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This is somewhat on-topic since it deals with the contrast controls on late-1940's sets, but I've noticed that high settings of the contrast control resulted in severly-overloaded video and loss of sync on ALL of my sets when connected to my home's amplified antenna distribution system. Setting the control fully CCW would kill the sound and video on most of the sets. I found that these sets all lacked modern AGC circuitry and were designed to make the most of fringe reception. In fact, the alignment instructions for the 721TS call for connecting a VTVM into the circuit near the contrast control and setting the control to give a reading of -3V as a preliminary step in the video IF alignment procedure.
To minimize overloading on all sets, my display room is now fed from the same distribution system via a switchable attenuator and that has cured the problem. Channels I've received are Cleveland channels 3, 5 & 8 and then by rotating the antenna I can pick up 2. 4, 9, 11 and 13 which are from other cities mostly to the east.
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