CT-100 on eBay

[jeyurkon]

Originally Posted by NewVista

Like Bob says, won't get true test of (purple) gas indication until all voltages present or a tesla coil is employed.

The blue glow is from the gas being ionized. If you have emission you've got electrons moving and the couple of hundred volts used when testing a CRT accelerates them well above the few eV ionization potential of the gas.

If there were no emission, then yes the only true test would be the Tesla coil.

[jeyurkon]

What's the purpose of the 2,700 ohm resistor in series with the Red cathode of the 15GP22?

Hope I'm not drifting too far off topic.

[kx250rider]

If I'm not mistaken, that's because of the early type phosphor's inability to produce equal amounts of light among the 3 colors. The resistor compensates so as the chassis can put out an equal amount bias, while the TV watcher "sees" equal amounts of light.

Charles

[miniman82]

Roundies have cathode resistors too, there are 3 on some so the guns produce equal amounts of saturation. Example: on sets which use the 21CYP22, the red is driven at full potential, where the others are reduced somewhat.

[John Folsom]

Charles is correct.
From the RCA Color Television Home Study Course, Lesson Six

[old_tv_nut]

Originally Posted by John Folsom

Charles is correct.
From the RCA Color Television Home Study Course, Lesson Six

That's very interesting - I never noticed it before. Apparently besides the higher drive for red, they needed to make the current/voltage characteristic of the red gun more linear. Perhaps the much greater red drive made it more strongly non-linear than the green and blue, which would mean the midtones would not track the lowlights and highlights.

[Username1]

Well thats essentially correct. The rgb bias settings are a low grey starting point, and the drive adjustments are for the greater beam current, or bright white adjustment. The drive adjustments are kinda like a linearity adjustment for the tube/phosphorus combo. as overall brightness is increased. I think starting with 2X beam current on the red gun for white, by adding a resistor in the cathode circuit would actually be a current limiter. So the question is would this be limiting the max current of the gun? Someone would have to measure what is going on at several current levels of each gun while maintaining white screen to see what this resistor is doing. Current limiting on a gun driven at 2X the current of the other guns for white....

[old_tv_nut]

The theoretical relationship of gun current to drive voltage is a power law with a particular exponent, which has the same shape even if the three guns are driven more or less hard. The need for a series resistor indicates that the curve of light output is not exactly a power law, because the resistor will change the shape of the curve and make it more linear. This could be either an effect of the gun curve itself, or a non-linear effect of the phosphor. I think it is the gun, as phosphor effects usually consist of a loss of efficiency at high currents.
There is also some difference between grid drive (used for RGB in the CT-100) and cathode drive of the luminance (used in later sets), but apparently the later sets also found it useful to put a resistor in one or two of the cathodes even though they used cathode drive.

[vintagecollect]

There's some old threads or discussions on how the new improved red phosphors were more red orange than true red. From what I heard the red phosphor was rare and hard to obtain? I really wish these weren't such finicky crts. Chris got a great deal, set had older restoration, picked up set locally and avoided costly shipping expense. That brass replacement knob IS actually solid machined brass and is quality. The finish is really a mood point, this tv is far from a basket case. Nice score.

[old_tv_nut]

The standard NTSC red primary is a bit orangy - for example, traffic signal red is outside the gamut of NTSC.

The original red phosphors were close to NTSC standard. The all-sulphide tube (zinc cadmium-sulfide for red) was probably the most orange, and got worse at high beam currents. The vanadate rare-earth red was pretty close to standard. The europium red that eventually became the norm is a bit orangy again, but not as bad as the sulfide.
Vanadium and europium are "rare earth" elements.

[ohohyodafarted]

RE: Beltron,

I just wanted to relate an experience I had with a 21cyp22. Upon testing it with a B&K 465 crt tester, the tube was devoid of all emission. I had very little hope of bringing the tube back to life.

I hooked up the Beltron and set the filiment voltage at 6.3 as normal. I waited for 10 minutes and only the green gun moved just a tad to about 10% of full scale. The red and blue showed no signs of life at all. I figured this tube was a gone'r.

I next moved on to the cleaning procedure. IN this mode the Beltron is used to "wake up" a sleepy tube. The procedure is to boost filiment voltage to 12 or 13 volts and let it cook for about 20 seconds. Once again no luck on any of the guns.

