RCA 21CT55 #8802897 is Dead

[Tomcomm]

Well now that the heat of this thread has subsided and a lot of good FBX info conveyed, I wanted to repeat portions of my initial post that described where I was coming from with my chassis-only 21CT55. Why I feel no guilt regarding my methodology in pursuit of realizing this ancient set’s picture-quality potential.

.................I decided in October 2007 to reactivate the beast after 40 years in attic storage and use it as a test bed to determine and demonstrate in photos, the picture quality this 55 year old RCA CTC2B chassis was capable of producing. As such, it was not constrained to replicate the original circuitry in every detail or even operate to the '55 RCA spec. I jacked up the +400v B++ supply to above +420v with silicon diodes and got a horz boost of almost 1KV. I intend to start reducing the CRT Ultor HV from its present 34KV to the 28KV range while doing a full convergence at each reduction level to determine if I loose ANY picture quality.

Note: All screenshots in the Photobucket below were taken at the reduced ultor of 27kv and a B++ of 420v. No abuse here. It has run at these settings from the screenshots of Nov 16, 2008 until the FBX blowup of March 21, 2010. At no time did the FBX coils get above 135 deg or the B++ fuse current above 260ma. I’ve never opened the HOT cathode to measure its current so I don’t know its value. The CRT is a 21FBP22A, the most advanced and possibly most rugged of the latest RCA roundys

Yes, I brutalized this valuable historical item but there are at least five other 21CT55 restorations on this forum alone. I believe I've succeeded in my goal of demonstrating the 21CT55's ability of displaying exceptional picture quality…………………….


That said, I realize the only reasonable chance of resurrecting this CTC2B chassis is thru the knowledge and skill of Vk member John Folsom who remains optimistic that he will produce a replica of RCA’s second color FBX, possibly this year. I am so grateful that he consented to give it a try. Truly this effort rivals the rebuilding of the 15GP22……as far as I’m concerned. Thanks John for your interest and dedication ………Tom

[miniman82]

Originally Posted by Tomcomm

At no time did the FBX coils get above 135 deg or the B++ fuse current above 260ma. I’ve never opened the HOT cathode to measure its current so I don’t know its value. The CRT is a 21FBP22A, the most advanced and possibly most rugged of the latest RCA roundies

See, this is where you loose me.

Let me just preface this by saying that I don't personally care what you do to your set; it's yours, and you're free to do with it as you please. But you can't say the things you just said, while in the same breath claiming 'no abuse here'. Doesn't make any sense. As I said before, I've never encountered a schematic text that instructed techs to do anything with reading current from a fuse holder. If you want to know HOT current, you read it from the HOT cathode. Since you didn't, I can promise you that it was most certainly higher than it ought to have been. Despite your assurances that the coil never got hot (again, no 'specs' as to what is deemed 'safe' as far as temperature is concerned), it's my conclusion that running the set the way you did is what caused the failure.

I won't speculate on how 'rugged' one tube is over another. I can only tell you that the RCA manual I have in front of me has the 21AXP22 CRT operating at a 25kv ultor, maximum designed voltage being 27.5kv (see http://frank.pocnet.net/sheets/137/2/21AXP22.pdf). The 21FBP22 is designed to operate at 20-25kv, and has the same maximum ultor rating. Any time you exceed a manufacture's recommended ratings, you are asking for trouble. That's all I'm going to say about that.

the only reasonable chance of resurrecting this CTC2B chassis is thru the knowledge and skill of Vk member John Folsom who remains optimistic that he will produce a replica of RCA’s second color FBX, possibly this year.

And I hope he succeeds. I can only hope that you learn from this experience, and check HOT cathode current correctly next time. These old parts were never designed to be pushed beyond their original limits, which many will agree were probably being pushed by the factory when operated within design tolerances.

Again, no bad blood. I just hope that you look for improvements in other areas in the future, because pushing the HV high again will likely lead to the same result. Best of luck.