Could also be a bad bulb socket. If you've tried cleaning it already try this fix. Stole it from the pinrepair website. I used this tip to "fix" several bad sockets on my SI pin.

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Welcome! But I'm going go to go back on my above suggestion. I popped the hood on my Bally SI and discovered I installed lots of 470 ohm 1/4 watt resistors on my sockets to eliminate flicker. My tip above is geared more toward a socket with an intermittent or bad connection which would manifest itself in an incandescent bulb being dim compared to it neighbors, or simply not working at all. Check out this short video I just shot. Thanks for the topic and getting me off my ass and into my game room to make this easy repair!

Edit- fixed the sound in the vid.