The glass score display tubes used on most pinball machines have a limited life and will eventually "gas out". This is not only inconvenient it is now days hard to find replacements and they can be very expensive.
For this reason we should try to extend the life of our displays for as long as possible. Many of these displays run at high voltages of around +100V DC and -100V DC. This allows them to be nice and bright when the pinball is being used in an arcade, but these days many of these machines are now installed at home.
Reducing the voltage by just a small amount will give a dramatic increase in service life. In this example I will be changing the zener diodes on a Data East 520-5000-00 power supply board. By changing the 100 Volt 1 Watt 1N4764 zener diodes to 91 Volt 1 Watt 1N4763 diodes we will effectively reduce the voltages being delivered to the displays. On this particular board the diodes in question are D9 and D10.
You can see D9 and D10 highlighted in the image below. Before changing these the measured voltages were around ±103V DC, after swapping them for 1N4763 the voltages were reduced to ±95V DC. The difference in the display is barely noticeable.
It should be noted that if your display is already flickering or faded then it is probably already starting to fail. In this case changing to a lower voltage may actually make the symptoms worse or may cause it not to display at all. In this case either your board needs servicing or the displays need replacing. If you do replace the displays I highly recommend changing the diodes at the same time.