Just how bad can an Ion burn be?
 

This bad.

Dumont 12JP4 installed in a Hoffman CT-800

Every 12JP4 I've had the misfortune of coming across has had this burn to some degree or another, this one is the worst.

It also had the worst emissions so probably a high hour tube, I cleaned it with the Beltron to get some brightness out of it, it really just made the spot look worse.
It's Green because it's a Hoffman.

http://videokarma.org/attachment.php...1&d=1491268776

A bit off topic, but...
...there is some strange bug in my brain that always causes me to, whenever I look quickly at a picture of a GE 801 TV, I always see it as having a big ion burn on the screen on the left.

https://s-media-cache-ak0.pinimg.com...2290efe8d4.jpg

Rip.

Thanks Dave, I needed a laugh.
:D

TV monster is bored. TV monster wonders what other living rooms are on. :D
http://www.videokarma.org/attachment...5&d=1491268752

Ion burn seems to be a recurring problem with DuMonts and Andreas.

For me, anyways.:smoke:

Ion burn is just a consequence of not having an ion trap or an aluminum backed screen. The ions are too heavy to be deflected along with the electrons, so you get a big nasty spot on any trap-less, non-aluminized magnetically deflected CRT. All 12JP4, 15AP4, 20BP4, 12AP4, and 9AP4s will burn sooner or later.

I seem to remember reading about an "X" shaped ion burn in a very good book by Robert G. Middleton, saying that it would happen on rectangular screen sets. I've never seen one nor ever heard anyone mention one before. Has anyone seen one?

Pretty, sure whoever you heard that from must have been mistaken. First off as Ben already said ions can't be deflected (too much mass), so the burn is limited by the scatter pattern of the electron gun. By the time rectangular CRTs were in consumer sets ion traps were standard, and aluminization was catching on...

Nope, nope.....had the book (sort of) handy. See?

Well, you can deflect an ion, just do it electrostatically. Charge predominates, and since an ion would have a magnitude of charge greater than or equal to the elementary charge, they'll zip right along with the electrons. This is why a great number of 7JP4s show decent emissions but look like garbage; they have a "whole-screen" ion burn.

Aluminization was becoming standard, the ion trap had been a standard for many years. In addition, I can't think of any mechanism that would lead to an X shape, aside from some element of the gun acting as a sort of crude, unwanted collimator for the ion beam.

That page from the book is puzzling.

Edit: If it is more likely on a rectangular tube, then it likely has something to do with electrostatic focusing or self focusing elements. Weren't the vast majority of the 1946 to circa 1950 round B/W tubes magnetic focus?

It's strange that Dumont, the Cadillac of Televisions, went with such an oddball CRT.

Not only is it not Aluminized, it's also made of thin glass and has a bulbous pre war shape, it's only advantages I can see is it's lighter, and they didn't seem too concerned about weight.

Others like Zenith and GE were using Aluminized tubes as least as far back as 1948. I wonder who had the Patent on that?

It's an oddball because Dumont made their own CRTs. My guess is Dumont was still using their pre-WWII (pyrex) tooling and methods for a while post war.

IIRC Dumont's CRT tech was state of the art pre-war (they had the biggest domestic production CRT 14"). Post war I think they were the first with a 20" CRT so they still lead, but as others made advances they were slow to latch on and mix in other's tech.

DuMont was building radar and CRO tubes for the war effort. Most of these tubes are just P4 versions of those same tubes. I have a collection of in-house datasheets with developmental numbers from '44 or '45. It's easy to match the 12JP4, 15AP4, and 20BP4 to the developmental types. They also pioneered the lackluster 7EP4 (rapidly displaced by the 7GP4/7JP4). Some of the developmental types died off however. It's interesting reading.

I must have had that book at one time. The page looks familiar and I too remember about "X" ion burns. I think it is because the ions are deflected some and they kind of lag behind the electron deflection, the sweep being rectangular. Maybe the rectangular tubes tended to have stronger fields since the deflection angles were greater because of the more recent design that the round.

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?

Or if the tube hasn't been used much, you should expect an incredibly small spot in the center.

My hunch right now is that the ions are scattering off some element of a particular gun type, and it's creating the characteristic X pattern shown in that manual. I'm not necessarily talking about 'static vs magnetic focus here, but rather some difference in physical construction. I've seen 10BP4s with very different looking guns that functioned identically. This gun type probably became more popular around the advent of the rectangular tube. Note that the page from the book doesn't exclude the possibility of a round tube developing the X pattern burn, it only says it's much more likely to be seen on a rectangular tube.

I'm not buying into a mis-adjusted ion trap hypothesis. A mis-adjusted trap would only allow the electron beam to overheat an element of a gun, perhaps burning a hole in it. I doubt very much that any trap magnet can produce the field strength necessary to redirect the ions toward the screen.

I did occur to me that the ions would be hitting the wall in a bent gun tube and since the ion trap magnet isn't strong enough to effect the ions much, they would never reach the screen. Even if the trap was mis-adjusted. I remember seeing a picture. maybe in that same book, of a gun element with a notch in the side of the aperture, produced by electron bombardment from a mis-adjusted trap.

Other than a picture (or artist conception) in one or several tv repair books that many of us have seen, has anybody here actually seen an "X" shaped ion burn in real life? Under what conditions?

jr

Is it possible that the phosphor damage in an X shape is really NOT an ion burn? I can't think of another reasonable explanation, but it wouldn't be the first time one of those repair books misattributed a symptom to the incorrect cause.

I have a CRT that had worked for a long time with a misaligned ion trap magnet. The result is a sickle-shaped ion burn:

http://i042.radikal.ru/1704/51/5a923caeaaa6.jpg

Although, it isn't very noticeable on a real picture:

http://s001.radikal.ru/i193/1704/5e/a08b00f80d0e.jpg

The bad thing is that the misdirected electron beam may bombard and overheat an element of the gun, causing release of gases from its surface and hurting the vacuum, thus even more ions are produced

Nope I can't see any ion burn in that picture. :D

This may answer why 7JP4 tubes were not rebuilt in the past (from what I have learned). If the phosphor is dead, it may be too costly to recoat on what, at the time was becoming an obsolete tube.
I guess I'm lucky with my 10 and 12 inch tubes. I see no ion burn on any, even the really high hour.
Would a double ion trap vs a single make a burn difference?

Yes you are, because the only thing responsible for ions is the quality and depth of the vacuum inside a CRT. That's why some straight-gun CRTs work for decades with no ion burn, while some others catch a horrible one in a year or two.
A fellow CRT rebuilder says that their shop has a very positive experience of rebuilding tubes with neither ion trap nor aluminizing. They use some modern, "very advanced" vacuum pumps, and ovenize CRTs very hard before evacuating.

CRT rebuilder?? Wow! Do they do any rebuilds for customers outside of the country?

jr

I would be very leary of a Russian rebuilder. Look at the "quality" of Russian small signal tubes...