On Jan 22, 11:09*pm, wrote:
Arfa is dead on about that resistor. The source resistor is about the
same thing as an emitter resistor to a bipolar. There is also usually
a zener from gate to source, it may be shorted and if it is the drive
circuitry might be fried. In fact if the source resistor is open the
drive circuitry is usually fried.

That's why we change fuses LAST.

Now as for a replacement. This is a switching FET. You must have the
same or better gain in case it is a self oscillator, switching times
must be faster or at least equal, not knowing what frequency it is
supposed to run at. That is after all the maximum ratings have been
met or exceeded.

Do not change case styles, especially to a metal tabbed one. It is not
only a pain in the ass but might be illegal. If you are doing this in
your basement it is unsafe. This is a hot side component. If it is all
plastic and you get the insulator kit, and put in a non all plastric
device, there could be an insidious shock hazard. You certainly blow
the UL or CSA or whatever rating, and if this is done at an ISO
certified company it is so close to illegal you can smell it. It would
certainly be grounds for dismissal. So if it is plastic, stick with
plastic.

Just remember, after all the Id, Vd all that max, even if the
replacement device meets that criteria there is still transconductance
and switching time. If you have higher gain and faster switching time
you should be alright. Of course use the right type of transistor.
Some are optimized for switching, you don't want the ones that are
optimized to be used in the linear mode, such as an audio output.

There is a safety factor, mainly keep the same case style. Other than
that you can always upgrade a semiconductor device (in the US) because
of a mandate in 1976 that safety of a product cannot depend on the
failure of a semiconductor device.

There were TVs with absolutely no high voltage shutdown circuits at
all, the idea was, well the horizontal output will fail before it gets
that high. Well some didn't.

Just recently I had a dog case where the other tech replaced a damper
diode. This application requires a 1200 V doide. He put in a 400 V
diode, and the thing lasted for like a month. He must have read the
numbers wrong or something, and then by chance got a diode that was
able to take it. When it came back he replaced the diode again and it
failed immediately. So did another.

I finally had a look, saw where it was and said wait a minute, you
can't.......I went to the computer and found out for sure, it was the
wrong diode.

But you can't count on it failing, that is the rule.

Check that continuity from the source to the negative end of the main
filter, then make sure it reads open circuit, or close, from source to
gate, positive probe on the gate. Follow the foil from the gate and
there will be a resistor, make sure that is not open, and check where
it comes from for a shorted condition between it and hot ground or the
Vcc or Vdd of the chip. It a transistor check if it is shorted.

Also with as much unplugged as you can unplug, check all the
rectifiers on the seconday side for shorts. Many times the current
limiting works fine when one of the output voltages is overloaded, but
I have seen more than one design where if one of those rectifiers is
shorted it does not work, letting it fry another chopper transistor.
Happened more than once, so it is worth checking.

At any rate, have fun with it. Good luck.

Also I would check anything that drives an optocoupler. Alot of times
that fails and leads to overvoltage, frying it. Run your eyes down the
foils from the pins, if there are any electrolytic caps connected,
check them.

After all of this, you might not have a fireworks display. I would
suggest firing it up on a variac, but there are alot of designs that
simply will not start that way, so forget it.

JURB

Thanks for the great info - I learned a lot. Cheers.