DC restoration (can o' worms opened)

[David Roper]

The Emerson 610 is one of my favorite electrostatic sets. There are a couple of videos uploaded to YouTube showing operating sets. One shows what looks like very typical performance of an entry-level set, but the other video is extraordinary not only for image sharpness, but what is apparently a constant black level with genuine fades to black. Intrigued by the difference, I took a look at the schematic to see which one was performing as designed.

I posted a message on the linked video asking the uploader if he had added a diode when he restored his set. This was his surprising response:

I must admit I didn't spend too much time so far with that tv.
Unlike most other sets from that period its video signal is fed to grid 1 of the crt and not to the cathod, at grid 1 the video signal is mixed with a low dc from the high voltage oscillator so a stable dc is
present
at grid 1. But thinking of it, you are right! The basic black level of the picture is not recreated through this measure as the dc level is always the same.

Where should I place that diode you propose? From grid 1 to ground as seen in other tvs? But there the video signal goes to the cathod...

I saw the connection of G1 to the HV oscillator in the schematic but didn't think it had anything to do with DC restoring. Does it? Can the difference between the above linked demonstration and this one be chalked up to nothing more than misadjusted brightness?

I leave it to the experts.

[Kevin Kuehn]

Could the G1 with with the addition of the bias possibly be acting as a grid leak detector, so the picture average adds or subtracts from the DC bias? Just a WAG as I'm no expert.

[ChrisW6ATV]

Originally Posted by David Roper

Can the difference between the above linked demonstration and (the second one) be chalked up to nothing more than misadjusted brightness?

I leave it to the experts.

I watched both, and I cannot tell if one is really doing a better job with DC restoration than the other. The "good one" looks like it has some too-dark objects in some high-contrast scenes, which will help with dark areas in other scenes, and the "bad one", besides having ambient light that makes it harder to evaluate, is not using a very good program example. That program seems to have about the same amount of light and dark areas in every scene (and is a very old video itself apparently), so it is hard to judge.

DC restoration is a great can of worms to open! I have never got the exact hang of how it works when it is good, and what is wrong when it is missing. I love how good my RCA 8TS30's picture looks with its good DC restoration. All the black-and-white sets I ever saw when I was growing up had none whatsoever, and I still remember being amazed watching my brother's Sony KV-1920D 19" color set that he bought in early 1977. "Wow, night scenes make the TV screen go DARK! And, the screen goes black between commercials!"