CTC4 Convergence Pots

[ohohyodafarted]

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