 24 and 25 when idling with no load? Measurements of circuit conditions will lead you to the cause a lot faster than random parts testing, particularly since you probably do not have test instruments to test components in a meaningful way, except DC resistance.

What drop do you measure across the emitter resistors of the power amp? Any higher or lower than the others on the same side?

Until you find out what the circuit is doing do not change any more parts without a good reason, it introduces more ambiguity. The unit does not need re-engineering, they have worked for a long time as intended.

[Enzo]

And don't assume it is a bad part, while it may be a bad part, it can just as easily be a cracked solder joint or an open trace on th board.

[R.G.]

I find it very useful to eliminate possible biasing issues with solid state amps by shorting the bias provision between the bases of the drivers. In this case, I think it's Q14 and Q15. This will cause gross crossover distortion, but it completely removes any issues about the bias string going nuts and probably stops any tendency to thermal drift as well as killing a number of RF runaway problems. If the bias change completely stops the fuse issues, then it's very likely that the problem IS the bias string. When you've found the real problem, fix it, and then take the bias short out.

Oh, and ... light bulb limiter. 8-)

[daz]

Oh geez....it was.....get ready........don't make fun of me.....PILOT ERROR ! I didn't realize it, but the speaker i tested it with was 4 ohm. Amp says 8 ohm min, and isn't SS much more sensitive to mis-matches ? Now it could have been a couple other components i found that were suspect, but more than likely the speaker. Just banged on it for 10 minutes and no issues. (adjusts tie ala rodney dangerfield) geez, it's tough bein' stupid.