Yeah, that was one idea I tried early on for the focus pot repair, but the India ink I tried was a bit too conductive. I laid out a strip on paper and got less than 100K resistance. Now I was able to use it to repair the red screen pot since it arced and burnt at the end of its track where it was riveted. The resistors are available at Mouser, but they're about $20 a piece. I have only one meter (Keithley 196) capable of measuring resistor values this high, but it's in need of calibration for the resistance function and I'm not sure how accurate the readings really are. I calculated the voltages I should get from the schematic, so I'll try making measurements later to see where I'm losing voltage. The voltage at the 6BD4 grid is low (200V, needs to be closer to 300-400V) to get it to start conducting and lowering the HV to spec.

Did you contact John Folsom? He may have a VCT. SEE THE BOTTOM OF THIS LINK: http://www.videokarma.org/showthread...=270450&page=5

Yep, already tried emailing John Folsom, he replied that he has no more transformers available. The one in my set has either shorts or leakage from secondary to either ground or the primary. This one doesn't look fun to take apart or rebuild. Lots of varnish and many thousands of turns of fine gauge wire. I probably won't get an exact turn count out of this. Also looks like the laminations are stacked in alternating directions, so they'd have to be pried off one by one.

Sure is nice to see what certainly appears to be a good CRT! Major obstacle overcome. Thanks for sharing your progress and pictures.

The apparently weak CRT turned out to be a wiring error on my part in the color video amplifier section. I had a coupling capacitor essentially shorting blue and green signals together as well as shunting the green signal to ground. Correcting that as well as going through a few adjustments in the chrome section allowed me to get a color picture at last. The set still does not like displaying very bright scenes, but these sets probably never got really bright pictures to begin with, so I will just keep the brightness down. Color lock is still a bit weak. Still need to come up with a solution for the vertical convergence transformer to get this working like it's supposed to.

Very nice improvement.

And "getting there". I will admit that as a young adult, I really looked down on the round color tube. To make it even more disgusting, I had no knowledge of the 15GP22 Kinescope! Now, I can really appreciate seeing one displaying a color image. So very cool.

I remember the hype about "Rare Earth" phosphorus and thought that this was a remarkable discovery. But red became Chinese Red, not blood red if I under stand correctly. Seems that the 15GP22 and 21AXP22 were the cat's meow colorwise?

Wikipedia has a pretty good writeup about CRT phosphors. The original red wasn't very efficient to begin with, then 80% or so of the electron beam current never got past the shadow mask anyway to even light the phosphors. From what I have read, the original NTSC phosphors were used in the 15GP22, 15HP22, 19VP22, and early production 21AXP22 tubes. The later reds had a bit shorter wavelength and so appeared a bit more orange. The green phosphor was changed somewhere down the line as well, but I don't know when (probably starting with the 21CYP22). The original green is actually the same as the P1 used in old oscilloscope CRTs. Later color TV tubes first used cadmium doped phosphors, then later on the rare earth (yttrium and europium I think) phosphors.

The overall impression I get having read various articles about the subject is that the 15" color sets really were the wrong product at the time. This is when 21" B&W sets were common, and you were going to pay lots of money to get what is effectively a 12" round CRT color picture. Add to that the weird and expensive construction of the 15GP22 and that's probably why these sets were sold at fire sale prices soon after being introduced. The CBS 19" CRTs seem to be even less common than the 15" ones. It took RCA with the 21AXP22 to get color TV sales off the ground, and it still took over a decade for color TV sales to reach 50% of total.

Well the 15G development started around 1950-52 and at the time 15" was the standard screen size so it is understandable why they picked it.

The 19" color tubes only had a few months before being obsoleted by the 21" tubes.

In that case the 15" tubes just ended up obsolete by the time they were ready for the market. I'm curious as to why RCA thought the best thing to do was use the separate internal faceplate in the 15GP22. It's either flat or very nearly so which introduces pincushion distortion. Maybe they thought they'd get best results getting phosphor dots on that kind of faceplate?

This set has been a real interesting experience for me. I never thought I'd run across something like this becoming available, much less with a good CRT.

I believe the flat internal phosphor plate was used in the 15GP22 due to technology being unavailable to create a stable curved shadow mask. The flat mask could be under tension to prevent deformation with temperature. A domed mask had to be mounted on temperature-compensating mounts that moved the mask back or forward slightly depending on temperature (and resulting contraction/expansion) in order to maintain color purity.

Yes, and in addition I believe that the phosphor dots were merely silk screened on the flat plates used in the 15GP22. The photo process to deposit the phosphor on a curved surface, using the shadow mask as a “negative” for exposure had not been perfected when these tubes were manufactured.

jr

"merely silk screened" doesn't quite describe it.

Zworykin and Morton describe the process in "Television" 2nd Ed. 1954. The description seems to be adapted from a 1951 paper by Barnes and Faulkner, but I believe it is the same as in the 15GP22.

Summary:
Produce the original image for etching the mask by exposing a photographic plate to a fine wire grill twice, with a rotation of 60 degrees between exposures.
Careful processing and copying reproduces a negative optical mask with round dots instead of diamond-shaped.
Expose a thin super-nickel sheet coated with photengraving enamel through the negative.
Wash away the unexposed enamel and etch the exposed metal to form the apertures with sharp feather edges.
Heat this mask and attach to the support frame while hot, so it becomes under tension.
For phosphor patterning, prepare a stencil from the mask:
[One stencil will be used for all three phosphors, with a precision displacement between applications.]
Place a Kodalith plate at the screen position with respect to the mask, and expose with a point-source carbon arc lamp placed at one gun position.
Develop the Kodalith plate.
Sueegee a protective coat of lacquer and wax onto the developed plate.
Coat with photosensitive gelatin emulsion, and expose through the plate onto the emulsion with a carbon arc lamp.
Wash away the unexposed gelatin.
Transfer the wet stencil from the plate to printing mesh and dry it.
Squeegee the phosphor suspension (phosphor powder in ethyl cellulose and amyl alcohol) onto the flat CRT glass screen.
Remove the stencil and clean it.
Allow the CRT screen to dry.
Bake the plate to remove the ethyl cellulose.
Repeat for the other two phosphors, with the stencil precisely positioned.
Bond the phosphors to the plate by misting with potassium silicate solution.
Float a nitrocellulose "blanket" onto the plate and evaporate an aluminum layer onto it.
Finally, bake to remove the nitrocellulose.

The thing to note is that the shadow mask was used as a negative, but with the intermediate stage of producing a silk screen instead of direct deposit of the phosphors.

Sheesh, its a wonder they ever got any 15GPs out the door w/all THAT going on...

Wow! thanks for the process details... incredibly more complex than I had imagined when I heard that the phosphor had been “screened” on to the plate. :thmbsp:

jr