No ions are being deflected by an appreciable amount in an electromagnetic deflection tube, period, unless an electrostatic focusing element is screwing with them.

A proton is about 1836 times as massive as an electron. (An electron is about 0.511 MeV over the speed of light squared, a proton is about 938 MeV over the speed of light squared, a neutron is about 939 MeV over the speed of light squared.)

I'm not sure which ions you would expect to find in a CRT, but let's assume it's something relatively light, comparable to an alpha particle at a mass of 3.727 GeV per speed of light squared. Now we're talking about objects on the order of 7294 times as massive as the lowly electron. Electromagnetic deflection from a TV yoke isn't going to do jack to a proton or neutron, and certainly not something similar in rest mass to an alpha particle.

I've heard the theory about Ions ruining 7JP4's before, maybe theoretically possible but I have my doubts that it happens in practice.

I've had plenty of tired 7JP4's, but they pretty much were all bad the same way, very dim and going negative at higher brightness.

Maybe I've just never come across one with a burned screen that still had good emissions since high hours and burned screen would go hand in hand.

Still I would expect an Ion burned tube to be burned only where the image was, in other words if you increased the scanning height or width it should be okay at the edges.

I have seen one 10BP4 (or maybe it was a 12LP4) where you could tell where it had been scanned most of it's life, if you rotated the yoke you could tell the difference in the color of the phosphor. I'm guessing that was simply worn phosphors and not Ion burn.

Ion bombardment definitely does do damage to the screen. Whether you notice it is a different story. Since the ions are deflected as well, the damage that would normally be a big nasty dot in the center of the screen is spread out over the whole screen. To get adequate brightness out of a damaged phosphor back you crank up the controls. Of course the cathode is wearing simultaneously as well, so eventually, the pairing of a damaged phosphor and a weak cathode gives the tell-tale shimmering, and eventually negative, picture. If you could isolate the effect of the ion bombardment though, let's say with some super long lasting cathode material, you would see detriment to the screen.

You're also correct in saying that on a 7JP4 you would expect to see the ion damage in the region of the previously scanned raster.

On the 10BP4 or 12LP4, it was assuredly damage from the electrons themselves; the ions were shot off into the wall of the neck. Remember that an electron in the region of ~2 keV in a lowly 7 inch Transvision to ~75 keV in some projection sets can and will do incremental damage to the phosphor. Ever seen a burned 5TP4? That was done by a beam of electrons at 25-27 keV over some extended period of time.

You don't suppose that the anode voltage in conjunction with the shape of the CRT envelope has some influence on the ion defection pattern? There's got to be some electrostatic effect involved between the anode and ions.

It's possible, but then why the corners? The whole bulb needs dag, inside and out, and all of the exterior dag should be at ground, and the interior at whatever your ultor voltage is, say 20 kV for a big glass 27 inch rectagular tube.

There's only one solution: when CRT rebuilding gets off the ground, have Nick rebuild an un-aluminized 17 incher with a straight magnetic focus gun, and run that S.O.B into the ground.
Then rebuild a second, identical bulb, with a straight electrostatic focusing gun. Run it into the ground.

I strongly suspect the 'static focus gun will show the X and the magnetic gun just a spot...

The difference in mass/charge ratio between the electron amd any ion is just too great for a magnetic explanation to make sense IMO.

Have we considered ion production in the space between the deflection yoke and the screen? For sure, there were a number of un-aluminized rectangular crts produced in the 50s that had an ion trap, but the screen possibly could have been bombarded by ions produced past the electron gun. :scratch2:

jr

What's going to produce the ions? Are we assuming that the electron beam interacts with residual gas, and ionizes it post-deflection? That's an interesting angle, but I don't see how the X pattern results...

All of the ions in the tube would be produced by residual gas interacting with the electron beam, so I'm guessing helium and hydrogen nuclei are the most likely ions encountered. They're the lightest gases; most likely to "sneak" in through a leak. There might be some diatomic oxygen, diatomic nitrogen, and a bit of carbon dioxide left in the tube as well that could cause issues.

Yes, but I don't have a clue as to how the x shape might be produced.

jr

Perhaps a bit of argon, as well?

.

I was thinking the geometry of the envelope could have some influence on the distributed static field. A round envelope would seem to have the most even distribution.

I have a non-aluminized rectangular CRT that was rebuilt with a straight gun. It has the usual round ion burn.

https://c1.staticflickr.com/8/7391/1...ffa09ab9a4.jpg

Bob,
The rebuilt rectangular tube is magnetic focus, no?

There were also straight guns with diagonal-cut(slash field) type of ion traps, so you can't assume they were not ion trap guns strictly by the lack of bend in the gun. But that one Bob has apparently was not rebuilt correctly.

The 10BP4 in my RCA T-100 looked like that. The scanned area was visibly browner (or less blue) than the portion that was never scanned. Looked weird until I put the set back together and rotated the CRT to match the brown area to the CRT mask. In the second photo, I taped the CRT to mark the edges of those zones, which were more obvious to the eye than in photos.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

http://antiqueradio.org/art/RCAT-100PurpleCRT01.jpg

http://antiqueradio.org/art/RCAT-100PurpleCRT04.jpg

Ah, electrostatic. I hadn't thought about that. I guess that would account for the small spot?