I was given a P220f that had a blown LM2412 video driver (dead short
from Vcc to Ground). One of the transistors just in front of it was also
blown. Replacing those and cranking the screen control around produced a
picture that was washed out and overloaded, but all three colors were
there.

In the process of trying to find out what else was wrong with it, the
replacement LM2412 failed in exactly the same way -- sudden dead short,
letting the magic smoke out -- and I wasn't even poking around the video
at the time, I was plugging the monitor cable into the computer. I guess
it's possible that the "transient" from application of video is what did
the deed.

I suspect that the LM2412 failures are somehow related to whatever the
real problem is, but without a schematic, I'm reluctant to keep on
feeding the thing 2412s in order to find out.

Any suggestions, or, can anyone provide a schematic of the video module?

thanks, Isaac

"Isaac Wingfield" wrote in message
...
I was given a P220f that had a blown LM2412 video driver (dead short
from Vcc to Ground). One of the transistors just in front of it was also
blown. Replacing those and cranking the screen control around produced a
picture that was washed out and overloaded, but all three colors were
there.

In the process of trying to find out what else was wrong with it, the
replacement LM2412 failed in exactly the same way -- sudden dead short,
letting the magic smoke out -- and I wasn't even poking around the video
at the time, I was plugging the monitor cable into the computer. I guess
it's possible that the "transient" from application of video is what did
the deed.

I suspect that the LM2412 failures are somehow related to whatever the
real problem is, but without a schematic, I'm reluctant to keep on
feeding the thing 2412s in order to find out.

Any suggestions, or, can anyone provide a schematic of the video module?

thanks, Isaac

Replace the 4N35 optoisolator in the power supply, it's likely the voltages
are creeping up, usually this blows out the vertical output chip but I've
seen other things fry.

See if you can find the datasheet on the video amp IC on the CRT neck
board, Then check the voltage on the Contrast pin. Most are in the
range of 3.5 to 4.0 volts. If yours is way less and you cannot increase
it by using the Contrast control, I suggest you experiment by manually
increasing the contrast voltage.

What I do is to locate a source of 5 or 12 volts on the board, connect
a variable resistor box between the voltage supply and the contrast
pin, then gradually decrease the resistance (increasing the contrast
voltage) until you get a good picture. Monitor the contrast voltage as
you do this. Most IC's peak out at about 4 volts.
Once you have a good picture, solder in a small fixed resistor in
place of the variable one.
Hope this helps....
John