A while back, I posted about a high voltage failure problem on an NEC AS120
monitor.

The symptoms were as follows:

An audible snap, followed by the sound of high voltage bleeding off.
Display would go blurry and grow in size as it fades out.


Most suggestions pointed to the flyback transformer.

I had a hunch it was a failing transistor or some current limiter circuit on
the edge.

But I went and picked up an expendable AS120 on eBay for cheap. And I
checked it out for a day. No functional problems, except that I didn't like
the quality of the video analog circuits on the earlier mfg date. It had
some ringing in the signal which made it look like cheap TV with the
sharpness turned up too high.

So I transplanted the flyback transformer.

Powered up the monitor with the known good FB and let it run about a minute,
when... snap! and the raster went out, just as it was doing with the
original FB.

At this point, I think it's got to be the horizontal drive transistor, or
maybe a current limiting circuit somewhere, but I am not sure. I was going
to swap the whole motherboard, but for the poor video quality issue with the
oversharpening on that earlier rev video drive circuit.

My question is this: since the FB is good, what else is likely to be causing
the HV to trip off like this?

I could swap out the power devices surrounding the FB with those from the
replacement, but I want to make sure I don't overlook anything that I didn't
think of. (It takes quite a while to tear down the monitor to get at the
motherboard.)

Additional info, if it might provide a clue:

When it 'trips off', I can bring it back by cycling the power off/on about
20-50 times. At some point, the monitor will just remain on. And sometimes,
if I never turn it off, it will remain operational for days, even weeks
without an incident. And then one day it trips off, usually after I've shut
down the computer and turned off the monitor for a while prior. Then when I
power it all back on, I sometimes get 5 minutes' use and the HV trips off.
Monitor is still powered on, just no HV.
Cycle power off/on up to 60 times and at some point it latches on and stays
on and I can get some work done.

The monitor is clean inside & out; there is no dust causing shorts or arcs,
so contamination is not a factor. I also cleaned the replacement FB with
acetone and dried it throroughly before installing.




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Best Regards,

Mark A. Weiss, P.E.
www.mwcomms.com
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