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A friend of mine brought me an ailing LCD projector; he said that
during a football game it went "pop" and everything went dark. We
powered it up and everything appeared to work normally, but there was
no light output. The bulb appeared to be fine, so I told him we could
probably expect to take it apart and find pieces of transistor all over
the inside.

Sure enough, the power supply has a bipolar power transistor that freed
one of its leads in an effort to escape. I normally do not deal with
power circuits, but I am wondering if we can just replace the
transistor (I would be inclined to replace all 4 of them) and see if
that fixes things. Is this reasonable? Is there anything else I should
replace? Should I do this in a well-ventilated area free of combustible
materials with a fire extinguisher handy?

My second question: The exploded BJT is a Sanken 2sc4297, absolute max
collector current of 12A. I am tempted to replace it with a 10A
Panasonic 2sc3975, which is otherwise mostly identical. Is this a bad
idea? I have no idea how much current actually goes through the bulb.
If anyone can suggest a source for the original Sanken part, I'd
appreciate it. I've gotten Allegro to send me free samples before, but
I don't know how many times I can get away with it.


Don't know whereabouts you are, but Dalbani in the UK keep it, and ship
worldwide.

www.dalbani.co.uk

Is the power supply that has blown, the one for the lamp drive circuit ? I'm
assuming that it is as you say the rest of the unit seems to work normally.
You might be lucky, and it be no more than the transistor. I'm not sure how
you determined that the lamp is ok. These are normally a plasma discharge
device, that requires careful start, run and shutdown management, so have
their own power supply, sometimes referred to as the " ballast unit " by
some manufacturers. It runs under the control of the system micro.
Basically, a high voltage is put across the lamp to start the discharge, and
the current monitored. Once the plasma arc is established, the voltage is
dropped to a maintenance level of around 90v if I recall correctly.

Be sure that you don't touch the lamp face, as this can lead to black finger
marks forming. If you do get it going, and run it with the covers off and
safety over-rides defeated, do not look directly at the lamp, particularly
through the optics.

Arfa