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  #61  
Old 06-26-2009, 06:39 PM
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Possible 21CT55 Warmup Fix?

Thanks for the replies regarding my warm up problem. I've been running silicon diodes in place of the seleniums from the first time I fired up the 55 and got only 390v of B++. Lost a lot of heat in the bargain. I replaced the rectifier’s 80uf big can electrolytic with a 100uf at 475v. The B++ goes to 455v at turn on and settles back to 410V operating. A Powerstat manually keeps the AC at 113vac resulting in a B++ of 410v which produces exactly +900v of boost and 30kv of ultor.

Since the boost voltage is used in the vertical oscillator height control, any change in boost voltage or the resistancs of the 2.5meg height control, both height and linearity greatly effects screen's appearance but more importantly vertical convergence accuracy! The height and linearity are both controlled with a cheap open terminal dual pot. The 5k linearity portion was open originally and I replaced it with a single pot mounted in a new control panel hole. When I hit the surviving height pot with Quick Freeze, it collapsed the vertical screen and destroyed vertical hold. As it dried out the screen circle almost returned to normal. I replaced it with a sealed military grade Allen Bradly 2.5meg pot I had from the ‘50s. When I adjusted it to the circle dots, linearity was restored and convergence improved significantly. Was this the fix? Time will tell when I fire the 55 tomorrow morning. All Hail Marys are welcomed.

I started using Quick Freeze on the rest of the vertical circuit resistors and caps but didn't finish yet since I was running my last can out. No further temp sensitivity so far. I took Bob’s suggestion and ordered five cans for the same $12 shipping as three, from Southern Hills. My nearest electronics store was a long trip and they wanted $8 a can. Santa Ynez is great to live in but major stores are at least 60 miles RT in any direction. ....Tom
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  #62  
Old 06-26-2009, 06:50 PM
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Glad you're getting close, let us know how she runs tomorrow! That dang freeze spray has found many a gremlin for me. Good stuff.......
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  #63  
Old 06-30-2009, 05:56 PM
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21CT55 Vertical Warm-up

The replacement of the height pot and .047uf amp feedback cap made a major improvement in vertical height and linearity stability. The sequential pictures show the cross hatch and circle pattern improving during warm up. Although better, it is still entirely too long a time for performance demonstration.
The black felt pen dots applied directly to the CRT screen represents the ideal circle locations and are very useful in judging the results of any circuit modification. My 55 can be expected to present a useful screen for casual viewing in about 10min after turn-on and reach maximum picture quality for critical screen shots in about 1hr. Convergence has not been optimized since vertical performance is the issue here.

Is my 55 unique with this problem or do all CTC2B chassis and maybe all RCA CTVs up thru CTC10 have this same problem to some extent? I would like to see sequential screen shots of member's early RCA's during warm-up. Ideally they would use the same DVE DVD cross hatch and circle pattern I use, and would apply the felt pen dots directly to the CRT glass for reference.....Tom
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Last edited by Tomcomm; 06-30-2009 at 06:10 PM.
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  #64  
Old 07-04-2009, 02:47 PM
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21CT55 Warn-up

Bob G........Somehow I didn't realize you were the "Bob" of the recent, most impressive full restoration of all 21CT55s. Since you did a complete recap I would expect yours to have the least problem with warm-up drift. I assume you Quick Freeze'd all resistors in both deflection systems and the complete convergence chassis? I can imagine your frustration when you fire-up your beautiful 55 for some TV-wise visitor and it comes up greatly miss-converged. I have the same problem but I never have demonstrated my 55 to any knowledgeable visitor, so waiting 30 to 60 minutes warm-up before taking critical screen shots is no big deal. I've asked this forum's members that have early RCA tube CTVs if they had similar problems and received zero replies. Must be you and I have a unique problem or others are unaware they have a problem or have little interest in the subject?

I checked my power transformer and horizontal output transformer tempts after over 3 hours operation on 113vac mains input on a hot day. The B++ was 410v, Boost was 900v and ultor was 30kv. The horz output tube input control was set for minimum drive and the horz B++ current was 320ma, horz output transformer was 113deg and the power transformer was 140deg.

I will continue freezing all critical resistors and caps looking for sensitivities as soon as my five cans of quick freeze arrives next week. Of course I will report any interesting results.....Tom
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  #65  
Old 07-04-2009, 03:50 PM
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  #66  
Old 07-04-2009, 06:48 PM
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Quote:
Originally Posted by Tomcomm View Post
I've asked this forum's members that have early RCA tube CTVs if they had similar problems...
As I am sure everyone is aware, the CTC2B chassis is an outgrowth of the CTC2; the two chassis have far more similarities than differences.

