Color crt question???

[ohohyodafarted]

The ctc4 I just picked up has a fairly decent 21axp22 in it. The only types of 21axp22's I am familiar with have always had a sort of light gray color phosphor when you look at the crt when not operating.

The 21axp22 crt in the ctc4 I just got, has the typical olive green color phosphors when the set is not operating.

Does anyone have an explanation as to why the phospors would be olive green instead of light gray?

Is it possible that late era 21axp22 replacement crt's from RCA were made with the newer type olive green phosphors that we are all familiar with on most round color crt's?

Inquiring minds need to know!

Bob

[Pete Deksnis]

It is said that early 21AXP22's have the same 1953NTSC color gamut as a 15GP22, but the later 21AXP22's do not. I do not know the answer, but you may have hit on some real evidence. Can you find a date code on your tube?

[ohohyodafarted]

Hi Pete,

Where would I look for the date codes?

Also the gray phosphor tube is in the newer ctc5 and is coded as a 21axp22a, (I believe it to be the original) where as the olive green crt is in the ctc4 and is coded as a 21axp22. I suspect that the ctc4 has had the original crt replaced at some time, because my first ctc4 that I had back in the 70's had the original factory crt and it had the gray phosphor.

I wish the gray phosphor crt was the good one, because there is a big difference in the way the color looks on a tube with the gray phosphor. the colors seem more vibrant to me. That is what I remember most about my original ctc4 that had the original crt.

Here's hoping that I can find a used gray faced 21axp22 to restore the ctc4 to it's original state.

Even if I could find a dud with the gray phosphor, I would be willing to pay Hawkeye the $700 he wants to do a rebuild on one with the new long heat up and cool down oven cycle, utilizing the oven cams that John Folsum commissioned.

We could use it to test out the new long cycle protocall for rebuilding the 15gp22 crt's

TTYL,
Bob

[stromberg6]

The 21AX in my now working CTC-4 appears to have a slightly greenish tint to the screen. It is a rebuilt without the "A" suffix, and produces excellent color, deep reds, brilliant blues and greens and will converge ok after all other minor cabinet repairs, etc, are done and it is in a more permanent spot. Perhaps it is one of the earlier tubes with a proper color gamut, as Pete notes.
I am trying to reduce the size of my screen shots so I can post a couple on the forum, but so far no luck. I will keep on, though.
Enjoy your 4! I am having a ball watching mine. Let us know how the re-cap goes.
Kevin

[bgadow]

My -5 has a definite gray screen; I don't remember if the tube is an "A" or not. I am pretty sure it was replaced way back, though, as the markings on the base say "RCA Service Company" which I assume might mean warranty? The next time I pull that set out from the wall I'll check the numbers.

[ohohyodafarted]

OK Pete, here is what I found,

Olive Green phosphored 21AXP22, SN: AA60243, No date code

Gray phosphored 21AXP22A, SN: AC28825, Date code 57-4

Hope this helps,
Bob

[stromberg6]

The serial number on my 21AX (no "A") is CM13643. It is a "Monogram"-labeled rebuild, and the number is on the socket of the replacement gun assembly. It has a green tint to the phosphor screen, though slight. The "A" tube I have originally was from the 4, and it has a grayish screen. It is not a rebuild.
Kevin

[old_tv_nut]

a timeline of some display developments from SId

http://www.sid.org/archives/0%20-%20...xhibit%206.pdf

shows rare-earth vanadate red introduced in 1964 and rare-earth oxysulfide in 1967.

I'm thinking that the olive-colored screens might be all-sulfide tubes. I'm ordering a 1993 paper from SMPTE on the history of TV display phosphors that may shed some light on this (no pun intended!).

[old_tv_nut]

A paper by Larach and Hardy of RCA in the July 1973 Proceedings of the IEEE lists 10 different red phosphors that had been tried up to that time in various color CRT applications. It states that the zinc orthophospate:manganese NTSC red was replaced by all-sulphide tubes for a period of 4 years (1961 -1965), after which the rare-earth red was introduced. The sulfide red was more orange than the NTSC red or current rare-earth reds.

