color problem with IBM p70 workstation monitor

[PgrAm]

Hey I recently got a nice IBM workstation monitor (a sony trinitron underneath) but it is having some issues displaying color correctly. When I feed in a VGA signal via the included 13w3 adapter it fails to display properly. Here is the result:



If I disconnect the red channel from the input I get a much more normal image, minus the red channel of course (ignore the dark bars, photo exposure issues)



Any idea whats going on? I already disassembled the monitor, inspected the board traces and did not see any obvious damage or bulging capacitors, already tried cleaning up and reseating the tube contacts. Maybe someone with CRT repair experience can give me some insight on what to check?

 

[PgrAm]

I did some more research and poking around and I'm starting to suspect the issue is a Red-Cathode to grid short, but I cant seem to measure such a short on the tube itself, though the issue seems consistent with my symptoms (one gun affecting the others, causing inverted colors). Tried tapping on the tube to dislodge contaminants but no luck yet.

 

[iz8dwf]

I did some more research and poking around and I'm starting to suspect the issue is a Red-Cathode to grid short, but I cant seem to measure such a short on the tube itself, though the issue seems consistent with my symptoms (one gun affecting the others, causing inverted colors). Tried tapping on the tube to dislodge contaminants but no luck yet.

If you suspect such a short, and you can't measure it when the tube is cold, then just leave only the heater connection to the tube, let it warm and try to measure the short. If you still can't find a short, in my experience that's not the problem then.
I would look at the red signal entering the tube socket's PCB and how it looks after the amplifier stage if you don't have an internal tube short.

Frank IZ8DWF

 

[PgrAm]

I hooked 6.3v to the heater and measured again, can't seem to find and significant variation in resistance with the red cathode vs the green and blue. So I guess there isn't a short on the tube so I'll be looking at the neck board for issues. I've never repaired a CRT before (I'm used to working with audio circuits, including tubes) so I'm not sure what I'm looking for, should I just hook up my scope to the tube socket and look for distortion? What kind of voltages might I expect to see at the red cathode?

I'm pretty sure the caps are still good, they're all high quality Panasonics and nichicons, and comparatively not that old.

 

[iz8dwf]

I hooked 6.3v to the heater and measured again, can't seem to find and significant variation in resistance with the red cathode vs the green and blue. So I guess there isn't a short on the tube so I'll be looking at the neck board for issues. I've never repaired a CRT before (I'm used to working with audio circuits, including tubes) so I'm not sure what I'm looking for, should I just hook up my scope to the tube socket and look for distortion? What kind of voltages might I expect to see at the red cathode?

I'm pretty sure the caps are still good, they're all high quality Panasonics and nichicons, and comparatively not that old.

I suggest that you try at least getting a schematic for it if it's your first CRT monitor repair. Voltages can be high, even in the thousand volt range, depending how they did polarize the cathode and the control grid. But well, yes, I'd use a scope on the color signals entering the socket PCB, with
a completely Red (then Blue, then Green) image and see how they go through the drive circuits.
As for capacitors, I would agree they're mostly ok, but I always take the time to check ESR. Also check BJTs and diodes junctions with the unit powered off.

Frank

 

[GiGaBiTe]

Have you ruled out the signal cable? I've had a few monitors where the signal cable was damaged internally from being bent at too sharp of an angle, causing various signals to short together and the monitor does strange things like this.