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Originally Posted by andy
The contrast problem you're describing is caused by a lack of DC restoration on the video. Basically, the video is AC coupled, so all the DC information in the video is lost. The circuit needed to restore the DC level is actually very simple, but it was usually not included to cut costs.
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The problem might be lack of DC restoration, but I would also suspect a lack of video AGC (Automatic Gain Control) on that set. Many/most TVs from that era either had no AGC, or a less-functional form sometimes referred to as "simplified" AGC. [I'm not sure when improved, "keyed", AGC was introduced.] The result is that the contrast level will change as the signal level changes (i.e. as you switch channels or adjust the antenna). If your set does have at least some form of AGC, there should be an adjustment for it on the back of the set somewhere; that might help a little bit, but don't expect it to be perfect.
In any case, if the problem is poor/no AGC, then you'll see the contrast change dramatically if you adjust the antenna or move it around.
An easy way to tell if a TV doesn't have DC restoration is to see what happens during fade-ins/fade-outs between scenes in a show, or any program material that has large areas of solid black or solid white with some other motion (for instance, a white credit scroll/crawl on a black background). If fade-outs fade to a mid-grey instead of black, or white-on-black titles end up looking like white-on-grey (with the background grey level constantly changing as the amount of white text on the screen changes), or black segments between commercials turn grey instead, then you don't have DC restoration. Most TVs made in the mid-50's through the early/mid-60's don't have DC restoration, but a lot of older sets did.
BTW, regarding that GE UHF converter calibrated in MHz rather than channel numbers: I actually have one of those too, and I don't know why GE decided to calibrated the dial like that either.
Then again, the FCC way-back-when actually assigned "channel numbers" for all the assignable frequencies on the FM broadcast band (didn't the numbers start with 200..?), but I don't think very many FM radios ever actually had those channel numbers marked on the dial. Did any FM radio stations ever actually announce their channel number duing station IDs rather than their broadcast frequency?