What does it look like on? I'm dealing with a similar issue, also alphanumeric. One display is fine, the other is faint and splotchy looking. 1st step I am replacing R48 and R49 per the pinrepair guide.
Check the displays and if there are little bright speckles like tiny diamonds when the game is on, this usually means the display is outgassed. Also there is a little are below the visible section on the display with two tiny wires. If some white dust is seen, that's also a good indicator of an outgassed display.
On my display, the speckles are there whether the display is on OR off. I have not looked for these "two tiny wires" yet.
The top one works, with segments a and b not working. The bottom one is completely dead. I'll post some pics with it on, but I don't think the bottom display changes at all when it's on.