Many of the troubleshooting topics here suggest using a bulb tester to help with the diagnosis. It's a simple and handy tool to help you get to the bottom of your issue. Using a bulb tester has been described here before, for example:

https://pinside.com/pinball/forum/topic/em-pinball-circuits-basics-to-not-so-basic/page/2#post-3184715
https://pinside.com/pinball/forum/topic/reading-gottlieb-schematics#post-2634425

But how many have gone out and made one? I'd like to encourage you to consider it and have written up a couple that I've made to help you get started:

https://www.funwithpinball.com/resources/bulb_tester

Bulb tester (resized).jpg

It's a great way to use up some of those used bulbs and sockets you have lying around.

If you have one or end up building one add a photo here. I'm sure there are plenty of good ideas out there.

/Mark