RCA 21CT55 B8802897 is Alive!

[Penthode]

I have seen this phenomenon on black and white and color tubes over the years. I suspect the tube is going a bit gassy and the cathodes do contaminate. The higher temperature cleans off the cathodes. That's my guess.

I suspect that over time the emission will diminish somewhat and you will have to burn the cathodes again.

[stromberg6]

Thanks for the update! Always looking for your posts on the test bed, as they really help others, even those who don't have a CTC-2B.
Going to start on two 7's this winter. Should be fun!

[Tomcomm]

Thanks for your renewed interest in the "21CT55 Test-Bed". Today I turned it on and still got immediate full brightness with perfect focus and boostless CRT heaters at 5.94vac! I agree with Pentode that running the 21FBP22A at 7.64VAC for 2 hours seems to have accomplish a very gentle CRT rejuvenation. It probably cleaned the cathodes and possibly burned-off any control grid contamination. The cut-off glow is virtually gone!


Hey, if all I have to do is occasionally run the Big Sucker at 7-8vac for a couple of hours to restore performance, I think I'll keep it....Tom

[John Folsom]

Tom, IMO you are asking for another flyback failure running at 30-32KV. That flyback was not designed to run with that much HV. The 21FBP22 has a maximum limit of 27.5KV anode voltage.

[miniman82]

He won't listen, I've already told him. Nevermind the fact that if the max ratings are exceeded, x-ray production goes beyond limits deemed acceptable.

[Tomcomm]

John.....I appreciate your concern for my old 21FBP22A and my new Flyback that's used on at least CT16 thru CT20s. I consider my 21CT55 a bench-top test-bed rather than a candidate for the usual chassis restoration including the mandatory complete recap. My stated long term goal for this test-bed is to demonstrate thru screen-shots the picture quality potential of the wide bandwidth I Q demodulation and low-level matrix processes of the CTC2B chassis teamed with what I consider the best roundy CRT, the grey screen 21FBP22A rare earth. Since I don't consider my 21CT55 a sacred icon of early color TV technology, I am at liberty to take reasonable design risks in the crucial horizontal deflection and EHV ultor system. The original 58 year old FBX that failed developed a short in its primary as the result of rotted insulation due to simple old age rather than over voltage in its secondary.

That said, the replacement FBX's secondary is not the universal winding of the original but a uniform wound solenoid stack of at least 30 to 50 layers! It was completely encapsulated in a multi layered rubbery boot to keep moisture out.

RCA used this FBX in the last 6 or 7 CTCs of the roundy series. I've been using it in my 21CT55 since December 2010 at EHV of 25kv to 35kv with HOT currents of 195ma to 205ma. In all cases it's coil temp has never exceeded 120degF with no fan at room temps of up to 70degF and 112degF at room temps of 95degF with a small fan directed at the FBX. The 21FBP22A is rated at 27.5kv max.This was probably dictated by barely detectable X-ray emissions rather then electrical breakdown within the CRT. It too was subjected to the same sustained 35kv as the FBX secondary. Standard high production engineering practices at RCA would not permit fielding of a consumer product without a proven safety factor of at least 20% to 30%. So 27.5KV times 120% says this FBX CRT combo should run all day with an ultor of 33kv! My performance ultor is now 30kv. The 21CT55 produces this ultor voltage with only 200ma current at the B++ fuze location which is the same current as the HOT cathode current.

Since I feel obligated to push the envelope of the 21CT55 I employ continuous monitoring of all critical parameters during all running of the TV. These include mains input held at 115vac, measure of B++ voltage, HOT current, Boost voltage, Ultor voltage, focus voltage and even CRT heater voltage at the tube socket.

Most importantly, I never allow the TV to run with screen display when I leave the room. I have a switch on the chassis that disconnects the B++ into the horizontal section but leaves all other sections active with tubes on. I hope this drone leaves you and miniman somewhat more comfortable that I'm not unnecessarily destroying a 21CT55 I've had since 1964.

[miniman82]

John,


Told ya so.

