You could lightly tap on the CRT neck with a piece of wood or a plastic handle screwdriver and see if it changes the open condition.
Vertical loss will be in the vertical circuit. Old capacitors, failed couplets, and The Predicta's weak tube sockets.
Just looking at the CRT pinout and you could check for continuity with a multimeter between pins 2 and 6 as they're both G2 to confirm open.
