[Kevin Kuehn]

Originally Posted by old_tv_nut

I think you misunderstood. In an ideal case, the brightness voltage and voltage on the 0.1 uf cap would be completely steady, so the sync tips would be clamped to a constant voltage no matter what the video content is.

The indication of the effectiveness of the DC restoration is the DC voltage measurement at the cathode. With no DC restoration, the change from bright scene to black scene will be minimal - it will just be set by the brightness pot. But with the DC restorer, the DC voltage will be high on the black scene and lower on the bright scene. Looking with the scope, set to DC coupling, the dark parts of a bright picture will be the same voltage as the black of a black screen (or almost the same, depending on how complete the DC restoration is).

Looking at your screen pics, you can see plainly that with the test pattern, the blacks are properly black, and with the black image they are still properly black, not gray, so the DC restoration is doing its thing. If you get an old movie that has spooky night scenes where nearly everything is dark, the picture will actually be dark like it should be, instead of foggy medium gray. This will not be visually subtle!

Your measurements at the wiper would indicate that the black level probably shifts about 8 volts when going from bright test pattern to full black. This is small compared to the full 50 v p-p video signal, so is much better than the non-DC- restored original.

If you look with a DC-coupled scope at the cathode, you will be able to see how little the black level shifts compared to the huge changes without DC restoration.

Thanks for explaining in greater detail. I installed a 1 meg bleeder across the diode which resulted in less change at the brightness wiper, and the voltage change at the cathode being greater. Seems like what was lost at the brightness wiper was added to the cathode. I find it very difficult to monitor the black level of a playing video on the scope. Using the B&K test pattern was the only way I could see it change, or not.