21CT55 #2628 restoration

[Zenith6S321]

I'm finally beginning the restoration of my RCA 21CT55. I had intended to begin it about three years ago after I restored some vintage test equipment. Then I ran across Phil's excellent restoration web site. Then I found the Early Television Foundation site and realized my set was kinda rare. I had always thought it was special, but I had no history of early color TV development to know how few were made and that it was based on the CT100 CTC-2. Then I found Videokarma and learned how much I did NOT know about restoring old TVs. The depth and breadth of the knowledge of the members is a treasure. SteveK sent me a copy of the 21CT55 RCA Service Clinic document that is very helpful. I then decided that instead of starting with the 21CT55, I would warm-up with early B/W sets that I was extremely happy to find were actually available. I studied all of Bob Andersen's YouTube restoration videos and learned a lot. So I hunted, bought, and restored a RCA 721TS, a Dumont RA-103, and a RCA 630TS. I mixed those restorations with a Zenith 6S321 Stars-and-Stripes radio that I have used since the 60's, a 1931 Philco Model 90, 1928 Atwater Kent Model 49, and a 1924 Atwater Kent Model 20 radio. As I learned, I picked up additional test equipment along the way. Here is a picture of my restorations arranged in my living room with my 21CT55 waiting patiently. And a few pictures of the 21CT55. Its time has come.

Dave

[miniman82]

Heh, looks like the HV cap in your blew at some point. Interesting way to make the repair, with them all arranged in a circle like that laced with dope.

[Zenith6S321]

Originally Posted by miniman82

Heh, looks like the HV cap in your blew at some point. Interesting way to make the repair, with them all arranged in a circle like that laced with dope.

I was college kid at the time (1974) and could not afford the right part. It functioned ok, but had a lot of corona, hence the slathered on corona dope. I hope to find a better replacement, but I have not started searching yet.

Dave

[ohohyodafarted]

The original gray 30KV doorknob on my CT55 also went bad. I found a suitable 40KV doorknob on ebay. Mounting was slightly different, but was easily modified to work like the original. I also see that the discharging assembly that grounds the doorknob has been removed. I installed a low RPM cooling fan on the HV cage to keep the flyback cool.

Here is a link to the restoration pages of my CT55 on my web site.

http://antiquetvguy.com/Web%20Pages/...CA-21CT55.html

Good luck and welcome to the CT55 club!

[Zenith6S321]

Originally Posted by ohohyodafarted

The original gray 30KV doorknob on my CT55 also went bad. I found a suitable 40KV doorknob on ebay.

Bob, thanks for the ebay tip for a capacitor. Would a 2700pF 40KV work ok as a replacement for the 2500pF 30KV?

Dave

[Zenith6S321]

I've done some cleaning to the chassis. I used naval jelly (thanks bandersen) for the cleaning and to treat the rusted areas. The chassis "before" picture is above, here are some "after" pictures. Also is a picture of HV capacitors that I will replace with the 40KV 2700pF door knob capacitor to replace the mess. I have not yet figured out how to clean the HV cage, any suggestions? I compared my flyback with the two Bob shows in his restoration page: http://antiquetvguy.com/Web%20Pages/...CA-21CT55.html
I think mine is the same replacement Bob is using.

Dave

[Zenith6S321]

I bought this set from the service manager at Electronic Specialty Company in Charleston, WV in 1974. My dad worked there as the accountant and I worked there a couple of summers during college. The service manager told my dad that the set was originally purchased by a doctor at West Virginia University through Electronic Specialty Co. They maintained it over the years and when it blew a flyback after the CRT had been replaced the service manager got hold of it when they bought a new set. He replaced the flyback, but had not completed the rewiring. The selenium rectifiers had been replaced with diodes before I got it. I bought it for the $75 from the service manager who had lost interest in the project. I found the 30KV capacitor shorted, so the first thing I did was fix that and a couple bad filter caps. That got the set working but it did not see much use. The CRT is a 21AXP22A dated 1957. My CR-70 shows its got emissions in the good range, so I am hoping I will not see purple in the neck when I get HV to it.

It also has a few of the white peaking coils that Nick found were bad in his, so I expect to be replacing mine.

Dave

[Zenith6S321]

Thanks for the link Bob! All advice is very welcome. I have the shorting bar along with others bits and pieces. When I got the set it had a chipped brightness knob, was missing the fine tuning knob, the volume knob, and the channel number insert. Here is a picture of the knobs I have, and another of a new brightness knob, a knob that I think is the correct volume knob, a fine tuning knob that I hope to modify to work, and an original "spare" peaking coil the service manager said I would probably need.

Dave

[Sandy G]

Can't wait to see the "Glorious Lollipop Color"....(grin)

[miniman82]

Yep, carbon tracking can cause a lot of heartache. I once had a Philco 17MT80 roundie with intermittent reds, which I traced back to a carbon track going to ground from the plate of the red output tube. I was able to grind out the carbon from the PCB with a Dremel, then fill it in with epoxy. Whole PCB would have been a loss if I hadn't been able to fix it.

[Zenith6S321]

I've been working on measuring and replacing out-of-spec resistors and all the paper capacitors. I found that the vertical blocking oscillator transformer secondary is open. I melted it out of its tar hoping to find a simple loose connection. No such luck, the open is deep inside somewhere. I was able to find a replacement from oldradioparts.com. Here is a picture of the old and NOS part. It has a -2 where the original has a -1, but its resistance measurements match the Sams. I also have found that some of my sets resistor values match the Sams values where they differ from the RCA schematic values. And also some in my set differ from both schematics. Here is a picture of the HO screen resistor which Sams and RCA (R269) list as 11K but my set has a 10K. Its soldering looked original, but I could be wrong. I guess they are production run changes. Is it best to start with my sets original values and see if I need to change from there?

I have also found all but one of my sets white peaking coils are open. I got some replacements from oldradioparts.com but had to get some from digikey. Can modern coils of the same inductance be used as peaking coils? I think the old ones as Pi-wound, no clue on the modern ones.

[old_tv_nut]

11k sounds picky - 10 k should be good.

New peaking coils should be OK unless they have so much stray C that they self-resonate in the video band. Try them, they probably work.

[miniman82]

Meh, people told me not to use the ferrite cored ones from Digikey, but they work just fine in my CTC-2 and 2B.

[Zenith6S321]

Cool, thanks for the information guys. I'm eager to fire it up, but I still have to rework the HV cage. Any good way to remove corona dope?

Dave

[Zenith6S321]

I replaced all the bad peaking coils and then compared all of the wiring and component values against the schematics. Then I installed the F104 300mA fuse ... and ... another carbon tracked part let out some smoke. Here is a picture of the burst amp transformer with a carbon track. I used a dremel tool to remove the carbon and trim that area away from the case and re-installed it.

I then turned my attention to the HV cage that needed to be completely rebuilt. I decided to remove it from the chassis to try to clean out the goo left from the original doorknob cap meltdown. Here is a picture of the mostly empty cage. I noticed my flyback had an opening right under it, but the cage did not. So I added an opening for better cooling. Then decided to add a fan. Here is a picture after the cage cutout. Here is a picture of the rebuilt cage with all new HV doorknobs. And picture from the bottom of the rebuilt chassis.