How Come Early CRTs contained more Ions?
 

...thus requiring an ion trap on the tube neck? It's my understanding that later crt's contained few or no ions. I don't understand why this is. :scratch2:

Ions are atoms of material off the cathode that ride along the electron beam. Electrons are deflected by magnetism, but ions aren't. Older tubes had their electron guns "bent" so they didn't shoot straight. The ion trap had magnet(s) that bent the beam back straight during operation of the tube. Since ions aren't bent by magnetism, they went off at the preset angle and impacted the side of the tube neck, doing no damage. Newer tubes have the electroluminescent phosphor coated on the back side with a thin coating of aluminum, which deflects the ions. Their electron guns are aimed straight at the screen, so no ion trap is required. Someone else may give a more technical explanation.

My 2cents (or less) worth

I believe the biggest change was aluminizing the screen, as the aluminum layer reduced the ion impact on the screen. This may not have been 100% effective, but it helped a lot. Ion trap use started to diminish when aluminized screens became prominent.

Other improvements to reduce the number of ions include:
1. Getter improvement from SAES and other suppliers.
2. Funnel flashed (rather then neck flashed) getters to provide more surface area of the getter flash.
3. Pump cycle improvements, improved diffusion pump "oils".
4. Improved RF processing (Bombing) while the crts are being pumped.
5. Improved raw materials/cleaning methods.

jr

I think jr pretty much covered it.

But, not resisting the urge to be pedantic :blah:; the ions are not from material off the surface of the cathode. They are produced from the residual gas in the CRT. It's impossible to achieve a perfect vacuum. The majority of the ions are produced when the electron beam hits a gas molecule. Some may be produced by the heat of the cathode.

Ions are bent in a magnetic field. Just not much. The force acting on them is the same as for an electron of the same velocity, but they have many thousand times the mass of an electron and are thus deflected less.

Electroluminescent has a different meaning than would be used in a CRT, even though the same material may be used. I don't completely agree with the wikipedia definition either. Most sources define it as a phosphor being excited by an alternating electric field or current.

Pumping speed can only buy you so much. The RF induction heating that was mentioned is an important element. Even in a very high vacuum the materials used in a CRT will only outgas at a certain rate unless they are heating to help drive off the gasses. The heating cycle of the envelope is important also.

Which translate to time and money. CRT manufacturers don't want a product to die so quickly that they get a bad name, but they could probably care less whether it dies in 5 years or 50 years of use.

The getter flash helps them to minimize how well they have to outgas the CRT.

This guy avoids having to due a getter flash for a vacuum tube by seriously degassing the tube before sealing it.

http://paillard.claude.free.fr/

O.K. I'll be quiet...

John

Wow, great replies. Thanks. So it is due to the gas inside the crt...I had a gut feeling it might be related to that, being that vacuum pumps probably got better as time went by...My oldest b/w sets have the ion traps in the neck but I never questioned why they did. No wonder why they can make or break the picture. On my Admiral, even the slightest adjustment hurts the contrast ratio.

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I've understand that the ION Magnet should be adjusted to the brightest image. I backed mine off slightly from maximum brightness to eliminate shadows on the edge of the picture tube. I assume this is OK considering that the ions still will not hit the phosphorous because the electron gun shoots out deflected?

Then I have heard that if you don't adjust the ION magnet just right, it will mark the center of the picture tube.

Comments?

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Possibly you could re-position the yoke slightly to obtain reduce the "neck shadow" and then obtain a brighter picture by re-adjusting the ion trap to its optimum position, but I don't think that a slight mis-adjustment of the trap magnet will allow ions to strike the screen. A VERY MUCH stronger magnet would be required to bend the ion beam enough to travel straight down the gun.
BTW The screen is coated with "phosphor*" not the element phosphorous, many confuse the two terms.

*For example, "white" P4 Phosphor is actually a mix of "blue" Zinc Sulfide (silver and zinc doped) and "Yellow" Cadmium Sulfide (silver doped).

jr

It's true that the ions won't hit, but perhaps he's referring to the effect in the attached snippet of an article on ion-traps.

The electron beam can sputter material from the gun electrode which will damage the screen.

My 10MP4 has a small nickel sized area that appears to have been damaged in this way. It's barely noticeable, but there.

