Thanks, gonk - the S-video cable turned out to have a broken pin. Alas, this was one of those Outlaw sent me a year ago for contributing to tsunami relief. I probably busted the pin while struggling to connect it in a hard-to-reach place at the back of my cable box. On my shelf rig I can turn it about 90 degrees but no farther, so connection changes are a real hassle.

Right now I have a regular 75 ohm coax video cable between the SA cable box and the Panasonic plasma TV. Works fine. Thanks again.