a lot of monitors have a small trimpot neat the fbt that sets the
overvoltage trip point - it could need a tweak.
there is also sometimes another trimpot nearby to adjust the hv output ,
this could also simply need a tweak.

whenever you tweak a pot, mark the original position with a sharpie so you
can align it back to the original position in case you adjust the wrong one.

"Mark & Mary Ann Weiss" wrote in message
news |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.
|
|
|
|
| --
| Best Regards,
|
| Mark A. Weiss, P.E.
| www.mwcomms.com
| -
|
|
|
|