Restore 21CT55 to schematic or as built?

[Zenith6S321]

This is my first post, but I have been reading posts for a bit. Great forums! I am starting the restoration of my RCA CTC2B 21CT55 chassis serial number B8802628. I bought it in the early 1970's and am finally going to attempt to restore it. I bought capacitors and resistors based on the Sams parts list. I started comparing the convergence chassis wiring against the Sams schematic and find missing/different components. I expect this is due to running production changes. Is it generally recommended to restore a chassis to as built or to the latest dated schematic? Anyone know if the Sams 21CT55 schematic represents the "first set" or "final set" design? Any guidance would be greatly appreciated.

[Phil Nelson]

I go with what I find in a TV, if it looks original. My theory is that it must have worked when it left the factory.

I would be especially cautious about changing components in the RF and IF sections.

Just my $0.02.

Phil Nelson

[andy]

...

[Steve K]

When I restored mine I went by the RCA manuals and schematics. I don't remember any differences. Remember, Sams often has mistakes. I'd be happy to look up something for you in my schematics or send you a copy if you would like.

Steve

[Steve K]

Andy:

I didn't mean to repeat what you said, you posted as I was typing.

Steve

[reeferman]

Stay as made. Rca built it, not Sams.

[Zenith6S321]

That all sounds like very good advice guys.

Phil, thank you very much for your excellent web site covering the CT100 and your restoration. I also had problems with intermittant loss of color synch in this CTC2B chassis, some 20+ years ago. Now that I can afford the proper test equipment I look forward to trying to track it down. Although the set displayed a very nice picture when last turned on 15+ years ago, it required lots of tweaking as it operated. The 21AXP22A and the flyback were replaced by the previous owner (TV repair shop manager).

Andy, Steve and Reeferman, thanks for the warning about Sams accuracy. At the least I will update my Sams schematic as I recap and verify resistances and check the coils for continuity (maybe I will also try to find an inductance meter).

Steve, I have searched the web for a copy of the RCA service manual/schematics to no avail (except for Sams). It would be very helpful if I could get a copy of any information you would be willing to share!

[old_coot88]

Ditto on Sams being prone to errors.
Bill(oc)

[Phil Nelson]

Originally Posted by Zenith6S321

I also had problems with intermittant loss of color synch in this CTC2B chassis, some 20+ years ago. Now that I can afford the proper test equipment I look forward to trying to track it down.

If you find any good tricks, I'm all ears. I do need to revisit that issue. Just got distracted with some other projects while waiting for my brain to grow a couple more hat sizes

Phil Nelson

[Zenith6S321]

Steve McVoy, PM sent.

Phil, I have no illusions that I can solve such a challenging problem, but its sure fun to investigate. Back in the 70's I used my old Knight Kit emissions tester, EICO low frequency O-scope, and EICO VTVM to fix all my neighbors B&W tvs and did manage to get the 21CT55 working. Everyone that saw it found the picture quality amazing compared to the 70's vintage picture quality. I never was able to get very good convergence at the edges of the screen. I think I recall that the HOT coupling capacitor was leaky, which may have caused the first flyback to give out? I did have to replace the 30KV HV cap, as it was shorted. The 21AXP22A had been replaced with the 21AXP22A. A 21AXP22/21AXP22A data sheet PDF says "The 21AXP22A features an internal neck coating having high resistance which eliminates the need for an external resistor between the anode power supply and the tube to proctect against damage by a momentary arc." But the CTC2B Ultor lead includes a resistor between the CRT and the HV section. Is it possible that the internal CRT resistor reduced the current draw and allowed the HV to exceed the 30KV cap rating? I did have problems with arcs from the exposed HV part. I now doubt the accuracy of my old VTVM and HV probe back then.

[Zenith6S321]

KX250, my set had a replacement 21AXP22A CRT and flyback in it when I got it, but it had not been in operation after the flyback went in. Plus it needed some electrolytics before I got it working again, way back then. Steve is sending me the RCA info for the set, so I'm going to use that for reference (especially in the flyback/convergence/HV section) while the rest I will keep wired as is, but with new parts where needed.

Sandy, I agree as witnessed by a, then, college kid spending $75 in the 70's to buy an old broken TV.


Findm, I'm way down south in St. Mary's county. I will post picturess when I have something "photogenic".

Stromberg, I'll keep an eye out in that area of mine then, thanks.

[old_tv_nut]

Originally Posted by Zenith6S321

Steve McVoy, PM sent.
Is it possible that the internal CRT resistor reduced the current draw and allowed the HV to exceed the 30KV cap rating?

I don't think so, since the current load from the CRT normally goes to zero on a black screen, and the regulator takes it all.

A shunt regulator works by measuring the current at the bottom of the flyback and keeping the total constant. Total current is divided between the CRT and the regulator, depending on how much the CRT demands. If the regulator fails, you could get an overvoltage, but not if it's working normally.

[bgadow]

Welcome to VK! What part of the "free" state are you from? Very desirable set you have...you shouldn't have any trouble getting the answers you need on here.

[Steve K]

Zen...

Send me a PM with your name and address and I will send you copies of the service info.

BTW, I pulled out the Sams and the convergence chassis is different from what is in the RCA schematics.

Steve

[Zenith6S321]

PM sent, thanks Steve!