John

Thanks. :yes: I keep having people with degrees as high as PhD's ask me to coat viewers with Red Phosphorus when they mean Red Phosphor. They think I'm strange for expecting them to know that much chemistry. I'm the one who should be confused, chemistry was my worst subject.

Yeah, it's like "shock absorber" which in reality is the rebound damper. The spring is what absorbs shock. Bill(oc)

I noticed in the Zenith Service Manual for my porthole, it's indeed listed as beam bender instead of ion trap.

Same for my Sylvania. They list it as a beam bender, though in a few places call it an ion-trap.

John

The "phosphorous" vs "phosphor" thing reminds of of the old George Carlin skit: "it's not a HOT water heater; it's just a water heater!!!!" :D I shouldn't laugh though: I'm no chemist. Phosphor compositions & associated jargon confuse me pretty quickly.

So has anyone seen a tube where the beam-bend was so far off that it melted the grids?

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Aw, gee, Andy-now I gotta try that! Pity the next broken-down 80s set that comes my way...

Well I was talking about melted guns/grids myself...I've seen burnt phosphor. When I was trying my hand at a DIY oscilloscope I burnt plenty of it. I remember a Panasonic 12" b/w SS set from around '76, and a Mexican-made 13" Zenith knob tuner from 1988. Without the yoke it doesn't take much to ruin that center spot!!!

Loss of difflection in projection sets will cause CRT damage (x3) nearly instantaneously. Some have built-in safeguards.

I recall reading in a very old (circa 1950s) television repair manual that misadjustment of the ion trap can also cause ion burns, some of which were very severe. The manual had a picture of a CRT screen that had suffered a very large, circular ion burn, and also a photograph of another screen that had an ion burn in the shape of the letter X. The manual stated that the round, large ion burn was more common in round CRTs, while the X-shaped burn appeared mostly in rectangular tubes.

The bright white line that appears across the CRT screen when vertical deflection is lost will burn a line directly across the middle of the tube, if the brightness isn't turned down to zero--fast. However, I have often wondered about the line that shows on a color CRT when the service switch is on (in the service position), which of course kills the vertical sweep. Is that not a potential danger to the CRT phosphors? I would think it would be. The only way I can imagine this not to be a danger to the screen is if the brightness level of the setup line is very low (just bright enough to see), and there is no way to increase or decrease the brightness of that line with the TV's own brightness control.

"I recall reading in a very old (circa 1950s) television repair manual that misadjustment of the ion trap can also cause ion burns, some of which were very severe."

Not everything that is written in these old service books is true. Go back and look at posts # 8,9, and 10 of this thread.

jr

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I wonder if someone once called it the "ion-trap magnet" (as in "the magnet associated with the ion trap"), and that quickly got shortened to "ion trap?"

(Just as people refer shortly to a transistor radio as a "transistor.")

If the water heater is bought from a reputable dealer it is a perfectly cool water heater. If bought from someone driving a box truck selling out in a supermarket parking lot it may be a hot water heater.

Phosphor I think is the correct term.

All the stuff mentioned in replies in this thread are (pardon my pun) on the beam. The ion spray from the cathode hit some part of the neck of the tube, or some target electrode, and did no harm.

The service life of those old CRTs was at the same time unpredictable. While there was a day when R&R CRT was normal procedure for the average tech today it is usualy R&R TV set. I had my share of the former. And now realise the latter.

Those of us that have to do this to restore an antique TV have still to do this. Otherwise it is not feasable.

I hadn't thought about that. When I when back to look at the service manual, when they don't call it a beam-bender they call it an "Ion trap magnet." They always included magnet. It does make more sense.

John

I will try to simplify and summarize in a simpler way. Anyone who believes that a mis-adjusted ion trap will not damage the face of a crt is sadly mistaken. All repair manuals of the time HIGHLY stress this, and they are what I used to find in the library as a kid. Needless to say, I have always been careful with the adjustment of the ION TRAP magnet. I think if you try to think too deeply into the engineering theories of the monochrome cathode ray tube, then it just becomes too confusing.

Just follow the instructions in any good old manual and everything will come out fine. Old TV manuals show up a lot in Goodwill stores with bookshops in them.

The best advice I could give anyone here that is new to the ion trap is: if you remove one, mark an arrow pointing toward the faceplate of the crt. This is so the polarity of the magnetic field will not be reversed. Take a FINE sharpie and just trace the positioning of the magnet on the glass of the crt neck. Then take a telescopic look down the gun and take note of the clock positioning of the magnet. This way you can't go wrong.

