Westinghouse K840CK15 Restoration

[etype2]

Thank you, Wayne, Tom, consoleguy67, Electronic M.

“ They simply used existing primary color phosphors when they designed the 15GP22”… My understanding was the phosphors were designed to come as close to NTSC color specifications, after all the 15GP22 was the first color CRT introduced to the public.

“ Because the NTSC correct phosphors we're horribly dim and terribly unbalanced … “ I believe it was more of a problem of the shadow mask design. The first shadow mask blocked 86% of the electron light. RCA could have designed a more efficient shadow mask without changing the color phosphors, but I’m not sure it was technically possible at the time. It is true that later RCA CRT designs were brighter, but at the expense of color accuracy. As Wayne has explained many times, the reds became more orange and greens became more yellow. Precisely why the Chromatron and beam index tubes were developed, to overcome the inefficiency of the shadow mask.

Edit. Sidenote. Muntz tried marketing a Chromatron television.

[old_tv_nut]

Originally Posted by etype2

Thank you, Wayne, Tom, consoleguy67, Electronic M.

“ They simply used existing primary color phosphors when they designed the 15GP22”… My understanding was the phosphors were designed to come as close to NTSC color specifications, after all the 15GP22 was the first color CRT introduced to the public.

“ Because the NTSC correct phosphors we're horribly dim and terribly unbalanced … “ I believe it was more of a problem of the shadow mask design. The first shadow mask blocked 86% of the electron light. RCA could have designed a more efficient shadow mask without changing the color phosphors, but I’m not sure it was technically possible at the time. It is true that later RCA CRT designs were brighter, but at the expense of color accuracy. As Wayne has explained many times, the reds became more orange and greens became more yellow. Precisely why the Chromatron and beam index tubes were developed, to overcome the inefficiency of the shadow mask.

Edit. Sidenote. Muntz tried marketing a Chromatron television.

It wasn't quite as you have stated.

The NTSC primaries were chosen to match available phosphors, not the other way around. RCA would have preferred to use an even wider color gamut as achieved with the trinescope prototypes, but the original phosphors were quite good. There was some confusion with the blue. They had to abandon the deeper blue sulfide phosphor that is used today, due to copper contamination turning it green, plus there seems to have been a typo in documents somewhere that didn't even get the blue coordinates exactly right for the FCC specs.

Also, it is correct that the original phosphors have horribly unbalanced efficacy. It was tolerable in the 15GP22 mainly because the small screen size meant a reasonably bright picture at reasonable gun currents. You are correct that shadow mask transparency was improved with the 21 inch tubes, but this was more than countered by the need for increased gun current to get reasonable brightness. The current ratios gradually improved but were still nowhere near unity until rare-earth reds came along. In the meantime, the ratio problem was reduced by going to a very cyan white point ("9300K + 27MPCD") which simply meant that the red gun was driven harder but not enough to make a 6500K daylight white.

There is also a psychophysical effect of increased "colorfulness" with increased brightness, which tipped all consumer preference toward brighter pictures instead of measured color gamut.

Also, the main change in gamut was from the yellower sulfide green and to some extent the deeper blue sulfide blue (more saturated purples and magentas). The reds through the years were all close to NTSC with the exception of the sulfide red in the early 60s. Original NTSC red is slightly orange to begin with; it can reproduce tail-light red but not traffic signal red.