Video Amplifier Problem?

[Dave]

I have an NEC Acusync 50 that I believe has a video amplifier problem as the
symtoms best fit into this category. The monitor displays an image, but the
image displayed is very dark and can't be brightened with the contrast
settings. As some have suggested, the problem might be with the CRT, but I'd
like to note that only the image from the computer is dark and the on-screen
display panel still retains its normal brightness. As such this indicates to
me that the prolem is in the video amplifer and not the CRT. Is this
correct?

Being a bit inexperienced in monitor repair, I do not know exactly where to
find the video amplifier within the monitor, nor do I know how to test it
with my DMM to verify that it is working correctly or incorrectly. I
therefore either need a detailed schematic of the above monitor, or a repair
manual before I can attempt repair. If anyone has either of these available,
or is willing to work with me via digital pictures of the monitor, the help
would be greatly appreciated.

Regards,

Dave

 

[Chris]

All 3 colors? Although I'm not familiar with the model, I'd have to say that
it's pretty strange for all 3 amps to go bad in such a way and still
maintain some kind of color balance. I'd start looking for something dealing
with contrast or ABL. Is there any control of the contrast? Some ABL
circuits work by adjusting contrast. ABL is usually monitored on the low
side of the HV windings of the FBT so you might want to consider experienced
help. A leaky diode here will make the ABL think there's excessive CRT
current and cut off the contrast. It's a common problem with some Sony's a
few years old so why not NEC?

 

[Dave]

You may be right about the problem, but again I don't have much experience
in monitor repair. I did however find a blurb of information about my
particular problem on the FAQ but nothing to suggest where to really begin
testing the monitor for failure. The information that I found said that
since all three colors were equally affected that the problem was most
likely a power problem to the video amplifier. If all the amplifier circuits
shared the same power source then I could easily see this as being the cause
of the problem. Again though, I need someone with a bit more experience to
help me with the repair.

Regards,

Dave

 

[Tom MacIntyre]

You may be right about the problem, but again I don't have much experience
in monitor repair. I did however find a blurb of information about my
particular problem on the FAQ but nothing to suggest where to really begin
testing the monitor for failure. The information that I found said that
since all three colors were equally affected that the problem was most
likely a power problem to the video amplifier. If all the amplifier circuits
shared the same power source then I could easily see this as being the cause
of the problem. Again though, I need someone with a bit more experience to
help me with the repair.

Regards,

Dave

The CRT filament voltage may be low.

Tom

 

[Tom MacIntyre]

The CRT filament voltage may be low.

Tom

Of course this is incorrect, as the OSD is fine. Ooops...

Tom