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  #1  
Old 08-26-2011, 12:55 PM
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Bob Galanter
 
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CTC4 Convergence Pots

Hi Gang,

In the process of restoring the Hallicrafter CTC4 clone, I discovered that the pots on the convergenc panel were all worn out. Most of the values were way off, and as a result it was completely impossible to do a good convergence. The best course of action was to completely rebuild the convergence panel.

Many of the pots were common types with standard values and were easy to obtain from various vendors such as Mouser. However there were two variations of pots that presented a problem.

The first issue was the screen pots. They are 500K, with plastic shafts that mount with twist tabs. These pots ride at "Boost" voltage of around 1KV. Standard pots are rated for 600VDC. RCA built a phenolic insulating panel into the aluminum convergence chassis and then used twist tab pots with plastic shafts to protect against getting a shock from the high boost voltage present on these pots. Although 500K pots are available, the only ones available have the conventional mounting with a nut, and this mounting type would defeat the phenolic insulation scheme RCA devised to isolate the pots from the chassis. Also most of the 500K pots I found have metal shafts, which also present a shock haxzard. So rebuilding the old pots was the logical option. I was able to scare up a bunch of 500K CTS pots of the same design and I transplanted the new elements into the old pot housings. Problem solved.

The real big problem was the 100 ohm, dual concentric pots. These were pure unobtanium. I searched for weeks until I located a source of 100 ohm CTS pots that were usable for transplanting the resistance elements. It takes a bit of doing. Only the front resistance element, is a direct, "drop in" replacement. The original rear resistance element is slightly different, and the new 100 ohm resistance elements need to be modified to work properly. The notches around the edge are wrong and must be filed to fit properly, and the hole in the center is too big. That hole needs to be "bushed" to the proper diameter or the wiper will not rotate correctly. I accomplished that with some special phenolic bushings.

After rebuilding all the bad pots and replacing the rest with new ones, the convergence panel worked just like it was new again. If you have problems getting good response from your CTC4 convergence panel, I suggest looking at doing a rebuild like I did. It worked wonders for my CTC4.

If you have a need to rebuild either of the screen pots or the 100 ohm dual pots on your CTC4, see the web page I posted on my web site. Perhaps I will be able to help you.

http://antiquetvguy.com/Web%20Pages/...tsForSale.html
Attached Images
File Type: jpg CTC4 Pot800x600.jpg (63.5 KB, 30 views)
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Last edited by ohohyodafarted; 08-26-2011 at 11:55 PM. Reason: Added link to web page
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  #2  
Old 08-26-2011, 04:48 PM
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ChrisW6ATV ChrisW6ATV is offline
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Bob-

I am happy to hear you had success with the rebuilding, and glad that you have shared your experience with the TV collector community. Your results are good news for others who have similar problems in the future. Especially, the news that you have rebuilt the concentric pots.
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Old 08-26-2011, 06:11 PM
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Hi Bob,
Thanks for the info on the convergence pots. I also am having trouble getting good results, and will inspect the pots in the convergence chassis. RCA solved the design problem the best way they could in 1955, but it would have been great if they had ability to use wire-wound's like the ones in later sets. Nothing wrong with a good challenge, though, especially when a good result is achieved.
Kevin
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Old 08-26-2011, 09:09 PM
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Can't wait to install mine, convergence will then be 95% of the PITA is used to be. lol
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Old 08-26-2011, 11:57 PM
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Bob Galanter
 
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Thanks everyone for the kind words. I added a link to the web page about the pots in my original post. I guess I should have done that earlier.
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Old 08-27-2011, 12:01 AM
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Bob Galanter
 
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Quote:
Originally Posted by miniman82 View Post
Can't wait to install mine, convergence will then be 95% of the PITA is used to be. lol
Nick,

I had a chance to rebuild 3 original OEM RCA units for you yesterday, so all you have to do is yank out the old ones from the set and trade them in to me for 3 that are already to go. Sort of like getting a rebuilt alternator for your car and handing in the old core. They are ready any time you are.
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Old 08-27-2011, 01:04 AM
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Homina homina homina! I'll have to get to your place soon, then!
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Old 08-27-2011, 11:31 AM
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Kevin Kuehn Kevin Kuehn is offline
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Bob, Nice job on rebuilding those pots. That sounds like it was a major chore.

Am I correct in assuming that you ended up using 600V rated elements to replace the 1000V ones in the 500K screen pots?
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Old 08-27-2011, 07:10 PM
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Bob Galanter
 
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Quote:
Originally Posted by Kevin Kuehn View Post
Bob, Nice job on rebuilding those pots. That sounds like it was a major chore.

Am I correct in assuming that you ended up using 600V rated elements to replace the 1000V ones in the 500K screen pots?
Yes Kevin, the replacement elements are basic 600volt CTS brand carbon comp units. The main issue is keeping the original twist tab mounting the same. The twist tabs often break when removing the old pots, so it is a good idea to have some NOS twist tab pots that you can canabilize to transplant new twist tab mounts onto the rebuilt pots in order to remount them on the phenolic insulator like RCA did.

WRT the 100 ohm dual concentric pots, the twist tabs are of no consequence because these pots ride at low voltage and instead of using twist tabs I convert the mounting to a conventional threaded sleev and nut style, and the hole in the convergence chassis is already large enough to accomodate a lock nut type of mount as shown in the photo above.

Good luck!
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