HELP: NEC AccuSync 120 HV Failure, but Not FB Transformer!

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.

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.

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.

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.

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...