Fender Acoustasonic sfx shorted? Ugh!

[g1]

Possibly one of the upstream transistors breaking down after on awhile. Have you replaced the driver (Q27)?
Also, the MPSW series are run pretty hot and can be problematic (Q17 & Q19).

I'd highly recommend you not install all 4 outputs til you get it sorted out. Just one NPN and one PNP. It will at least cut your losses in half.
Leave out the one with the poor insulator, and it's mate (either Q29 or Q33).

[The Dude]

If you have a spare meter laying around, it's a good idea to monitor bias as you bring up line voltage. If bias is high and out of range, it's a good indication there is still something wrong in the circuit.

[Helmholtz]

If the amp is stable when on the LBL, check for DC at speaker output. Also look for components getting hot.

[Indyryder02]

Already replaced the plastic with the silicon insulators I salvaged.

[Indyryder02]

I have not checked dc at outputs... not used to approaching guitar amps like solid state stuff receivers etc.
mid I can remove a pair from that side. And rest with DMB. Should be ok. Must be something charging up or building up current.

[Helmholtz]

And rest with DMB. Should be ok. Must be something charging up or building up current.

What is DMB?

Semiconductors have increased leakage current at higher temperatures. Increased leakage current further increases component temperature and this in turn increases leakage current etc.
The result is thermal runaway, which may look like a short and can melt the chip.
This is my explanation of the delayed rise of current.

Reasons can be e.g. a leaky transistor or diode, too high bias, insufficient heat sinking...

[Indyryder02]

Makes sense.. haven’t done any type of biasing.. as I cAnt get it to even hold long enough for that. So it’d have to be one of the drivers that was leaky?

[g1]

One other thing, Q15 and Q16 are thermal trackers for the bias. If you removed it for testing, it needs to be mating to the heatsink in the same way as the one on the good side.
So if they are using thermal paste, it has to be used there.

[Indyryder02]

Those are actually prob the only two I didn’t remove and test.
Something is fooling me and me meter for sure! Haha

[Indyryder02]

Removed the failed OT’s. Tested all of the surrounding components in and out of circuit... only thing I’ve was Q19 getting hot. It tests ok though with diode tests and my B-K 520B
Would love to find a failed component! At least one suspect!

[g1]

Removed the failed OT’s. Tested all of the surrounding components in and out of circuit... only thing I’ve was Q19 getting hot.

That is normal. Q17 should also run hot. If it does not, it may have a problem.

[Indyryder02]

Q17 seems to test ok as a transistor.. it does have a higher HFE than Q19. 85 And 138.. could that be off enough to cause a bias issue? I’ve only been diode and lean testing the transistors.

[g1]

Does Q17 run hot? If it does, it's probably ok.
Something is breaking down as the amp warms up, probably a transistor. Maybe just a bad solder connection. The diode check function or transistor tester will not provide the kind of voltage making the bad part break down. So they will all measure good out of circuit. That is why it seems fine when first turned on.
These kinds of faults can be quite difficult to find.

[Indyryder02]

I would agree! Quite! I think 17 was also getting hot and I just didn’t spot it. (Took a thermal pic, shows equal heat at both)
I think. As I’ve done in hifi stuff before. Replace the drivers and bias trannies at the same time as the outputs.. prob would have prevented.. but I’ve been fine plenty of times before not replacing em.
I agree. Has to be something going out of whack once up to voltage.
definitely sick of this one! I have new parts on order.
will replace drivers as well as bias and pres if needed.
bring it up slowly on the DBT. And check bias as I do. Until im sure it’s all running right. And hope for the best!

[Indyryder02]

Update. Finally got some more time with the amp.
brought it up with without the two failed trans. Q31 and 35.
checking bias voltages... at TP 73 and 75. Pretty erratic.
73 seems to remain in the ok area. -1.2ish. But 75 climbs up to almost 5 v at times...
Going backwards and testing.. leads my suspicion to Q15
on the heat sink. Is this actually the bias trans that create the bias voltage? I can’t really test it in circuit as it in the heat sink.
but I suspect it’s wonky. While testing voltages.. I cold sprayed and voltages really when out. Up to 10v on tp75..
feeling like I’m on to something.. does this info help clear anything up? Plan to replace Q15 and retest.
thoughts? Suggestions?