I took the battery clip off the non-working CPU board and put it on the working CPU board, and installed fresh AA batteries in the hopes that the adjustment failure message would go away. Alas, it did not, so I will need to search for a new angle to try and get this game working 100% again.
Just for fun, I decided to try and see how many points and waves I could accumulate in 10 minutes time. So, I got a timer, set it for 10 minutes, started the timer, and then pressed the 1 player start button. The instant the timer started beeping, I let go of the joysticks. In 10 minutes, I had scored 760,625 points and reached wave 29. Boy, am I out of practice…