When I need to view my CT-100 (CTC2 chassis) critically, I never do so without a bare minimum of 30 minutes warm-up, with one hour typical. In particular, fine color balance is not achieved without the warm-up time. I often wonder if a 'time-warp-acquired' set of just-manufactured tubes from RCA Harrison would shorten the warm-up requirement. My sense is that the 55-year-old 6AN8's and 12BH7's require more time to stabilize than when they were new, even though the time on them is not great.

Pete
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  #67  
Old 07-05-2009, 11:42 PM
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Hi Tom,

Thanks for the info on the HO trans temp and B+ current. Yours seems to be running nice and cool. That gives me a good benchmark to shoot for. I just got back from a vacation and my freez spray has arrived, so I will try and find some time to look for temp drifting resistors this week to see if I can nail down the large shift in convergence at startup. The funny thing is that after half an hour of running, when the convergence has stabilized, I can turn the set off for 15 seconds or so and the convergence drift will re-occur. But it will re-stabilize in about 2 minutes.

John Folsom has told me that I should replace the flat green power resistors. John says that these type power resistors are known for being a problem. That is the next thing I will try.

Thanks again for the data on temp and voltage/current readings. I will post again as progress is being made.

Bob G.
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  #68  
Old 07-06-2009, 03:09 PM
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The original flat green power resistors are rumored to be a little intermittent, causing things to flash and change suddenly. They do not, however, drift wit temperature.
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  #69  
Old 07-08-2009, 10:12 PM
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Had to replace those green power resistors on our example before it worked well consistently. A known intermittent part. Beautiful set and landmark engineering for the era. Superb picture with that vintage RCA accurate lollipop color. One of the best pictures I ever saw on a color TV set after the pain of getting it right.
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  #70  
Old 09-23-2009, 06:15 PM
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Low level "glow" at CRT cutoff

I have this continuing problem with my 21CT55 after a year of operation.With ext video input from the DVE test DVD at blacker-then-black video level input and the contrast and brightness controls fully CCW, I observe a uniform, non-raster, very low-level white glow over the entire screen. This is only obvious in a totally dark lab on the RCA but not on the Sony monitors operating on the same video.

During critical PQ evaluation comparisons with the Sony with no room lights, this glow detracts from the absolute black of the RCA on demo DVE's video relative to the Sony. This is disturbing to me since I never watch LCD TV and suspect my RCA crt's black presentation is similar...yuk!

What to do?? I've tried a few diagnostics:

(1) I determined the 21FBP22A is driven well into cutoff by at least 120vdc.
Besides, the glow is not scan-line defined as if it was going thru gun optics.

(2) By under-scanning the crt with misadjusting the vertical linearity control,
the glow is confined crudely to the raster area of the crt but totally out of
focus. So its effected by the deflection yoke but not the gun optics?

(3) Could this be caused by secundary emmission from the gun elements?

(4) Could it be caused by X-RAY excitation of the gun elements? I am running an ultor of 30kv. The 21FBP22 is rated for 27.5kv so I reduced it to a min of 22.5kv with absolutely no change in the glow intensity.

(5) How about a small air leak? No violet glow in the gun assembly.

(6) I pulled off the crt socket and turned the power on and let the ultor come up. No screen glow so I pressed the crt socket back on. No immediate glow as the crt heaters came up to temp, still no glow. After about 3min running, a glow was barely noticeable. It got progressively brighter until after about 10 min it was at maximum. I directed a fan at the crt neck attempting to cool it but it made no difference.

Question: Is this low level glow I have on my 21FBP22A a common phenomenon observed on many early roundys but only noticed on very dark scenes in totally dark rooms? Since I have only one operational roundy and have not observed another roundy in 30yrs I have no way of finding out. I would be very grateful for any worthwhile replys regarding the cause of this glow and the prevalence of this abnormality in other forum member's roundys.
......Tom

Last edited by Tomcomm; 09-23-2009 at 09:43 PM.
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  #71  
Old 09-23-2009, 06:32 PM
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Quote:
Originally Posted by Tomcomm View Post
I have this continuing problem with my 21CT55 after a year of operation.With ext video input from the DVE test DVD at blacker-then-black video level input and the contrast and brightness controls fully CCW, I observe a uniform, non-raster, very low-level white glow over the entire screen. This is only obvious in a totally dark lab on the RCA but not on the Sony monitors operating on the same video.

