In article , "Dan K" wrote:

"jakdedert" wrote in message
.. .
Arfa Daily wrote:
"Dan K" wrote in message
...
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

Refailure of this kind is common. You really need to be able to run the
amp up on a variac to see what's going on without risk of re-frying all
the parts that you have replaced. It is possible that the transistors
could have failed as a result of having no heatsink on them, but probably
only as a result of something else still being wrong. With the amp
running at just above idle, the output stage dissipation should be
minimal, but not if there is a problem with the bias setting, if this is
variable, or bias supply if fixed. Once the transistors start to get hot,
they will quickly go into thermal runaway without adequate heatsinking,
and rapidly fail.

If you can lay hands on a variac, it might be worth having one more go,
but after that, without proper schematics, you will probably struggle to
get to the bottom of it, and finish up throwing good money after bad

Arfa
It might also be useful to post the brand and model # of the amp. Someone
might have experience with--or be able to supply a schematic for--it.

jak


Its a CSI P-240A, which is from Speco.

Its possible that you heated the parts. Try again?

I ALWAYS use a light bulb in series with power amp testing.
60 - 200 watt or so. I sometimes also use a variac in
conjunction with the light bulb. That would not have cured a heat
problem if that fried the amp, but it may have prevented sparks and
fire.

I laughed, sort of, after fixing a brute of complexity of a Crown
amp. After "fixing" it, I sat it on the floor playing some soft music,
and started walking upstairs, heard a pop, looked around at the bright
light comming from inside the power amp, then some smoke, and
that was that.


greg



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