All four displays used to work fine. Game was in storage for about 5 years. On starting it back up, 2nd display was out (planned to fix it later). After a few more months, display one is out. No clear signs of outgassing or gradual degradation, but did have some new/spare glasses here so I swapped both with no improvement. Displays three and four are clear and bright. High voltage power is clean and stable all the way to the display board.
I'm not seeing any single component failure that would lead to just the top two displays going out entirely. Any thoughts before I just buy an LED replacement?