Rebuilding electrostatic CRTs

[Dave S]

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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[N2IXK]

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.

[Eric H]

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

[timmy]

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.

[Dave S]

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.

[IsthmusTV]

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.

[Dave S]

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

[bandersen]

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

[Eric H]

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.

[N2IXK]

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.

[timmy]

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.

[wa2ise]

Originally Posted by timmy

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.

[Dave S]

Originally Posted by wa2ise

Was wondering if a green phosper tube's cathode and other guts could be a doner to rebuild a white phosper tube?

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!

[N2IXK]

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...

[timmy]

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.