I'd turn the game off. Then reseat all ribbon cables involved.

LTG : )

Check your display voltage too....
Should be -100V and +100V.

And post a picture of the CPU board so it can be looked over for battery damage.

There are two things evident in the image. It is not well lit and not all in focus so there are probably other things not visible.

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I think there is other evidence of alkaline corrosion (black darkened solder mask) that remains. It's obviously been previously repaired but I don't think the repair was a full and complete repair or replacement alkaline batteries have again leaked (in the past) and caused more corrosion.

You can measure the voltages at J5-3 (-100V) and J5-4 (+100V) on the power supply.
My game has a led board, so no hv-components, but the location is the same.

power_supply (resized).jpg

Alkaline corrosion is insidious. It is often overlooked and even ignored. Repair performed by those who don't often deal with it is often inadequate. The silicon might work on the surface but without full abatement the alkaline corrosion is a ticking time bomb. It continues to spread and will potentially cause failures in the future.

The display translates signals from the CPU board to the display. If the signals from the CPU are not correct it does not matter what display you put in the machine. The display will not show the correct result because the originating signals are not correct. Start with a good foundation.

https://www.pinwiki.com/wiki/index.php?title=Williams_System_9_-_11#Repairing_Alkaline_Corrosion

Anything labeled "B" is 103Z = 0.01uF ceramic capacitor +20%/-80%. You can substitute 103J (5%), 103K (10%) or 103M (20%) as these all have lower tolerance.

See above. Both the CPU signals AND display have to work correctly to show the correct result. The two ways to be sure are to measure the signals for correctness or swap a known good on either end (board or display) and see what happens. Swapping a known good is the fastest and easiest way to start the differential diagnosis process. High voltage plasma displays require the correct voltage from the power supply board otherwise they will not show anything regardless of the logic signal processing.

Your board shows more signs of corrosion in the image. The image is reasonably well lit and reasonably in focus. Of course, there is no substitute for having the board directly in front of you to make the best possible diagnosis. The RED areas I would classify as severe (obvious) corrosion. The ORANGE areas are lighter (but evident in the image) corrosion.

The effect of the corrosion is unpredictable. It could interrupt the signal or it could tamper with the signal such that it is inconsistent. Digital logic (TTL) has a well defined range of voltages. Anything outside the range is ambiguous and may result in unpredictable results. The corrosion may induce this ambiguity. You need to measure to be sure. Or ... perform alkaline abatement, repair and replace the affected components with clean components.

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They are all 6821 chips which are SUPER common and easy to find.

Stop trying to avoid getting real repair done to your board. No matter how you wish, swap displays and/or pray, that board, as stated before, is a ticking time bomb. It won't serve any good if you just replace the IC without replacing the socket. The only real repair that you can trust is to remove all resistors, capacitors and ICs, wash the board with all the the parts remove, take a bunch of pictures, note all the obvious traces that need repair, put in sockets and new ICs as much as possible unless they are unobtainium , new resistors, new capacitors including those missing, remove the battery connector and your batteries from there and place them remote if you don't install NVRAM.

And then, and only then will you be able to trust your board is not going to cause more and more issues as time goes by. It's like putting a Band-Aid on an infected wound. You might stop the bleeding, but not the infection. Instead of spending money on a display, spend money to send the board to a local pro to have it cleaned and fixed. Just a glance of the first picture of your board was obvious this board can't be trusted in its current state.

It is a bit on the high side.
It should be around 102.4V minus and plus.
You can only lower the voltage by replacing the zener diodes ZR2 and ZR4 on the power supply.
The original zener is a 1N4764 (100V), but can be replaced with a 1N4763A (91V) zener.

b558209cfa3300f9b6889039ecf5b49264027ee5 (resized).jpg

Is R3 burned?