Well at this point there seemed to be very little hope that this tube could be saved So I proceeded to the restoration stage were the Beltron removes surface grunge from the cathode material to expose good material,( provided there is still good cathode material to expose.)

I raised the filiment voltage to 13v, and As normally happens the 2 large blue pilot lights flickered and blinked, I processed each gun as recommended for 3 cycles. As I processed each gun the emission got up to about 97% of full scale. I was very encouraged..

When I went back to test mode all 3 guns were now between 95% and 97% of full scale.

Now for the real test. I removed the Beltron and went back to the B&K 465 tester. I like this tester for a more accurate measurement because it has a nice large 4" meter and it seems to give a better quantitive measurement of the emission than the Beltron does. The scale is calibrated from 0 to 1000 where 200 is the cutoff for good and bad. If I get a reading of over 500 and 10 seconds life with this tester I know I have a very good tube.

I set the cutoff bias and proceeded to test each gun. I was ellated to find that each gun tested between 650 and 750 and had a life test of 16 seconds on the weakest gun. I have rarely ever tested a tube that went over 800 on the B&K, not even new tubes.

Absolutely incredible for a tube that was for all practical purposes totally dead on first testing.

It is my opinion that even if a tube can be awakened through boosting filiment voltage, or just opperating for an extended period, that in itself is not enough to bring the tube back to optimum performance, because you are still trying to boil off electrons from behind a dead layer of cathode material. Sort of like trying to piss through a window screen. The Beltron not only awakens the cathode, it actually has an ability to microscopically re-expose the cathode face so that it will once again emit a proper flow of electrons. A meer awakening of the cathode will not produce an equivalent result.

I have used the Belton on 50 or more tubes since I got the unit several years ago, and in some cases the crt is just plain dead due to lack of cathode material remaining. But if there is still viable cathode material available, the Beltron can exposed the material and bring back nearly full performance to a tube that was otherwise for all practical purposes, dead. And I have never damaged a tube with the Beltron. I can't say that about the B&K. I zapped the filiments on two 10BP4's with the 465 trying to rejuvenate them before I bought my Beltron. Now I only use the B&K for emission testing.

IF this 15GP22 still has a good vacuum, and the tube has weak emission, it is very likely that it can be brought back to very good performance with the Beltron. When you get the chassis up and running, see if someone in your area has a Beltron that could come over and test and possibly clean or restore your tube. You will need to build a 20 pin to 14 pin adapter to use the Beltron 14 pin socket. It can't hurt and it will possibly restore the tube to a very good performance level. At the very least it will improve it.

Good Luck and keep us posted on your progress.

[Phil Nelson]

Originally Posted by old_tv_nut

The original red phosphors were close to NTSC standard. The all-sulphide tube (zinc cadmium-sulfide for red) was probably the most orange, and got worse at high beam currents. The vanadate rare-earth red was pretty close to standard. The europium red that eventually became the norm is a bit orangy again, but not as bad as the sulfide.
Vanadium and europium are "rare earth" elements.

Is there a source that tells you what phosphors were used in various early color tubes? Just curious.

Phil Nelson

[old_tv_nut]

There were a couple of papers on the subject, telling the sequence of development of each color of phosphor individually, but incomplete information on which ones were used together, and no specific information on years or tube numbers. The combos we do know are the original yellow-plus-filter for red, then the15GP22 phosphors, which correspond to the NTSC spec, then the all-sulfide tubes, and then the rare earth tubes, first with vanadate red and then europium oxysulfide red. But there are a string of reds listed between the 15GP22 and the all sulfide tube, and no hint of how long each of those were used, or if they were mainly lab developments.

[old_tv_nut]

By the way, I have often wondered why RCA didn't try to do something to equalize the phosphor efficiencies in the 15GP22 (by reducing the green and blue efficiency with some sort of mixture of inactive material) in order to balance the gun currents better. There is probably some practical reason not to try it, but I don't know what that would be.

[miniman82]

Me either, seems to me if you're gonna drive the red at max, why not drive them all at max? It's not like they stood anything to gain by not running the others at max, since any bad gun ruins the whole tube.