Audio amp step-by-step repair

[okw]

I replaced Q1, Q3, Q4 with output reduced to around 340mV DC.
BUT... I accidentally shorted the output pins. That blew the fuse. I replaced the fuse, and it blew again

[Audiorepair]

Oh well, at least you know its not the op-amp, the protection circuitry, or the driver transistors.

That kind of leaves what remains of the output Mosfets.



It could also be that this is a crappy design susceptible to DC offsets that they never bothered to fix.
And now, apparently, that's your job.

[okw]

As I accidentally shorted the output, it now blows the F1 fuse. Do you think one or several of the mosfets are shorted? Or anything else?
I need to sort this out before I troubleshoot the DC further.
If it is the mosfets, I would probably be better off removing all 4 mosfets, DMM testing them for shorts, and replacing the bad ones? Instead of replacing one at a time, powering the amp after each replacement (that could possibly harm the new one if the 2-3 others are shorted?).

[Audiorepair]

You can test for shorts in circuit first.

[xavier60]

The amp can be test run with one pair of good MOSFETs, just keep it lightly loaded.

[okw]

All 4 mosfets were shorted.
Would x140 and x240 be interchangeable? I don't have IRFP140/IRFP9140 just IRFP240/IRFP9240.

[xavier60]

All 4 mosfets were shorted.
Would x140 and x240 be interchangeable? I don't have IRFP140/IRFP9140 just IRFP240/IRFP9240.

IRFP240/IRFP9240 should be ok for testing the amplifier but not at full power, they have half the current rating.

[okw]

Of course, I forgot to check. I thought I bought these to the BA115HP amp we are discussing, but I bought them for a SVT 3 PRO. This amp has a problem it all of the sudden dies. It could happen after a few minutes of playing or it could be good for months. It has under and over voltage shutdown circuits marked in its schematics. Could it be any of these?
And would the fan keep running in case of under / over voltage? I don't remember if the fan stops also. Could be a good clue to watch out for that.

[xavier60]

Of course, I forgot to check. I thought I bought these to the BA115HP amp we are discussing, but I bought them for a SVT 3 PRO. This amp has a problem it all of the sudden dies. It could happen after a few minutes of playing or it could be good for months. It has under and over voltage shutdown circuits marked in its schematics. Could it be any of these?
And would the fan keep running in case of under / over voltage? I don't remember if the fan stops also. Could be a good clue to watch out for that.

The Shutdown disconnects the output only while either or both stress condition is detected, not latching. The fan will continue to run only while high temperature is detected. What voltage is the fan?

[okw]

Sorry for the late reply, but the SVT was put in storage. I have it on the test bench now. The shutdown actually happened after just 4-5 minutes. Sine 1.0Vp-p input, tube gain (plate voltage) @ 10, gain @ 2-3, master @ 2-3. 8ohms 100W resistor over the output, oscilloscope connected.
Suddenly the signal died. Quickly turning the tube gain (25 or more, either direction), restored the signal. But the signal dies again after a short time.
I measured both output and effects send (preamp out). Preamp outputs signal, while output dies, so I guess preamp is good, but power amp fails?
 
Is this of any help?

[okw]

After i accidentally shorted the output of my BA115HP and blew the FETs (3 out of 4 was shorted), I finally got around to replace them, along with Q3, Q6 and Q9. I don't remember if I replaced transistors Q1 and Q4 before or after output short (I think before).  After replacing the FETs, they blew again along with the fuse

Could Q1 and/or Q4 be damaged in the mishap, and now causing the FETs to once again blow? Or could the replacement of all transistors and FETs make the bias adjustment so way off it blew the FETs? Which direction (CW / CCW) will reduce the bias on AF1?
I know on the SVT3 PRO, bias adjustment is extremely sensitive so it's easy to see smoke.

I have a lot of all transistors (Q1/3/4/6/9), but only four IRFP9140 and three IRFP140. I wanna make sure they don't blow again (delivery of new ones take time).
Before the mishap I replaced C11, C20, C31 and C44.

[okw]

I removed Q1 and Q4 and if I'm not mistaken, these were swapped. Could that explain the blown FETs?