I have not been able to find a reference to the screen color of these variations, although the article says the body color of the sulfide red was originally orange, and this was fixed along with some screen deposition problems before the sulfide red went into use.

This article also lists 5 variations in green phosphor and 3 in blue.

[kx250rider]

I know that when a B&W CRT faces the sun for years, it turns yellowish. Maybe a similar thing with the color tubes???

Charles

[old_tv_nut]

Correction:
I re-read the 1973 paper , and it shows the sulphide red slightly on the more orange side of NTSC red; the vanadate rare earth red (that was used for a short time) slightly less orange; and the yttrium red the most orange of all. The modern value for sRGB primaries would indicate that the latest version of the yttrium red is somewhat less orangy, but also slightly desaturated, at least as practically used in a tube.

Still no info on screen color when the tube is off.

[Eric H]

Does the color of the screen actually have anything to do with the phosphors or is it the tint of the glass that makes them the olive green color?

My CTC7 has a 21CYP with a 1960 date code and it has a light gray face.

[ohohyodafarted]

I am just taking a guess, but here it goes. up until about the early 60's as near as I can remember, after the period when they were making 21cyp22's, the color of the phosphor compounds deposited on the screen became olive green in color. I don not have any idea why this is or what it relates to or even if there is any significance in what the phosphor compounds were made of. All I remember is that the earlier crt's typically had a grey color to them. I do not believe that there is any relationship to the color of the glass faceplate. I am fairly certain the faceplates are clear or very nearly clear.

When you think about it, the probable reason they went to the olive green color phosphor compound was to increase contrast. When the phosphor was not excited, an olive green color is darker than a gray color phosphor. I guess the ultimate color phosphor would be black, provided it wouldn't inhibit the emission of light when excited.

As I recall there were other methods of increasing contrast too. There was a black stripe crt in which there was some sort of configuration where there was a stripe of black between the phosphors to increase the contrast. There may have even been a method where they deposited black between the color phosphor dots, but my mind is foggy on that and maybe I imagine it.

Perhaps some of the other guys around her with a better memory can enlighten us about using black to improve the contrast.

[old_tv_nut]

It would be nice if the phosphor reflectance could be made dark independent of its light-emitting properties, but that's just not the case. CRT faceplates have always used a neutral tint if any in order to improve the contrast (except for the very earliest experimental tubes, some of which needed filters to help get a decent red and suppress the much more efficient blue and green). Some modern tubes use a different colored pigment on each phosphor to reduce its reflectance while letting its primary color through.

One possibility, which I have not heard mentioned, is that when going from NTSC phosphors to sulfides, a darker faceplate was used - so a slightly greenish appearance just became darker. The 1973 paper also says that the green phosphor went from silver-acivated zinc-cadmium sulfide to copper-activated zinc sulfide in 1968, which had "a more nearly white body color". However, I think I recall that Zenith-Rauland was using cadmium up until the 1970s. Manufacturers eventually eliminated cadmium because of environmental restrictions.

Regarding black matrix tubes, this was one of the big inventions fo the mid-70's. All tubes use the black matrix technique today to reduce the screen reflectivity. Along with this came the concept of "negative guard band" for better purity.

The early tubes had positive guard band - that is, the holes in the shadow mask were smaller than the phosphor dots. If the beams didn't approach the screen at quite the right angle, there was still some room for them to be off center and still hit the right phosphor dot. However, this could lead to white-field impurity if the dots were not uniformly efficient. When black matrix was added, someone got the idea of making the holes in the black matrix smaller and the holes in the shadow mask bigger. Thus, the electrons hit all of the dot and some of the surrounding black matrix, and their aim can be off more without causing part of the dot to be unlit or wandering onto the wrong color dot. This makes the purity adjustment have a larger tolerance - purity tends to stay good up to the limit of misadjustment and turns bad more suddenly at the extreme.

Combining negative guardband with in-line guns that don't care about vertical displacement of the angle, allowed the introduction of self-converging deflection yoke/tube systems that did away with all the convergence circuits and need for setup in the home. This was a big part of keeping the price of a TV constant or falling in current dollars (and always falling compared to inflation).

[old_tv_nut]

I read the above and realized I slipped a cog in a few places - please re-read as edited!