Up to Potential

We've had a Gottlieb Solar Ride pinball for a while now, and it has had consistent problems with the displays.  They have ghost numbers - the score on the first player display will appear on the other displays as the numbers increase.

Yesterday I traced it down to a failed zener diode / resistor causing the +8V reference voltage to go away.  From the Futaba web site:

"If the filament potential is lower than the anode and grid cut off voltage, thermionic electrons can reach the anode and cause illumination of the phosphor. The filament bias voltage should be increased to prevent this problem." 

Naturally.