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I dug out the field service guide schematics for the CTC-5 and -7. The contrast control in the -7 is coupled through a capacitor, so it won't change the DC gain of the video output. This would make it more stable against video output tube variations, but it also means you get a variable percent of DC restoration with the -7 circuit, depending on the contrast setting. The contrast control in the -5 is directly coupled to the video output cathode, so it varies the gain of the whole signal, AC and DC alike. However, when turned towards max contrast, it will also magnify any DC video voltage errors that are due to the operating point of the video output tube.
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