RCA 21CT55 #8802897 is Dead

[stromberg6]

Hi, Great that the flyback enables a picture. I may be wrong, but I seem to recall that a 6JE6 is a 6DQ5 with a different base, having very similar if not identical specs. I'll look at my RCA tube manual later and compare the two. If that's the case, you can just plug the 6DQ5 into the socket with no mechanical changes.
Looks really good so far!
Kevin

[Tomcomm]

Hey stromberg6......You're right on, the 6DQ5 and 6CB5 are electrically identical! The local elect store has them for $22. Mechanically its perfect, all I have to do is interchange the 2 grid pins with the 2 screen pins. In fact RCA used the 6DQ5 with my new FBX thru CTC12 then switched to the 6JE6 in the CTC15 with the same FBX. If this doesn't calm the ultor out of the mid 40KVs, I'm at a loss.
Thanks a lot......Tom

[doogie812]

I remember in the later days of the roundies and even after RCA had the HV rectifier drop into the top of FBX. Many of those fried because of dry or contaminated dielectric grease or loose cage screws. Out of scrap I had used a CTC-11C FBX in a CTC-16. My earliest experience with roundies was with a CTC-7. If your chassis uses a 6BK4 shunt regulator I would imagine a later xformer could be used with a little circuit modification to make your set functional. Question; What CRT do you have in that thing? It looks to be a glass bell. If I remember correctly the 21AXP22 was metal.

[stromberg6]

Originally Posted by Tomcomm

Hey stromberg6......You're right on, the 6DQ5 and 6CB5 are electrically identical! The local elect store has them for $22. Mechanically its perfect, all I have to do is interchange the 2 grid pins with the 2 screen pins. In fact RCA used the 6DQ5 with my new FBX thru CTC12 then switched to the 6JE6 in the CTC15 with the same FBX. If this doesn't calm the ultor out of the mid 40KVs, I'm at a loss.
Thanks a lot......Tom

Your welcome Tom. I just looked at the RCA manual, and the specs for the tubes 6DQ5 and 6JE6 for practical applications are identical, only a few minor differences, and the 6DQ5 looks like an excellent sub for the 6CB5, but for the connections at the bases. Interesting that the pin outs are different, perhaps to prevent quick swapping, protect patents, or to generate service revenue when uninformed techs subbed one for the other lol.
Let us know how the swap works. Hope it all comes together the way you want.
Kevin

[Tomcomm]

doogie812.........The FBX I'm using was intended to have the EHV rect on the top of its HV doughnut. Any loose fit would produce a continuous arc that could take the FBX out. The CTC2B FBX had a HV lead and tube cap. I cleaned all the HV grease out and removed the little hooky spring. I then quickly soldered a tined spot on the large output contact area and tack soldered a EHV rated lead wire and cap going to the original EHV rectifier. I then completely filled the output cap cavity with RTV silicon. Works fine, but at an occational 40+kv non loaded run with no 6BK4 reg the little 3A2 EHV rect arcs across its full 2in length. Im using my original 21FBP22A rare-earth gray screen I installed in place of the 21AXP22 in '64. This is the last and IMHO the best roundy CRT. RCA rates it's ultor at 27.5kv and their data sheet states "Brilliance and definition decrease with decreasing ultor voltage".

[miniman82]

Originally Posted by Tomcomm

an occational 40+kv non loaded run with no 6BK4 reg the little 3A2 EHV rect arcs across its full 2in length.

And what are you hoping to accomplish by doing that?

Im using my original 21FBP22A rare-earth gray screen I installed in place of the 21AXP22 in '64. This is the last and IMHO the best roundy CRT. RCA rates it's ultor at 27.5kv and their data sheet states "Brilliance and definition decrease with decreasing ultor voltage".

