Cool looking effect, I don't remember seeing anything like that before. Looks like RED and GREEN are maxed out in brightness, which probably means that video amp chip on neckboard or some related component had died, its so bright, that even retrace lines can be seen. Don't keep CRT powered on like that, reduce brightness if possible. Is that the best picture you can get, what action caused it?

When I adjust the horizontal position, the bright part goes to the side and text appears more clearly as blue.

Don't know what caused it. I bought it used. Seller said that it was working on his Amiga, mut I'm not aware how long ago. He agreed to refund but I'd like to find out first if it could be repaired.

I also tried to hook it into a X-Ray machine at work and it looks the same.

Is it normal that the lines are a bit exposed in the connector? Could some of them be touching eachother?

The grey wires are probably all ground (not a problem) if that's what you mean.
So much energy going to one side of the screen, maybe the tube is out of position.

I don't know too much about how CRTs work, but I've seen bad ICs cause weird stuff like this before. Was the monitor shipped to you? The neck board (or a similar component) could be loose.

(Edit: You'll likely hear this a lot, but never attempt to open a CRT-based display if you do not know what you are doing.)

that looks like a dead yoke to me, maybe some windings are shorting...

give it a good ol' degauss

oeuvre wrote:

give it a good ol' degauss

How? I’m so afraid opening these things, I’m leaving it to those who know what they’re doing 😜

I want one of those NEC multisync 3ds for all my vintage computing! Sorry to see yours is on the fritz, hope you get it sorted.

keropi wrote:

that looks like a dead yoke to me, maybe some windings are shorting...

It can't be caused by a bad yoke, monitor still displays raster, just in blue only.

From your second photo, it can be seen that the upper part of the picture is stretched, which indicates fault in vertical output section, stretch is caused either by dry joints or failed caps. Red and green color beams should overlap each other where blue text is, to form white color, but now they are simply uncontrolled, aka no R-G signals reach red and green electron guns. Check neckboard, something as simple as loose/corroded connections could cause that or in the worst case, failed video amp chip or amp transistors.