Robert,
I see you got an answer to this same question on that other forum, however I, as you know have this same Admiral and although I experience slight the loss of all functions when the contrast control is turned way down it is not a problem because I never use the control that low. However the chassis that preceeded this one, the 30A13 has this problem really bad, turn down the contrast and sync gets lost, sound disappears and the picture goes away, I guess it is a manual screensaver control.
I guess one could design a blanking circuit fix for retrace lines by examining the schematic and poking around with the chassis on the bench. I've not felt it necessary.
Another cause for visible retrace lines is the lack of a DC restoration clamp circuit in the video output stage where it drives the CRT. Without a clamp circuit, and many of the earlier sets did not have one, it is a scientific impossibility to achieve full grey scale dynamic range without seeing the retrace lines. My 30A13 has a DC restorer (half of the 6AL5 used as the video detector) but my book covering the 21X12 is out in the shop so I cannot say. I suspect it does. Perhaps your restorer is faulty.
Rob