CTC4 Convergence Pots
In the process of rebuilding my first CTC4, I discovered that one of the weak areas of this chassis is the convergence panel. In particular, the carbon composition pots used on the convergence panel, have experienced a very hard life. Most of the pots can be replaced with off the shelf items that are readily available except for two items.
The first item is the three 500K screen pots. The issue here is that you need a "twist tab mount" style pot in this application unless you want to modify the aluminum panel to accommodate some sort of plastic insulator for standard pots which mount with a threaded shoulder and lock nut. The issue here is that standard pots are rated at a maximum of 600 volts, and the supply voltage to these pots is from the boost supply at about 1000 volts. Most standard pots also have a metal control shaft, and this also will not work unless you place some sort of plastic knob on the metal shaft of the replacement pot to insulate the user from shock. The best solution is to carefully remove the old pot and replace the carbon comp element from a donor 500K pot of the correct design. This is the route I took with my screen pots. If your original 500K, plastic shaft, twist mount screen pots are in need of rebuilding, I will be happy to rebuild yours for you if you decide not to attempt it yourself. Cost is $15 per pot and covers parts and labor.
The second item on the CTC4 convergence panel, which is usually in failure mode are the 100 ohm dual concentric carbon composition pots that adjust the vertical tilt and amplitude. These pots are very prone to wearing out and when they do, you will not be able to get the proper response from your CTC4 convergence panel. These pots are made from pure UNOBTAINIUM. I searched everywhere for replacements for these 100 ohm dual concentric pots. I even went as far as to contact the original manufacturer to see what it would take to have some made up. Unfortunately the minimum order was far too large for restorers like you and I. The only solution was to try and find donor pots to use as transplants for rebuilding the original pots. Finding the correct donor pots was not an easy task. 100 ohm carbon comp pots of the correct design to use as donor pots, is almost as impossible as trying to find the original 100 ohm dual concentric pots themselves. After performing this rebuild on all of the dual concentric 100 ohm controls on my Hallicrafter CTC4 clone, I was pleasantly surprised how easy it was to bring the screen into convergence. With worn out controls, you will never get the proper response from your CTC4 convergence panel.
Even if one of the old sections seems OK, I still replace both resistance elements, so that each control will work and act just like new again. I highly recommend that you rebuild all 3 of the dual controls. Even if one or more of the controls seems fine, it's probably close to being worn out, if you have one or more which are already shot. So you may as well rebuild all of them at one time and save yourself the extra work of having to tear into the convergence panel a second time. The easiest thing is to rebuild the entire CTC4 convergence panel all at once and replace or rebuild ALL the pots and rewire the entire panel.
I eventually located a source of donor, 100 ohm carbon composition, American made pots, from the same manufacturer who made the original RCA part. Here's a photo of an actual RCA OEM dual concentric 100 hom pot that I rebuilt. I purchased a large supply so that I can offer rebuilding service for CTC4 owners who wish to get their CTC4 convergence panels operating like new again. If you wish to have your CTC4, 100 ohm dual concentric convergence panel pots rebuilt, I will be happy to provide this service for you. I will rebuild both sections with new 100 ohm carbon composition elements. The price for parts and labor is $25 for each dual control. If you are interested in having either the Screen pots, or the Dual 100 ohm pots rebuilt, send me and email through the link below and I will get back to you with shipping and payment instructions.