Again, you need to stop misquoting the tech sheets. 27.5 is design maximum, not a 'rated value'. Here's a link to the PDF: http://frank.pocnet.net/sheets/049/2/21FBP22.pdf

Any time you operate the tube beyond it's deign center values (found on page 4, which clearly state 20-25kv), you open yourself up to excess x-ray production, and an increased instance of electrical shock or tube destruction from arcing. I caution anyone on these forums from doing anything you're attempting here, I feel it's unsafe on a number of levels as you've already demostrated by destroying a flyback.

[Tomcomm]

Originally Posted by miniman82

And what are you hoping to accomplish by doing that?

.

Since I was replacing the original CT2B FBX circuitry with the entirely different FBX circuitry from a CT almost 10 years newer, I had to know how high the ultor EHV would go if the CRT was at cutoff and the 6BK4 HV reg would suddenly burn out. Surly this makes sense, even to you??

[3Guncolor]

Back in the day it was normal to run these tubes 21FJP22/FBP at 25kv. 20 to 24 would be on the low side.

[miniman82]

Originally Posted by Tomcomm

I had to know how high the ultor EHV would go if the CRT was at cutoff and the 6BK4 HV reg would suddenly burn out.

Same thing as any other HV section, voltage goes through the roof.... On my CTC-9, anode voltage shoots up to around 28kv without the 6BK4. But that's common sense, no? Shunt regs are pretty rugged though, why are you worried about burning one out?

[doogie812]

I thought so… Since you are using a glass tube you can nix the HV capacitor. Glass has a dielectric constant of 1. Doogie

[doogie812]

Make sure the dag is grounded.

[Tomcomm]

Since my CTC2B FBX failure, I have attempted to replace it with a “modern” roundy FBX used in the CTC 10, 11, 12, 15, 16 and 20. The support circuitry for this FBX is totally different from the original CTC2 and CTC2B I’m familiar with. For starters, none of the CTC10 and later have width controls and must rely on component tolerance to assure proper overscan. They incorporate a balanced pair of windings with a 10ohm wirewound pot to center the raster +/- 2in. Even the crucial damper circuitry is totally different, using an ferrite-cored vari-inductor and capacitor values to accomplish horz linearity adjustment with an “efficiency control” that performs some sort of resonance?? All of this design rational is unfamiliar to me since my only TV reference ends with Fink, 1957.

In my present FBX test-bed module, the vari-inductor has no effect on anything over its full range, not good. I have the wrong RCA coil part number but have ordered the correct part, got to wait till it gets here. Horz linearity is very poor and the EHV Ultor will climb well past 40kv, resulting in severe arc-over of the little 3A2 EHV rectifier. With almost all post CTC10 roundys using this FBX with many circuit variations, surely some members have experience with these “modern” FBX configurations. All replies welcomed……Tom

[Tomcomm]

I am attempting to replace my unavailable original 21CT55's CTC2B FBX with an available CTC11, CTC16, CTC20 FBX. The new TV's horz oscillator and FBX support circuitry is completely different from the original CTC2B's. The only part that seems to be the same is the yoke. Progress so far has been encouraging. With 115vac mains, I get B++ of 396v, 170ma at the fuse, 170ma at HOT cathode, and a Boost of 840v with a minimum Ultor of 27KV. I use the new FBX to deliver chroma burst keyer and color blanking pulses which generates good chroma channel performance. Only vertical dynamic convergence is presently operating, horizontal is non operational since I still have problems with the display's horizontal fold over. No sense working to align horiz convergence until I fix the fold over.

[miniman82]

I'm officially calling BS: there's no way in hell you have only 128ma on an output tube making that much high voltage from the flyback, it's simply not possible. You're fooling us and yourself if you think you have the right reading, I promise you it's a LOT higher than you think.

[RobtWB]

Nice job of circuit re-engineering / design.
From what I can see on my crappy old crt monitor you are almost there. The color looks exceptional, tweek the horizontal and you are home free.
For me I wouldn't get obcessed with voltages, currents, et cetera. The picture displayed tells just how close you are.