If unplugging J2 connector from the MPU and the '8' still stay on the displays when you turn on power, then there is a data issue on the displays. Please post a picture of the back of both displays.

Well that is electronics. Some if it is very sensitive, especially if unplugging/plugging in connectors when power is on.

Can you try only hooking one display at a time to what those results are?

I definitely would do a diode test on all the transistors before removing any parts.

Good. So each display has a digit locked on. I hope you find a at least one shorted transistor on each display board.

So basically sounds like a couple of the transistor leads were shorted to one another.

I sure do like free fixes.

Likely someone or some wires pushed on some of the transistors.

Enjoy!!

I would say that it worked out quite well !!! Never hurts to look over anything, even if you never worked on a particular type of game before. Apply knowledge that you learned on other games and see how you can narrow down the problem(s), just as we did above. We did not even need that user manual that you had resting on the game.

The next thing I would do is remove that battery off the MPU board and install 6116 NVRAM. It is an easy plug and play on this board since the old RAM at U4 is already in an IC socket from the factory on these 6803 games. Then the game will be protected for 100+ years and never need a battery replacement ever again.

https://www.pinitech.com/products/6116_nvram.php