RCA 21CT55 B8802897 is Alive!

[Steve McVoy]

I just sent our second 21CT55 to Russia for a TV documentary. It has a chassis that had been working well. The day the shippers arrived, the set developed two independent problems - both due to flat power resistors that had opened up. I would recommend changing them all.

[John Folsom]

Those flat power resistors are know to cause trouble in 21CT55s and CT100s.

[John Folsom]

Follow up report:

tested my RCA Cheltenham CTC-4, which has what I think is an original CRT in it. The CRT has the rCA logo on the base, and is engraved "21AXP22A/21AXP22". The painted on date code is unreadable.

With the brightness an contrast controls turned down, and the 3 G1 grids biased to ground potential, there is absolutely NO glow in a ptich black room, after 45 minutes of operation at 25KV ultor voltage.

This seems to confirm the data from Tom and Wayne that says Tom's glow is due to cathode contamination of the G1 grid. Tom, you need to find a good 21AXP22 for your 21CT55 chassis so it can be returned to it "proper" configuration!

[bgadow]

If it were me I would try to locate a really dead dud to send for rebuilding. Seems ashame to rebuild one that is still in pretty good shape. Just my opinion.

I know I have seen such a glow on some sets, though off hand I can't say just which ones. Mostly noticed when doing the setup procedure and all guns are biased off-yet I still have a faint glow on the screen.

[Tomcomm]

As it stands now, Bob G and Wayne B tests "noglow" but have recent reguned 21AXP22s, no chance of grid contamination from cathode deposits. John F's tests noglow from his original 21AXP22 in his CTC4! This crt is at least 10 years older than my 21FBP22A and should be more contaminated than mine, right. Pete D will test his 15GP22 just as soon as he gets it running again. I need more glow testing of non-reguned roundys like mine.

So out of four roundys tested, my 21FBP22A is the only "glower". Has it been this way when I got it 40 years ago but I never noticed or have I abused it by running over 32kv ultor the last couple of years? Anyway, the glow is only noticeable during very dark scenes when viewed in a totally dark room. I can live with this considering the only fix is to find a known good 21FB cheap (ha) or let Scotty rebuild one for $400 plus a couple hundred two-way risky shipping.

Rebuilding a 21AXP22 if I could find a core is out of the question. There would be the additional core charge and I would have to find an ultor hv boot or hobble up something and I'd be back to 1964 when I converted over to the brighter 21FBP22 in the first place. My 21CT55 is not a restoration, picture quality is the objective here, not proper configuration.

If I get desperate, I can always install my spare 21FBP22 green screen sulfide and hope its been spared the glow of the present gray screen 21FBP22A rare-earth..........Tom

[jr_tech]

My understanding is that grid emission is caused by evaporated barium from the cathode ending up on the inside of the grid cup, where it can become an emission source, when the grid reaches a certain temperature. Higher than normal cathode temperature can cause the evaporation of the barium as well as contribute to a hotter than normal grid cup temperature. Is the crt heater voltage running high in this set?

jr

[Tomcomm]

When I bought my 21CT55 in ’64 it had no booster on its 21AXP22. I converted it to a 21FBP22 when I compared its brightness to my CTC4 which I converted to a 21FB a few months earlier. I have never run a booster on the present 21FB or rejuvenated it, since it has always and still is very bright. I reactivated the 21CT55 about 1.5 years ago and ran the ultor at max since I had no HV vtvm probe and everything seemed fine. I have a Powerstat on the mains input in order to limit the B++ at 410v with the silicon rectifiers that replaced the big seleniums. This dropped the heaters from 6.4vrms to 5.95vrms so no over heating of the crt cathodes here.

I became aware of the below cutoff full screen glow a few months ago but since it only was annoying on very dark scenes during no light night viewing I tended to ignore it. From a cold start, the glow becomes barely noticeable in about 3 minutes and steadily gets brighter, reaching max in about 10 minutes. A relatively long time constant for the small masses of the crt gun assembly. Crt grid contamination from the cathode seems the most logical explanation for the glow, but I need more input from forum members that are running non reguned roundys…………Tom

[andy]

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[jr_tech]

3 to 10 minutes (sometimes as much as 30) seems to be in the ballpark for the grid structure to heat up. Remember we are in a vacuum here, so there is NO convection, little conduction, because the cathode is mounted on an insulator, so the grid cup is heated mostly by thermal radiation from the hot cathode (about 750 degrees C). If it is grid emission from G1, then lowering the G2s (R,G,B screen controls) should decrease the "background" that you observe when the set is warm.

jr

Are the heaters of all the small tubes in the set also running at 5.95 vrms? could cause a warm-up time problem?

[zenithfan1]

I have a few roundies with their original but strong CRTs, I'll check them out to see what they do and report back later.

[Tomcomm]

Well its been almost a month since I first reported to this forum the "cut-off glow" displayed on my original 21FBP22A and I described a simple test to determine if other members had similar conditions. So far I have received 4 replies, hardly an overwhelming response: 2 recent rebuilds, no glow, 2 originals, no glow. Mine is an anomaly, possibly caused by a shop poltergeist?

