If only one of your Bally or Williams gas plasma displays doesn't work, it
may be that the one is just bad. This is an easy method to check if a
display glass is good or not.
First, tear off the plastic-type rubber on the front of the glass and look
for a small wire, mostly at the bottom of the tube--not under. If this wire
(I think it's a fuse that burns when air is getting inside the tube) has a
grey spot, the display is bad and you must replace it. This is true for
numeric and alphanumeric displays.