Hi group

I have a 240w single channel solid state transformer coupled power amp that
isn't working. There is not a whole lot to the power amp board, but I do
not have a matching schematic. I did find a schematic to a more recent
version of this amp and its close enough so I was able to figure out the
main power path, but not the front end, feedback, and protection circuitry.
It looks like the company re-designed the front end, feedback, and
protection circuitry on this later model. The power amp board had 2 open
transistors, 3 shorted transistors, and showed signs of burns. The amp blew
fuses when turned on.

I replaced all the bad transistors and an open resistor (burned). I
reinstalled the large heat sink that connects to the two power transistors,
the two pre-driver transistors, and the bridge rectifier. I verified that
all transistors connected to the heat sink were electrically isolated from
the heat sink and powered the amp up. It blew a fuse.

I removed the heat sink and the power transistor pair and powered things up.
I found very high voltages throughout the output stages (like around 60 vdc)
with a large amplitude distorted signal riding on it that was the input
frequency I was feeding into the amp. The amplitude of this distorted
signal followed the volume control. I assumed that this particular amp
cannot be run w/o the power transistors, so I put them back in. I powered
the amp back on and it worked. The only difference being I was running
without a heat sink now.

There is a fan in this amp. One fan wire is connected to a ground wire that
is connected to the power supply. These two wires are connected via a screw
that screws into the heat sink. I found that as soon as this ground from
the power supply is connected to chassis ground (via the heat sink
originally) I lost all audio (but did not blow a fuse or apparently damage
anything from the momentary contact). Anyway, I was running this way...no
heat sink, fan wire not grounded, just audible output...thinking about what
I should do next when snap, crackle, pop I had flames shooting out of the
board. Both pre-driver transistors are physically cracked, I presume the
power transistors are shorted, a couple of resistors have burned up, and the
pc board really shows burn signs now. So, here's my question:

Could this failure be due to just running for 3-4 minutes at extreme low
volume without the heat sink? I would guess not, but I don't have a lot of
experience with class B power amps. If it could be, it might be worth
replacing the transistors and trying again, otherwise I think the garbage is
the best place for this amp.

Thanks

Dan