[John Folsom]

Ah well...concern expressed, now not my problem. :-)

[Einar72]

Dosimeter, anyone?

[RobtWB]

Hello Mr. Tomcomm,

Have you ever done any bench testing / experimenting with broadcast monitors such as Conrac or Sony CVM PVM series? Or are you only interested in the ctc2b/grey screen combination?

[Tomcomm]

After three garbled one-liner negative responses to my lengthy rationalization of my CTC2B test-bed effort, one reasonable response was finally posted. Yes, RobtWB I have a Sony PVM-1344Q composite / component pro monitor. I use it as a lab standard when comparing the picture quality of the CTC2B with a perfect comb-filtered pro monitor standard. I will use it after I converge the CTC2B and take screen shots with the new Sony DSR-HX9V camera, soon. Perhaps the three first-responders would care to elaborate on their vehement, specific objections with my present CTC2B operation at 30KV ultor, which is only 9% over max and runs only 200ma HOT current?

[miniman82]

Originally Posted by Tomcomm

Perhaps the three first-responders would care to elaborate on their vehement, specific objections with my present CTC2B operation at 30KV ultor, which is only 9% over max and runs only 200ma HOT current?

I for one already have, in your meltdown thread:

http://videokarma.org/showthread.php...hlight=tomcomm

John Folsom has much more knowledge on these old sets than I ever will, if you won't listen to him why would you listen to me (again)? Let me reiterate that your experiements are worthy, I simply fail to see what you hope to obatin by operating these parts beyond their rated limits. My 21-CT-55 makes a fine picture indeed when operated at normal current and voltages (when the tuner isn't acting up), just ask anyone who's see it. My intuition tells me that you're more likely to find improvements in picture quality by providing a better signal to the CRT, like gutting the decoder entirely and going with something more modern. I don't think anything you're doing with respect to HV production has anything to do with your results, or any improvement in picture quality. Just my opinion.

[John Folsom]

I will simply add that operating the flyback at elevated voltage levels increases the chances of flyback failure. I have no science to quantify the risk, so if you are happy so be it. I expect there is a very modest radiation risk, you should at least keep the HV cover on (shielding the 6BK4). There is probably some potential risk for a CRT internal arc over, but once again, if you are happy with the risk, so am I. As I said, I just wanted to express my concerns, and after that, it is not my problem to worry about. Have fun!

[zenith2134]

Hello- I've watched this thread with great enjoyment and also admiration for your skilled work... I have no idea how much of a risk you run with a nominal hv of 30kv but I doubt it's emitting significant x-radiation. It goes without saying that the shields around the rectifier should remain in place at all times. Remember the 70s sets like Zenith that would knock the necks off the tube bell, they were supposedly spiking to about 40kV for that to happen. And of course G-E in the later 60s.
If all seems well, it probably is. Smart to check HO current to make sure.

Nice piece of TV history!

[Electronic M]

Much of your video drive experiments I find facinating, and would like to learn more about.

However like Nick and John I would not try to elevate the HV. These sets are plenty bright at 25KV, and any gain from running it high does not sound remotely worth the risk to me.
I reciently revived a CTC4 and on the first run in which I had good video I let it run about an hour, and even though no damage was done, by not preforming the sam's horizontal adjustments, I was kicking my self when after powering down I found the flyback to be warm. If I can't get a fly to run cool and or within factory specs it prevents me from being able to fully enjoy a set as I become paranoid over the possibility that I may be ruining an IMPORTANT piece of a prized possession which I can't readily get a replacement for.

A flyback is like a good car if you treat it gently, and give it the proper care it will out last you, but if you ride it hard and push the limits enough it will eventually fail catistrophicaly.

I guess you are not subjecting it to the worst conditions a fly has endured, but you are still rideing it kinda hard. What you do with it is your business, but if you don't want to be looking for another flyback please make sure to carefully monitor it, and be prepared to pull the plug at the first warning sign of failure.

Good luck! Here is hoping you don't toast another rare fly.