Swapped CRT and a ?

[Dan Starnes]

About 18 months ago I located a NOS 21XP4 Crt. Figured I would used a snowy cold saturday to swap it out from the old Crt that had grown tired. This is on my Admiral that I have had for several years. Took about an hour to get it all swapped out and when I went to test I was amazed at how well it was set. So I worked on centering and the like when I noticed I had forgotten to place a spring tensioned metal band on the neck of the Crt (pictured, and sorry cell phone pics) When I placed it on the new Crt the images just went to white. I had raster, took it back off and it worked again. The crazy thing is, if I had put it on before I tested the set I would have never known what the problem was. Question is, what is the band and what does it do? This set used to be my daily viewer and since it works so well again, just might be watching it a whole lot more.

[DaveWM]

Ion trap, perhaps not required on new CRT, used to prevent brown ion burns on non aluminized CRT's

[Phil Nelson]

You can experiment by sliding it forward and back on the CRT neck, and rotating it one way or the other. Eventually, you should find a "sweet spot" where it makes the picture brighter than any other position.

Phil

[wa2ise]

If I Recall Correctly (IIRC), you wouldn't even see a picture on the new CRT if it required an ion trap and you didn't have it on. The electron gun aims the electrons and ions downwards so they'd hit the inside of the CRT's neck. And as electrons are way less massive than ions, it's easy for a magnet to deflect the electrons and barely touch the ions. I don't know if they placed getter material at that spot of the neck's glass to trap the ions, to take them out of circulation.

[jeyurkon]

The 21XP4A is aluminized while the 21XP4 isn't.

The Tungsol data sheet says that they both use an external single field Ion Trap. The GE data sheet says that the 21XP4A uses an ion trap.

The fact that the screen went white and had a raster makes me think that you were just saturating the screen. I'd try the trap again and turn the brightness down, if you haven't tried that.

John

[andy]

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[jeyurkon]

Do you think that you can take a photo of the neck of the tube so we can see what the gun looks like?

John

[jr_tech]

Sometimes a re-builder would notice an aluminized screen and rebuild the tube with a straight gun rather than a bent one. They would usually put a note in the box stating that the ion trap magnet should not be installed on the rebuilt CRT. My guess is that you have a 21XP4-A (Aluminized) that has been rebuilt with a straight gun.

jr

[jeyurkon]

Originally Posted by jr_tech

Sometimes a re-builder would notice an aluminized screen and rebuild the tube with a straight gun rather than a bent one. They would usually put a note in the box stating that the ion trap magnet should not be installed on the rebuilt CRT. My guess is that you have a 21XP4-A (Aluminized) that has been rebuilt with a straight gun.

jr

That makes sense, but I'm puzzled by the fact that he still has a raster with the ion trap installed.

John

[jr_tech]

This is a pretty good puzzle... hopefully a picture of the gun will reveal if it is bent or straight.

jr

[Dan Starnes]

Hmm,, My puter is in the shop and I cant upload pics from my digital camera onto this laptop. I will take the back off this weekend and take a pic once I get my puter back. Should I get a pic of the gun from a 12 oclock position, or 9 oclock, or does it matter?

[andy]

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[jeyurkon]

Originally Posted by Dan Starnes

Hmm,, My puter is in the shop and I cant upload pics from my digital camera onto this laptop. I will take the back off this weekend and take a pic once I get my puter back. Should I get a pic of the gun from a 12 oclock position, or 9 oclock, or does it matter?

The angle of the photo does matter. If you look at the attached document you'll see a gap between two electrodes that is at an angle. That's what we'd be looking for.

When you said there was a raster, were there actually scan lines or just a glowing screen?

Andy's explanation of the ion trap is correct and if it is not adjusted correctly it can cause one of the grids to overheat as he said. It can actually melt a hole where there shouldn't be one or enlarge a hole resulting in a loss of ability to focus. See https://www.msu.edu/~yurkon/Document...adjustment.pdf

It may be as jr_tech suggested and that the tube is rebuilt and they put in a straight gun. That wouldn't have been the best idea. The Silver Screen "85" tubes were aluminized to increase brightness and not to prevent ion burn. The optimum thickness of the aluminum is different for increasing brightness and preventing charging effects, than it is for stopping ions. Though it will still help.

John

[Dan Starnes]

Faint scan lines but a very bright glowing screen. I did not really mess with it when it did that. I will not operate the set till I take the back off and try the ion trap properly on it.

[jeyurkon]

Originally Posted by Dan Starnes

Faint scan lines but a very bright glowing screen. I did not really mess with it when it did that. I will not operate the set till I take the back off and try the ion trap properly on it.

If it was really bright, then I suspect the ion trap might be necessary and other adjustments are needed. I know others don't agree, and I might be wrong.

When you look at it again it might also help to know what the CRT tube base says. If it's the name of a re-builder rather than a name like Tungsol, then the straight gun theory might be right.

John