During critical PQ evaluation comparisons with the Sony with no room lights, this glow detracts from the absolute black of the RCA on demo DVE's video relative to the Sony. This is disturbing to me since I never watch LCD TV and suspect my RCA crt's black presentation is similar...yuk!

What to do?? I've tried a few diagnostics:

(1) I determined the 21FBP22A is driven well into cutoff by at least 120vdc.
Besides, the glow is not scan-line defined as if it was going thru gun optics.

(2) By under-scanning the crt with misadjusting the vertical linearity control,
the glow is confined crudely to the raster area of the crt but totally out of
focus. So its effected by the deflection yoke but not the gun optics?

(3) Could this be caused by secundary emmission from the gun elements?

(4) Could it be caused by X-RAY excitation of the gun elements? I am running an ultor of 30kv. The 21FBP22 is rated for 27.5kv so I reduced it to a min of 22.5kv with absolutely no change in the glow intensity.

(5) How about a small air leak? No violet glow in the gun assembly.

(6) I pulled off the crt socket and turned the power on and let the ultor come up. No screen glow so I pressed the crt socket back on. No immediate glow as the crt heaters came up to temp, still no glow. After about 3min running, a glow was barely noticeable. It got progressively brighter until after about 10 min it was at maximum. I directed a fan at the crt neck attempting to cool it but it made no difference.

Question: Is this low level glow I have on my 21FBP22A a common phenomenon observed on many early roundys but only noticed on very dark scenes in totally dark rooms? Since I have only one operational roundy and have not observed another roundy in 30yrs I have no way of finding out. I would be very grateful for any worthwhile replys regarding the cause of this glow and the prevalence of this abnormality in other forum member's roundys.
Maybe Scottie is knowledgeable? Anyone know him personally?......Tom
I have seen the glow on modern set's and it was the screen voltage set to high, But I would think you would see retrace. Try turning down the screen controls and see if your problem goes away. I have never seen this on my round color sets and I use them almost every day.
Ed
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  #72  
Old 09-23-2009, 07:57 PM
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It shows a warm-up effect, so it is not field emission, which would come on immediately. It's not secondary emission or x rays either, because the normal beam current is off. My only guess is that it could be thermal emission from G1 after it gets heated up. I second Ed's note to see if changing G2 voltage affects it. I have no idea if this was common.
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  #73  
Old 09-23-2009, 08:14 PM
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Last edited by andy; 12-07-2021 at 01:42 PM.
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  #74  
Old 09-23-2009, 08:29 PM
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A web search shows there were a number of papers written on techniques of constructing grids to suppress unwanted emission,which could come from evaporation of cathode material onto the grid.
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  #75  
Old 10-02-2009, 10:43 AM
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Cut-off glow sourse solved?

I contacted a crt rebuilder that was mentioned on the ETF site called Magnaview. Talked to Earl who has been rebuilding roundys for 25 years. When I briefly described my cutoff glow phanomination he immediately identified it as cathode contamination of the grid which produces unfocused electron emission when the grid gets heated by the cathode. This substantiates the speculation of John F, Wayne B and Andy. Earl stated emphatically that most all roundys have this glow after 10 years of operation. Its something we all have to live with as we and our roundys get older, something like arthritis.

I requested the forum members to test their roundys with a very simple procedure (even a caveman could do it). Activate TV with brightness, contrast and chroma at full ccw and view in a total blackout room for greater than 15 minutes. Got one reply that no glow was observed after 20 minutes. His crt was a very recent regunned 21FBP22 with no chance of any cathode buildup on its new grid. No one else reported observing the low level cut-off glow or if they did, would rather not comment??

I talked to Scotty about getting mine rebuilt with new original guns. He says no problem, $400 with my dud. He claims he has never observed my "glow" or even heard of the problem, but then he only scrutinizes his newly reguned rebuilts and I doubt in a total blackout room.

My 21FBP22 has not been rebuilt or had a brightener on it. It has good emission on all guns as long as the heaters run above 6.5vrms but falls off rapidly as I drop below 6.3vrms so the cathodes are pretty well depleted, near end-of life. I am now running a switched brightener which supplies 6vrms normal or 7.5vrms boosted at the crt. each gun's emission is way good and well balanced., no change in the glow intensity though.

Really the only time I notice the glow is at the start of the Digital Video Essentials test DVD during the "sunrise out of total night darkness" scene at the start of the video demo. This is a great DVD for setting up, evaluating and comparing the PQ of the early 50s 21CT55 with the 80s and 90s Sony monitors with comb filters. When they all look virtually the same, you know the roundy's PQ is truly optimized.....Tom

Last edited by Tomcomm; 10-02-2009 at 11:22 AM.
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