Sounds like blooming/breathing of the shadow mask caused by localized heating from the electron beam, distorting its shape thus affecting color purity. Try lowering the brightness/contrast a good amount and see if the problem is delayed or eliminated.

If the degaussing circuitry were faulty the color would be severely messed up and a simple power cycle wouldn't correct it.

You could build a degaussing coil and try that. Here's one in particular, but BEWARE as I haven't tried it and you could kill yourself. http://www.staffordnet.net/degausscoil.htm

It definitely relates to magnetism of the tube. A while back I held a strong magnet about 20cm away from the screen and it corrected the colour error. However, after removing the magnet, it got worse. I think the monitors perfom some sort of basic degaussing on power up, which is why a power cycle helps to correct the issue. The monitor is packed away at the moment, but if it helps I could take a photo.

Open it up and see if there are any coils in the corners around the CRT. Some monitors have these to control colour purity for each individual corner. If that's the case with yours, the circuit that drives these coils may be busted.

jwt27 wrote:

Open it up and see if there are any coils in the corners around the CRT. Some monitors have these to control colour purity for each individual corner. If that's the case with yours, the circuit that drives these coils may be busted.

I'll have to open it and check, but I do vaguely recall there being some spring-like objects or maybe a thick wire at the corners of the screen. I thought they had something to do with degaussing.

If this is the problem, is there an easy way to fix it?

I usually do the following:

1- Turn on the PC and the monitor.
2- Wait until the defect becoems visible.
3- Play around the monitor with some unshielded speakers from an old stereo until it desapears. Magnets of any kind work too.

I don't know if this ruins the monitor, only needed to do it once, so...

TELEPACMAN wrote:

I usually do the following: […]
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I usually do the following:

1- Turn on the PC and the monitor.
2- Wait until the defect becoems visible.
3- Play around the monitor with some unshielded speakers from an old stereo until it desapears. Magnets of any kind work too.

I don't know if this ruins the monitor, only needed to do it once, so...

I've tried that before. with a small magnet. it corrects the issue for a while, then it becomes worse. It's like there's something inside the monitor is magnetizing the screen.

As I've said before, there's a problem caused when the electron beam is too 'hot', it causes the shadow mask inside the tube to heat up in certain spots (usually where the brightest parts of the image are), making it physically expand, distorting its shape and messing up color purity in the area, when the stimulus is reduced or removed it cools down, returning things to normal. This phenomenon is known as blooming or breathing of the shadow mask. To check for blooming lower the brightness and contrast (or even the G2 voltage) by a significant amount and see if the problem is delayed in its onset or eliminated.

Since it's very likely that there are no electronics at all near the top right end of the picture tube I'm inclined to think that the mounting point for the shadow mask in that particular corner is being overly temperature sensitive and thus being warped by the 'heat' from the electron beam passing over it. This could very well be a manufacturing defect or caused by physical trauma to the CRT.

If this is the case the only solution would be to reduce beam intensity until the problem is minimized, which might or might not yield enough screen brightness to be usable.

Just checked google and I can't find any monitors with these coils I mentioned. One of my Philips monitors does have them though, they're wrapped up together with the degauss coil in black tape, and you can adjust corner purity with them through the OSD menu.

If the shadow mask is loose in that corner, there's no way to fix it properly. But if you can get the colours right using a magnet, maybe you could "fix" it by glueing a weak magnet to the CRT in the right spot...? 😀

What kind of monitor is it, anyway? Does it have a shadow mask or aperture grille, digital or analog controls, flat or curved screen, etc?

It's a cheapish monitor, digital controls, no OSD, no special features. Will probably open it up and take some photos and perhaps you guys can help. If not, It should be good as spares for the other 2.

CRT's are easy to get hold of here. You can get a good used one for $5.00. The only reason I'm bothering to try and do anything about this one is that it's brand new.