Rebuilding electrostatic CRTs
 

I seem to recall hearing (and I certainly could be wrong) that it was more difficult, perhaps impossible, to rebuild electrostatic CRTs like the 3KP4 and 7JP4. Does anyone have any details on this?

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I cant see why it would be "impossible" to do, but in general it wasn't done because electrostatic tubes were generally used in inexpensive sets, and the tubes themselves were relatively inexpensive back in the day, compared to the larger screen EM types.

Because these types weren't generally rebuilt "back in the day", there aren't likely to be any stocks of replacement guns floating around anywhere.

It should certainly be possible to rebuild the old gun with a new heater and cathode, though.

I think tubes like the 7JP4 may be made of Pyrex?

well i have a 7jp4 that was rebuilt at some time because the neck with the gun , the glass is a different diameter then the rest of the neck on the tube it self. so it was done and i would assume there were guns out there at one time. the tube i have i can clearly see where the two were joined together, infact i posted a pic of the tube and steve from ETM commented that maybe after all they can be rebuilt. but nos guns are needed or to rebuild the bad ones.

The plan as I understand it, is to gear up to be able to install new cathodes into the old guns, so I guess there's hope for those tubes.

I think another issue is that the phosphors can wear our sooner than the cathodes. I've heard tales of 7JP4's that show excellent emission on a CRT tester, but make very dim pictures in a set.

Good point. And I would expect that 7JP4s may be among the easiest of tubes to re-phosphor.

The phosphor wears out from ion bombardment because there's no ion trap and they aren't aluminized.

Most Ion damage I've seen has been confined to an area in the center of the screen, usually about the same diameter as the inside of the neck, the 12JP4 is probably the most notorious for this condition.

An example can be seen in this thread from Isthmus' Andrea restoration thread here: http://i1216.photobucket.com/albums/...pssy8t0jtj.jpg

I think the Ions only travel in more or less a straight line and can't be deflected to the outer areas of the screen, could be wrong about that but I don't think it would cause a completely dim tube.

In an electrostatic deflection tube, the ions get deflected along with the electron beam, so the damage gets spread out over the whole screen, gradually darkening the screen and destroying the phosphor.

Magnetic fields won't deflect the ions, so you end up with the classic "ion spot" in the center of the screen on EM tubes.

well the 7jp4 that i have was rebuilt and it started out perfect besides having the classic burn in the center but now putting it on a working chassis the brightness wont shut the image down and dont know why since it was in the closet for a month and a half then took it out to try on another chassis and thats what im left with some sort of problem with it as im told it could be a heater cathode short, how this could happen from not being used i dont know and not having and crt tester for this tube to see or get rid of a possible removable short, the tube is useless now.

Maybe run the heater off a separate filament transformer?

Was wondering if a green phosper tube's cathode and other guts could be a doner to rebuild a white phosper tube? Or would the cathode being briefly exposed to air ruin it? IIRC, cathodes have to be cooked when first in a vacuum, and you might not be able to do that again.

yes i heard that the cathodes need to be cooked to kind of activate after rebuilding. both 7jp4 and 7jp1 guns are identicle except the different phospor. switching guns between crts i dont think would work because of the cathode coating that somehow protects it from outside air. by the time the guns are swapped the cathode is history. its weird how that meterial is, so how was it put on in the first place when the cathode was new, do they rush coat it and then put the protector coat on that seals it from air ? just wish there was a way someone here can come up with his own way of reviving these tubes.

Cathodes have a carbonate coating which undergoes a conversion once high heat is applied during the cathode activation step of CRT processing, sometimes this is done while the tube is still being pumped but it doesn't have to be. RACS activated their cathodes after the tubes had been tipped off and cooled to room temp, said they never had any problem with tube poisoning. Once activated, if exposed to air the cathode must be replaced.

I wonder if that great big, ginormous 30" tube was electrostatic, or magnetic, & if it was, maybe that's why it was so short lived... Always heard those things were kinda primitive & a bit crude, despite their huge sizes...

The DuMont 30BP4 used electromagnetic deflection. DuMont did make electrostatic tubes up to 20", though.

The reason for the short life on 30BP4s was insufficient gettering. The tubes were not aluminized, so getters could only be used in the neck to avoid contaminating the phosphor with patches of barium. Unfortunately, neck getters alone weren't sufficient for a tube with that much evacuated volume and that many linear feet of glass to metal seal to potentially leak air molecules into the tube.

Once aluminized tubes became the standard, additional gettering could be used inside the funnel area to improve the vacuum integrity.

Good info, N2IXK!