I have assembled a new presentation of relative picture quality between my '54 21CT55 and a '92 Sony KV27TS27 with comb filter. All pics are 1024 X 768 and should fill the width of most computer monitors. Any differences in PQ is readily apparent on these larger displays. I use Outlook Express to produce an .....ENL file that permits comparisons by slewing between adjacent inserted pictures.

Anyone interested in viewing these latest Picture Quality comparisons can send me an email to this address: modcommlab1@yahoo.com

I will reply to you, using the BCC window, with a file attachment that will play in your Outlook.......Tom

[Tomcomm]

Recently I questioned the reasons why my 1954 RCA 21CT55 with its first full NTSC spected CTC2B chassis and the last roundy 21FBP22A crt produced picture quality almost comparable with my solid-state, comb-filter 1984 and 1992 Sonys. I checked all the RCA schematics from the CTC4 up to the CTC38, tube types and also the all solid-state CTC40 of 1970. All of them have narrow bw chroma processing and hi level crt luminance/chroma combining. Did RCA ever go back to wb IQ video processing? The dramatic difference in picture quality between wb IQ and nb R-Y/B-Y was obvious to me in 1964 as related in my first AK message of March 28, 2008:

……..When I bought my 21CT55 for $50 in '64, I brought it into the house after I got it working and ran it along side my Dad's old CTC4 he left me. Both had 21FBP22s so it was a good comparison of narrow bw color R-Y B-Y vs. full bw color I Q. The extension of full color into fine detail on the CTC2B was startling! Color persisted in fireworks until extinction which I never saw before. The CTC4 fireworks turned into white long before extinction. The CTC2B carried full color way into the shadows, the CTC4 went gray to black……I get really awesome displays on the 21FBP22A with vibrant, accurate color shading. Having the CTC2B's super wide dynamic range due to full bandwidth I Q demodulators feeding low level I-Q-Y matrixing to the CRT grids only. I kept the CTC4 in the house for the kids but brought the CTC2B back to the garage workshop for further study…….

I have assembled a Flickr photo post: http://www.flickr.com/photos/tomcomm/

After selecting a thumbnail, click on “all sizes” above picture to enlarge to full screen. Two clicks of the Browser’s “Back “ returns to thumbnails.

[andy]

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[old_tv_nut]

Originally Posted by andy

I believe RCA made a chassis in the early 80's with I Q processing. I think it was a CTC13X chassis, but I forget the exact chassis number.

RCA developed a clever way to drive the required I delay line single ended, thus saving a pin on the analog IC. However, when I measured the performance of this chassis for comparison with Zenith sets of the time, the I channel high frequency response was low, not much better than the residual high frequency chroma in the Zenith narrowband demods. The reason, I believe, is that it was too difficult to get the required flat and phase-distortion-free chroma response even in the Q channel, partly due to the difficulty of making an IF that was both sharp-cutoff at the sound carrier and phase-distortion free. So, I think RCA turned the I channel high frequencies down (or didn't peak them to normal level) to avoid annoying hue effects on edges. The other part of the problem was that the NTSC never specified the encoder Q filter closely enough. Some Q filters that would meet the NTSC/FCC spec would still leave visible Q sidebands beyond +/-0.5 MHz that would be unbalanced by the sound carrier trap in the receiver. By the time this RCA set was around, it was known how to make good encoder Q filters that would have a zero at 4.5 MHz and symmetrically at 2.7 MHz, but of course the specs would never be changed in the thousands of encoders out there.

Experiments I did with "ideal" I/Q pictures (using baseband filters and never going to RF) showed that the increased saturation on small bright details and increased sharpness of red roses, barns and rocks was noticeable in side by side comparisons, but also visible were things like the edges of typical yellow movie-title letters turning orange. With a 27 inch or larger set, you couldn't keep people from getting too close, the way a 12-inch console would. Zenith concluded that there was no way to get a good compromise between visibility of red details and visibility of artifacts, and that it would be hard to actually show any of this to customers. If you couldn't turn up the red detail, there was no point in going to I/Q demodulation.

[old_tv_nut]

Here's an example - look at the differnce between the vertical and horizontal strokes of the letter t in Beatles. Unfortunately, I can't turn off the dot crawl in this simulation, whihc would be less visible in reality due to alternating phase from frame to frame. The dot crawl (with a simple chroma trap in the luminance) will generally fall in the same area as the red-orangish discoloration of the yellow.

It should also be visible in the vertical vs. horizontal strokes of the B. I find I have to zoom out quite a bit, until the image is about 1.5 inches wide (at about 15 or 20 inches viewing distance), before the discoloration becomes really less noticeable. This shows that the ability to get away with less chroma bandwidth than luma and use narrower Q bandwidth than I, does not have a sharp dependance on viewing distance, but also depends on the viewer not having an original to refer to. When home computers came out however, the lack of chroma resolution on small text and artificial objects with edges but no texture and possibly little brightness contrast made the failings of composite video obvious.