Overloading video was also my thought when looking at those screens. Too much gain might mess up the sync, and thus sweep.
I believe the Riders manual for Admiral chassis 21C1 is at ETF:
http://www.earlytelevision.org/tv_sc...s_postwar.html
I don't see an AGC adjuster in a quick peek at the schematic, but the manual does have this note:
AGC may be as good a place as any to start checking. Some of my sets are quite sensitive to the signal strength from different sources. In addition to adjusting the Picture control, you might also try out different sources, if you have any (DVD player, VCR, digital converter, signal generator, etc.).
Phil Nelson