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.

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

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

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.

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

...

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

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

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

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

jr

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?

...

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/Documents/ion_trap_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

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.

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

I put the ion trap on and messed with it. Worked well at the 12 oclock position and slid all the way back, so it is there. Would not work any farther forward. Tv is working very well. And I forgot to see what the make of the CRT was dangit. Another cell phone pic, my digital was not charged.

I put the ion trap on and messed with it. Worked well at the 12 oclock position and slid all the way back, so it is there.

Does it work any better than if it's not used at all? If it doesn't work any better than when it's not used, then it's not needed and you should take it off.

The centering is better, could not really get to where I thought it should be before the ion trap. Also prior to the ion trap installed I noticed a slight cone head in some scenes, now that is gone and my height control is in the middle of its setting. Basically I had to adjust the centering, height and vertical, and I have a very very nice picture.

I guess I have to throw my hands up in the air. I can't tell from the photo which type of gun it is.

There are two possible orientations for the trap. It might have markings on it indicating which side should be facing forward, or it might not. If it doesn't you might have to try both orientations.

Here is one datasheet for that CRT. http://tubedata.milbert.com/sheets/093/2/21XP4A.pdf

The fact that the centering is easier is encouraging. Maybe someone else is more familiar with 21XP4A guns.

John

It looks like your crt gun has an electrostatic focusing element-did they make any bent gun tubes with electrostatic focus? I thought ion traps went out of style around the same time magnetic focus did.

Hopefully the last of my unhelpful comments.

This would be simple if we knew the CRT wasn't rebuilt because it would almost certainly require an ion trap. It's more complicated because a re-builder could have used a straight gun.

1. It could be a straight gun and the trap isn't required and only works when the trap is mounted out of the way. The washed out images were due to scattering off of an electrode.

2. It could be a bent gun and the trap is required. Having a raster without the trap would require that the CRT had been operated at high brightness without the trap and the beam then burned a new exit hole. Unfortunately, this is a possibility. With the trap in place it works better, but if there is an exit for the electron beam without the trap, the ions will still be able to exit that way with or without the trap.

There's still the possibility that you have the trap flipped the wrong way, but that doesn't change above scenarios.

leadike: It is electrostatic focus.

John

It looks like your crt gun has an electrostatic focusing element-did they make any bent gun tubes with electrostatic focus? I thought ion traps went out of style around the same time magnetic focus did.

I have seen bent guns with electrostatic focus... an example is this DuMont 14SP4 that I just replaced in a small DuMont portable from 1956 (or so). This gun has a combination of a VERY bent gun AND electrostatic focus. The gun looks like it was made in two different eras, ceramic supports and an older looking design at one end, and multiform rods and electrostatic lens at the other! The gun had the worst looking welds that I have seen in a CRT and was indeed OPEN at the cathode sleeve weld. The weld at the 1st anode (shown in the picture) was quite carbonized and fell apart when I touched it! :thumbsdn:

jr

...

Maybe I missed it but I think no one has mentioned something that might be important about the placement of the ion trap. The owner of the TV shop where I worked after high school had me pencil-draw an arrow on the ion trap pointing forward toward the screen before removing it.

If you get the trap on backwards, this makes the magnetic polarity wrong and the trap won't operate correctly. Don't know if this is what's happening here, but thought I'd mention it.

Hope this helps, Cliff

Thankyou all for the very helpful info. I have learned alot. The gun is straight and sorry for the low quality cell phone pic. I did turn the ion trap around before being satisfied, but no difference. When I was messing with the ion trap I could make the picture bend and in other positions it would not make a good picture. It did result in a sweet spot where it is now located. The tv has never performed better, picture quality wise, and I cannot complain at all about quality. I am picky when it comes to sound and picture quality.

It looks like your crt gun has an electrostatic focusing element-did they make any bent gun tubes with electrostatic focus? I thought ion traps went out of style around the same time magnetic focus did.

The 21ALP4A in my Viking (Eaton's of Canada brand made by Electrohome)
is electrostatic focus with ion trap. TV is from 1956 as far as I can tell.

...

That is a straight gun with the focus hooked to G2 internally, remove the ion trap and leave it off.

Thankyou all for the very helpful info. I have learned alot. The gun is straight and sorry for the low quality cell phone pic. I did turn the ion trap around before being satisfied, but no difference. When I was messing with the ion trap I could make the picture bend and in other positions it would not make a good picture. It did result in a sweet spot where it is now located. The tv has never performed better, picture quality wise, and I cannot complain at all about quality. I am picky when it comes to sound and picture quality.

I'm not sure I understand. Did you end up leaving the ion trap on or off? In either case, I'm glad it's performing well for you.

John

That is a straight gun with the focus hooked to G2 internally, remove the ion trap and leave it off.

Which CRT are you referring to? That doesn't seem to match the data sheet for any of the CRTs mentioned here, unless I missed one.

John

This reminds me of our discussion about the CRT and ion trap in my DuMont RA-103.

http://videokarma.org/showthread.php?t=246335&highlight=ion+trap

Executive summary: The CRT looked like the type that doesn't need an ion trap, but it made no picture without it. With the ion trap in the sweet spot, it looks fantastic, so I'm keepin' it :)

Phil "whatever works" Nelson

Yes, Phil.. seems we both experienced the same thing. And the ion trap is on mine.

It also reminds me of a joke an attorney friend told me. "If you want a dozen different opinions, ask a dozen attorneys."

You weren't lacking for opinions from us. ;)

John

Which CRT are you referring to? That doesn't seem to match the data sheet for any of the CRTs mentioned here, unless I missed one.

John

I am referring to the photo he provided.

Most data sheets on consumer CRT's used in the early 60's were never updated. The industry was moving so fast then, paper ended up in the incinerators.

I am referring to the photo he provided.

Most data sheets on consumer CRT's used in the early 60's were never updated. The industry was moving so fast then, paper ended up in the incinerators.

So, are you saying that a CRT factory could have produced a 21XP4 with a straight gun even though such a beast can't be found in any tube manual?.... I still suspect that this tube was re-built, and a straight gun was installed at that time.

jr