Somewhere I have an old article from Antique Radio Classified (or maybe it was Don Patterson's Radio Age?) about having a 3KP4 rebuilt; this was in the late 80s or early 90s, and the rebuilder was a former DuMont employee in NJ. The gentleman was getting up in years back then.

Yeah, Thanks for the info ! I THINK I've seen a pic of the 14" & 20" Dumont tubes... There's a pic from like 1938, w/this rather odd-looking gal sitting on a table, holding the both of 'em up... She's smiling, as if she was BORN to do that, one wonders if she knew how much DANGER she had, mere inches from her face/chest.

Like this?


http://www.videokarma.org/attachment...8&d=1301008843

jr

Aye-Yoop... Dat be De One..

I know a green phosphor 20" tube was used in a classroom demonstration oscilloscope that DuMont sold. Pic here:

http://www.myvintagetv.com/Scope/dum...cope%20233.jpg

NOT very likely those tubes were evacuated...

I would hope not, but on the larger tube I think that I see a couple of oval getter flashes on the neck, about an inch above the base cap. :scratch2:

jr

How would she even manage to hold on to those tubes like that? They would seem WAY too top heavy to be held vertical from the base end with screen up.:scratch2:

Early "photoshop" job? Or are the tubes actually supported by rods on the backsides of the necks?

Those are thin glass, so even though large, they don't weigh all that much.

I think they'd have no problem standing up like that if plugged into a socket that was attached to the table, she also has he hands near the base, possibly to stabilize them a bit?

Looks like they might use a unique base with edge contacts? So the end could be flat, might not balance too bad. The glass does look awful thin, not much more than a big light bulb.

I'm still a 'dumb guy' when it comes to all the hands-on details on this stuff, but as far as I can see there'd be no reason a green tube couldn't be re-phosphored as a white tube. Hadn't thought of that before. Everybody save those dud 3KP1 'scope tubes!

Unfortunately, once you let the green phosphor tube down to air to replace the phosphor, the cathode coating is destroyed by contact with the air so the cathode (or entire gun) will need to be replaced, as well...

or we could send a whole lot of crts that need something done to them to outter space with the astronauts and during a space walk they can perform what needs to be done since space is vacuum and upon taking the gun out, well it wont be contaminated. i know its wishfull thinking , lol,lol.... :D

Sounds like a plan, but settling new phosphor screens might be a bit of a challenge in zero-G. :D

well yes but the gun should be left outside the spacecraft to be replaced in the bulb after the phosphor is done inside the spacecraft, lol,lol :D

One of those spacecraft with an artificial gravity generator, right? :D

NASA has a large vacuum chamber they use to test satellites and space probes. Though I doubt you could borrow it...

Sealing the removed gun back onto the neck would be interesting challenge as well. How does one operate a glass lathe in a vacuum? :scratch2:

anything is possible these days, as we should look up to the stars for answers, lol,lol :banana:

Lasers!


Well, actually the normal way. The flames engulf the glass, so conduction of heat energy is the prevailing process, you just have to do some modifications to the torches themselves so they are prepared for exhaust into 0.0PSIA instead of ~14.XPSIA. (0PSIG is 14.7PSIA, Gauge and Absolute.)



If one were inclined, I believe it would be feasible to construct an inert atmosphere glove-box type system for handling such tasks down here at the bottom of ye-olde gravity well. Not sure what inert gas would be best to use, something easy to pump down but also easy to get "four-nines" purity economically. I haven't checked the Ar prices recently but they have to be cheaper than Xe. Argon should be easy enough to pump to a solid hard vacuum when all is said and done. Oddly enough, you can weld glass in an oxygen-free environment fairly easy. Go look up atomic hydrogen welding. Think of it as similar in principle to a plasma-cutter head, but works without oxidizer. (And much, much hotter!) Hot enough to weld tungsten...:yikes: Glass is a walk in the park!




As for phosphor deposition in free-fall? Centrifuge the jug, suction up the majority of the supernatant solution following successful precipitation/deposition, instead of decanting it off, then freeze the remaining film of watery phosphor in place thermally, and proceed to sublimate off the remaining water via hard vacuum. Then pick up as normal.:scratch2:

Before I started reading up on the subject (before websites like this one) I used to assume that all picture tubes were rebuilt in big vacuum chambers of some sort.

The closest thing that actually happened that way is of course NuVistor production.

(Edit) NoPegs sed it first (nuvistors).

I've seen photos of a big, demountable CRT setup they used in developmental work at RCA's Princeton Labs. The front pops open and they could insert whatever guns or screens they were testing and re-pull a vacuum and give it a go. Pretty slick!