[old_tv_nut]

This is likely a problem with the particular version of Zenith chassis - a countdown IC for V and H drive that doesn't recognize that the game signal is non-standard and doesn't switch to independent H and V. Manufacturers invented countdown sync circuits that were absolutely noise immune on a standard signal and could sync when the signal was so weak that your eye couldn't recognize it - then they got socked with games and VCRs with non-standard sync and had to degrade the circuits to follow the VCR head switch timing and non-standard game sweep frequencies. So, from time to time there were short periods when new sources came out and the TV makers had to scramble to cope with what the game hardware designers did.