The CTC-5 Contrast Control Modification

[Penthode]

Originally Posted by old_tv_nut

These sets were not designed to have 100% DC coupling, as you stated (even before the contrast mod). Trying to set the bias to maximize it is mostly futile. If you want 100% coupling (or full restoration), feel free to redesign the circuit - but this of course is no longer "restoring" the set to original operation.

I agree 100% with the desire for full DC response for picture quality, but I don't plan to try redesigning the circuit to get it on my '5.

I agree that the sets were not designed for 100% dc coupling. It was the inevitable cost trade off. And I am not suggesting that the set be redesigned.Although what I am trying to say is that those who make the mod are trying to do so.

I am now content undoing the mod and keeping the set as original. In the CTC5N Manual (T-6) instructions are given how to properly set up the background and screen controls as the set was designed. I find that this will help minimize but not eliminate the blooming problem when the contrast and brightness controls are not adjusted properly. Setting the background is reducing the 21AXP22 grid bias will compensate for the reduced cathode voltage when the brightness control is adjusted. I am more or less restating what is in the RCA instruction manual.

I perhaps did not make this clear, but setting up my CTC5 without the mod. as per the instructions, the dc level is not perfect but not bad. And I feel that it is better than before I undid the mod.