SWR Bass 750 Power Amp Issue

[Naoscaire]

Hey guys!
So I have an SWR Bass 750 amp on the bench that seems to have a strange issue in the power amp circuit. It originally came in for repair because the owner had decided it was a good idea to run it on a 1/2 ohm load for God knows how long until it finally gave up the ghost. As usual I replaced the outputs Q9 thru Q14, the drivers Q7 and Q8, as well as a few other components - Q3, R13, R14, R15, R18, and C7. Q3 had gotten so hot that the heatsink changed color, and R14 and R18 had unsoldered themselves from the board. So after all that I biased it using the crossover notch method, got a good waveform, and everything seemed OK until I realized that Q3 was burning up again, while everything else is running cool. I've been over this a hundred times already and I know that there were some design issues with this circuit, but this is killing me. Any ideas? Btw, this is the Revision 'E' power amp board but I did find one discrepancy - R15 is a 470R instead of 27R. Thanks guys!

[Jazz P Bass]

Kindly post the schematic.

[mozwell]

maybe one of these schematics
Is output at 0V with no speaker connected.
R15 as 470 ohm makes sense, as R13 is the same value, does R15 look to be "original"
are all emitter resistors ok R26-31, and R22, R24, R25, and D6
C13 may be better as a 100V part

[Enzo]

470 ohms is from the older versions.


Here are three versions. Yours may match one exactly, or may be close to one with parts of another. The design was ever evolving.

[Naoscaire]

Hey thanks guys for the quick replies! Sorry about not posting a schematic. I tried but my dinosaur of a desktop wasn't cooperating at the time. Hey mozwell, I've got 0V DC offset, R15 was for sure original, all the emitter resistors (or collector resistors in the case of R29-31) check good, and R22, R24, R25, and D6 check good but I'll test them again out of circuit just to be sure. Totally agree with you on C13 being a higher rated component. I believe it's good, but I'll change it out for an MPP cap anyway. Enzo, thanks again as always! The '750 Watt Power Module Rev E' shcematic is the one I have been using and everything matches up but the R15 being 470R instead of 27R. I'll keep at it and let you guys know what I come up with. Thanks again!

[Naoscaire]

OK guys, here's an update:
I checked R22, R24, and R25 out of circuit and they all read good as suspected. I replaced D6 with a 1N4007 and C13 with a 250V MPP cap for good measure. I was able to zero in on C5 and after a little trial and error, I decided to replace it with a 250p 2KV cap. This actually helped the situation considerably, and Q3 is now running much cooler at idle. Thanks again guys for all your help, and I hope this info will help someone else in the future.

[drewl]

I'm sure it will help me, as I have one here with every one of the Sankin 2SC3264's shorted.

Anyone know if SWR sells a replacement board?
It may be cheaper than rebuilding them....then again, it may not be.

[Enzo]

SWR is Fender, contact them and ask if the boards are still available.


And many times output amp boards come stuffed but without output transistors. Those are soldered to the board after it is mounteed, and they may be considered part of the heat sink assembly. You might want to ask about that too.

[imix500]

As of about a month ago they still had the boards available. They come partially stuffed missing the outputs and the bias transistor. Their stock of outputs is thin by Fenders admission, so they suggested getting them from another source.

[drewl]

Well, so far this one fired up okay after repairing the output amp.
Talking to the owner, they had been running it at 4ohms for years. Added an 8ohm 2x10 cab and it fried, even though the manual says it can be run @2.6 ohms.
Unless it was getting ready to die anyway.