Can we discuss DC restoration circuits?

[old_tv_nut]

Originally Posted by ChrisW6ATV

Your description is correct for the problem.

No, it's stated backwards, as I explained.

Originally Posted by ChrisW6ATV

Years ago, I had a Sears portable color TV with lousy black-level retention, but I could make it pretty good if I cranked the contrast way down. (I had to have the room lights off and it was essentially unwatchable, it was so dim, but it really did keep its black level consistent within that low range of contrast.)

If you look at the CTC-7 contrast control circuit, you can see that it changes only the AC gain of the video ouput. AC gain is greatest when the contrast is set to max and a large capacitor shunts the video amplifier cathode resistor. Therefore, the DC coupling percentage is greatest at minimum contrast (where the AC gain is small and closer to the small DC gain). Probably something similar was involved in your Sears portable.