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
-

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
| -
|
|
|
|

"Cobalt" wrote in message
news
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.


I wish it were that simple, but there are no pots on the PCB at all. All
there are is a lot of power transistors on heat sinks and many smaller
components, but nothing adjustable.
The problem occurs both hot and cold (at startup as well as after running
for several days non-stop.) I can't seem to locate it with hot/cold
application and it occurs too infrequently to reliably track down. But most
frequently, it occurs after coming out of standby mode. I had been using it
non stop for months after I disabled Windows standby for the monitor,
leaving the monitor in operating mode 24/7. But the problem started to
appear again this spring, even when not using standby features, so I'm
forced to deal with it now.

Couple of options:

I could put the FB back in the "replacement" monitor's motherboard and put
that it this monitor (mine is much newer than the "replacement" and MUCH
cleaner--like new in fact) and hope that no components on the older board
are worn or impending failure.
Thinking about the ringing problem in the video on the 'replacement' unit,
it may be confined to the module that contains the DB15 connector. It has
the RGB drive outs that feed the circuit board on the back of the tube.
Perhaps that module on the "replacement" monitor is poorly-compensated for
overshoot and if I use my input module, would still result in a good quality
display. (NEC did improve the video amplifier circuits after June 2000,
apparently, as my other two newer AS120's don't have the overshoot/ghost
images around small text and the overall display is sharper, more like an
LCD.)
Hopefully the motherboard has nothing to do with the ringing/text quality
and I can just replace it.

Other option is to change all the power transistors for the ones in the
"replacement" board. But the cycle of testing and the sheer amount of labor
(40-50 screws, & 2 hrs labor) to disassemble and swap a few transistor and
then test again, might be more than reasonable.

But the problem occurs more frequently when the monitor uses Standby and
comes out of standby, than if I just leave it running with no standby. There
has to be an answer in there somewhere....

--
Best Regards,

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

There is a small PCB that is edge-mounted near the rear of the motherboard
PCB and from what I can discern, it is some sort of High Voltage Detect
circuit. There is a pot on it, but I didn't notice it earlier because it was
completely covered with glue to seal it from adjustment attempts.

Not saying that I am even close to the problem, but this particular area is
looking mighty interesting... I tapped on that little PCB with the handle of
a small screwdriver, and the HV kicked off.

The next thing I did was try to remove some of the glue. It's not hot-melt
glue and it has a considerably resistance to heat---even from a 140W Weller
soldering gun, which I used as a heat source to try and melt off the glue. I
ended up chipping away at it a little. Just enough to free up the pot a
little. I got about 10º of rotation out of it, to the CW direction. And then
I powered up the monitor again. This time, tap on the PCB and it doesn't
affect it.

Next test was a 'sanity test'--rotate the pot back to where it was, and tap
on it while monitor running. And... the HV kicked off again. Rotate the pot
CW just a bit, power up and tap on it... doesn't kick off. 'Could be sheer
coincidence, but at the moment the notion of HV Detect being on this little
PCB and it's apparent sensitivity to vibration seems to suggest that this
board may have been tweaked to the edge of sanity, and just modest aging has
pushed the detection value over the threshold. So turning the pot moves us
back into moderate territory. The raster got a bit brighter too, so I turned
down the Screen pot on the FB to compensate.

Well it did kick off once, since playing with that adjustment, but
restarting it took drasticly less effort (1-2 cycles as opposed to 50 cycles
of off/on switching to get it to stay on.

I have a hunch this little PCB is an X-ray safety precaution. It probably is
there to shut down the HV if it becomes high enough to emit X-rays. So the
question is, why is the voltage excessive as to trip this safety mechanism?
Or perhaps the mechanism is falsely-triggering. I might take the other PCB
from the "parts" monitor and install it in this one and see what happens.
Killed an entire day trying to solve this bugger of a problem.


--
Best Regards,

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

I'm focusing all my attention on the plug in circuit board now. I downloaded
a data sheet for the chip that it contains. Apparently this is the PWM
horizontal deflection control circuit. So it is a VERY likely suspect.

Further testing has revealed that the pot setting (reference voltage) has
nothing to do with the problem. It controls the size of the raster.

I noted an intermittent mechanical issue though. Sometimes when I touch the
board with the back of a screwdriver, or even touch the pot in an attempt to
adjust it, the HV goes off. So I starting thinking "cold solder joints". The
board is mechanically unstable, being that it 'hangs' off the motherboard by
a tab. So I removed the motherboard and soldered all the connections from
this PCB to the main motherboard. Now I can tap, touch and adjust the pot on
it without the HV going off suddenly.

I'm testing further, but this ephiphany came about after I reassembled the
cabinet and the HV stopped working. The mechanical shifting of the chassis
could have disturbed the cold joint somewhere on the PCB connection.

This has been a deceptive, elusive problem, due to the intermittent nature
of the problem.


--
Best Regards,

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

Apparently, the soldering is not the problem with my AccuSync 120. Although
it behaved fine for two days (a record) after I redid the soldering on the
Horizontal PWM control board and it's connections to the mainboard, the
problem returned with a vengeance today about a minute after resuming from
standby. This is very frustrating, especially after my laptop PC started
acting up last night with all Admin Services disabling themselves for no
known reason. The list of broken equipment just keeps piling up while I
can't get any productive work done.

I decided to swap out the Horizontal PWM board entirely. I HOPE this is the
last of the problem. Another 75 minutes' labor to take out 60 some-odd
screws, remove cabinent, metal sub cabinet, video preamplifier, and
motherboard.

Keeping fingers crossed...