Andy, just letting you know that I tried your DC restoration trick with the diode and got very good results.
I used a couple of 1N34A diodes in series. I knew that they would have negligible capacitance of a few pF. I tested the reverse breakdown at just under 100V for both diodes. The video swing at the CRT cathode is about 75V p-p, so I figured two 1N34A in series would give adequate margin.
I also added your recommended bypass cap. The DC restoration as observed on the scope was quite obvious. As the scene content varied, there was nearly no movement in the average video voltage. The minor DC variation that could be seen, if observed carefully, was mostly due to the non-zero impedance of the brightness pot.
The picture was indeed greatly improved. Now the color saturation stays put on someone's face, even if the back ground brightness varies substantially.
My motivation for adding the DC restoration was that I was finding that color saturation was very dependent on overall scene brightness.
Surprising that something else in the set could not have been traded in the original design for such a cheap and very effective image quality improvement.
By the way, I did not see any retrace from lack of blanking. The CRT on my set is pretty strong. It could be that on a set with weak CRT emission, the brightness control will be turned way up, making it more difficult to blank.
I did not add the 100k resistor to the bottom of the pot to limit the max brightness. I suspect a 50V zener or NEON bulb may be better than the 100k to keep the pot impedance low for best DC restoration. I have not tried this.
My Portacolor set is a Bradford 1171D24 sold by W T Grant as WTG79814 with the 10HE Chassis made by GE in 1974. I have the Photofact folder 1394 in scan form available for anyone interested at
Bradford Portacolor.
Thanks for the original post. Saved me a lot of head scratching to figure out a good DC restoration circuit.
-Joe