Now it's a whole different story when you run into a situation where the magnet has been disturbed and is out of place or missing. Maybe we could make another thread out of that.

This is good advice, the trap alignment *procedures* in these old manuals are usually quite correct. Proper alignment of the magnet is important for several reasons, and the life of the tube can be shortened if it is not done correctly. Some stated reasons for proper adjustment:

1. Less than optimum beam current (magnet not adjusted for max brightness) means that the brightness control must be turned up higher than necessary, which can lead to premature cathode failure. This is my primary concern when adjusting the magnet.

2. Spot size/beam characteristics may suffer if the magnet is not adjusted properly. Best efficiency usually produces the sharpest picture.

3. It is possible to deposit metal from the anode to the screen if the magnet is not adjusted properly.

4. Ions can strike the screen if the magnet is not adjusted properly. No, I really don't believe this can happen, because the trap magnet is simply not strong enough to bend the ion "beam" enough to travel straight down the gun.

In many old sets that I have seen, it is very difficult to obtain "perfect" adjustment, due to a variety of conditions, such as weakened position magnets, magnetized yoke support and other nearby chassis pieces, non-uniform focus magnet field, and perhaps a few other problems. The inability to adjust the magnet to obtain max brightness without neck shadow or positioning problems indicates that "something is wrong" that can perhaps be remedied. I will try to correct any of these problems first. If I *must*, I will slightly mis-adjust the ion magnet or place a small permanent near the neck of the tube to correct the problem, and not worry about it...much.

jr

Hello Gentlemen,

Some pictures of phosphor burned CRTs

Courtesy Noriyoshi Tezuka's repair blog (Japan) translated into "Funny English" by Google:

http://translate.google.de/translate...Fvrc-tezuka%2F

It is interesting to see that serious phosphor burn is visible on the CRT even when not energized.

Best Regards

jhalphen
Paris/France

These are not examples of ion burns, but rather collapse of vertical sweep, destroying the phosphor by intense electron bombardment.

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Speaking of 7"crts... A 7JP4 has no ion trap, is not aluminized, was processed under primitive late 40s processing conditions, and yet, I have never seen an ion burn on one. :scratch2:
I may be very lucky... Or perhaps there is a better explanation for bonus points?...:yes:

jr

I have a 7JP4 with a spot burned in the center, that is otherwise bright.

Remember being a kid and watching the little white dot that would hang around for a few minutes after the 'family' set was turned off? This probably depends on your age. Anyhow, I think many years of that leaves a small burn in the center of the faceplate that is otherwise harmless to the tube.

I have seen many examples of ion burned tubes in old repair manuals. It seems like most were a larger circle, but of particular interest, now that I look back, is my memory of an example that was an X burned into the phosphor. I wonder how ions that supposedly can't hit the screen can do the damage I have seen, and read the heavily stressed warnings on....

I see the logic in the 'beam bender' theory, but I think there is a little more to it than that. I guess I should break out the books......

I also remember seeing pictures of CRTs with huge ugly "X" burns (rectangular tubes) and have wondered if perhaps these tubes were gassy enough to exhibit glow discharge, in which case the ion trap would do no good because the ions are not generated by the beam striking gas molecules, but are everywhere in the tube? :scratch2:

jr

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Did those tubes use lower very high voltage? maybe ions are not an issue at 5KV?

I have a 10BP4 that came out of my porthole; the screen has dark blotches without any real pattern; I'll have to dig it out someday and take a photo. That tube is entirely used up, with basically zero emission left.

The reason there were more ions in early non aluminized tubes was from atmospheric atomic bomb testing. The newer aluminized tubes keep the ions out of the tube the same way aluminum foil wrapped around a technician's head protects his brain from continuing education.

Electrostatic tubes do suffer from ion burn the same as magnetic tubes, but since the ions are deflected along with the electrons, it's happening evenly and extremely slowly over the entire surface, so you don't notice it. This is true not only for the 7JP4, but even pre-war tubes like the 5BP4 and 5AP4 which will never show the typical center ion burn.

The picture is of a 5" pre-war British set that was magnetically deflected. You can clearly see how large the ion burn circle is. This set ran with an anode voltage under 4KV.

Darryl

Does it keep the networks from spying on me too? :nutz: