I doubt all the displays are bad, too. I couldn't link directly to it, but check out section 5.15.1 at Pinwiki's Gottlieb system 80 repair guide (click on section 5, then search for 5.15.1 in that section): https://www.pinwiki.com/wiki/index.php?title=Gottlieb_System_80
Lots of good info there, hopefully it's something simple like a bad connector or something.