[Vectrex won't display the game, only 3 lines.]

I saw this vectrex for sale for $400 yesterday but it won't display the game. The audio comes through clearly and it's different for every cartridge so I know its reading it but it won't display anything but 3, sometimes 2 lines. The seller said it displayed some of the games when he first got it recently but now it won't. Does anyone know what is wrong with it?