Sony PVM color issue

[HatchedMango]

I have a Sony PVM 14M2U that looks perfect for the first 5 to 10 minutes it’s on. Then the screen slowly starts to turn more and more magenta (reduced green). If I turn it off for a minute and turn it back on it will look correct again but start to turn magenta (Edit: now that I look at it closer I think it's more purple) a little quicker than if I booted it cold.

It’s unrelated to input and affects the whole image evenly including the menu text.

I tested voltages with a multimeter on the neck board but the kg (green out to the actual tube) voltage isn’t changing with the change in the picture. It’s reading just under 140, not too different than the other colors. Am I correct in thinking this indicates the issue is either with the tube or the connection between the tube and neck board? Or does that voltage not necessarily indicate the green output from the neckboard is correct?

Any thoughts on what this could be? I’ve seen many examples online of completely missing or intermittent colors, but I haven’t seen anything exactly like this. I’m guessing my next step is to remove the neckboard and hope there is a contact issue with the tube. I assume if it’s the tube I’m out of luck.

[tongshadow]

When did you get it? This is textbook worn tube symptom.

[HatchedMango]

I got it a couple of years ago with this issue but I hadn't gotten around to figuring out what was wrong until now. Do you think something in the tube gets out of spec when heated up? It's interesting to me that the colors are spot-on for several minutes, I'd almost think a worn tube would have color issues right off the bat. I know very little about CRTs though, so my intuition on that is probably wrong.

[tongshadow]

These monitors have a self-regulating beam circuitry to keep the colors from drifting too much. As it gets warmer the monitor needs more emissions and that's when the cathodes dont have enough emissions for the green color. The green cathode is the most important one because green makes up for around 70% of the brightness.

So if I had to take a guess, as the cathodes heat up, the emissions arent enough to keep up with whatever the beam feedback circuit demands, so you get a pinkish picture due to the lack of green.

I would highly recommend using a tube tester to test for emissions before diving into the circuits.

[HatchedMango]

Thanks for the info. I'd need to get my hands on a tube tester. It sounds like something worth owning at the right price. Is that circuitry part of the tube or on one of the PCBs such as the neckboard? If it's on one of the PCBs, does it change the output to one of the RGB pins or one of the other pins?

[Josh128]

Probably not the same but does it kind of look like this? This is what a partially detached shadow mask looks like. When it warms, it changes color as purity drifts due to expanding mask. When it cools, its all back to normal, till it warms again.

https://www.youtube.com/watch?v=5l7eEcJTqcc

[HatchedMango]

No, I think it's different. I should take a video. It affects the entire screen evenly and ends up being pretty purple. There's no splotchyness to the screen at all.

[HatchedMango]

I do have a photo I took a while back. I intentionally turned the brightness knob up to show the color across the entire screen, and this is just the TV menu but you can see how it is even across the picture. I think the text itself looks more purple in person. When the monitor is just turned on this all looks completely grey and white without any tint. It's probably more purple than magenta if that matters.

[Josh128]

Interesting. So the background should be black, not purplish here? Seems like it could be some drift in the R or B cutoff levels? Does it carry over on a mostly lit image, say like SMW title screen? Drift or improperly set cutoffs will make blacks have a colored tint to them. Your whites still appear bright and I cant detect any burn in on this photo, so I would actually think the tube itself and the cathodes would be OK.

[HatchedMango]

Sorry, that image might be misleading. I turned the background (bright) up on the set so that you could see the uniformity. With the brightness knob set correctly, the background would be completely black. I should get my hands on a test pattern to maybe show a more useful image.

[HatchedMango]

It does carry over on bright images, it is just more pronounced in dark areas.