SA-W2500 Sub blowing fuse

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So, I have an SA-2500 that likes to pop the 2.8A fuse on the board. I replaced the fuse and with the speaker disconnected, and the volume at minimum, when connected to a current limiting bulb, the bulb is full bright.

I have included a schematic of the sub, along with highlighter indicating which components have been pulled from the board and tested.

Yellow = Test good
Pink = Fail.

The only component that I've found failed is Q704. Using the diode check on my multimeter, I found the transistor to check good as a diode between B and C and B and E. However, I have continuity both from C to E and E to C...

The power transistors (Q708 and Q709) have not been pulled, but check good in circuit....

I can't believe that only the one transistor could be causing this fuse issue...could be wrong though.

Any thoughts??? (schematic attached)

Check the resistance at the load side of the fuse to ground. It will tell you if you have a short since you said that the output transistor check OKAY.

I went a step ahead of your suggestion, the fuse is inline with the main transformer. I bypassed the power-on relay and powered the main transformer with the bulb current limiter in place. No illumination on the bulb, and all proper power outs from the transformer.

I then checked resistance between every combination of pins where the transformer connects to the amp board. I found no shorts, all readings were 1M or higher. Then again, this is with the failed transistor removed from the circuit. I'm tempted to power the main amp board with the failed transistor removed, but not knowing the reproductions of missing that transistor keeps me from doing it.
Someone has to be joking... one pair of TO-3P darlingtons for a sub ? No wonder it went pop...

Be warned, there are a lot of fakes of Sanken transistors out there...

Generally two things will cause output stage failure here - overheating, or the speaker shorting out. I'd suspect the former if run hard enough.
I know with a typical transistor you should read OL in either direction between C and E, but on both Q708 and Q709, when removed from the circuit, I'm reading 0.5v in one direction between C and E. Other than that, they check good between B and E, and B and C...

These measurements were done with both transistors out of the circuit
So, I'm going to have to consider the 0.5 in one direction a normal mode of operation, since one transistor is NPN while the other is PNP, and that 0.5 conducts in opposite polarity between the two transistors.

I went ahead and basically tested every single transistor in the rest of the amp circuit and found no failures. I even went ahead and pulled the two main filter caps and tested them, they also test good. Not 4700uF, but 4200 is close enough.

I also went ahead and gave powering the circuit with the failed transistor removed from the circuit. Ops check good. All relays clicked, and I had proper rail voltages. Bulb tester came on bright and immediately dimmed down.

So, I guess its time to bite the bullet and assume somehow this one single transistor went toast, and caused all the headaches, with no other failures within the circuit. I would have pulled the trigger a lot sooner, but its not like I can stroll into the local RatShack and pick up a 2SA1145 transistor. I don't want to have to place any more orders than necessary and suffer the shipping. I'll place an update when I get parts in hand as to success or not.
Wow...and Q704 was the one and only cause of failure. That little $0.14 transistor was the cause of too much headache! I was convinced that there had to be more to this. Guess not! Thanks for all the assistance, and participation! I really appreciate the time and effort you guys put in to these forums. I hope that down the road this thread provides some enlightenment to someone else with this discrepancy!
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