A very, very, VERY (did I say very yet) common problem is 'U20" on the MPU board. Symptoms are one switch row is not driven by this chip.
Noted . I keep forgetting to mention that this game worked fine when i played it on location last week
only takes a nano-second for a semiconductor to be damaged.....a flashover of a contact etc will do it. I would change it before spending any more time on it (make sure you fit a socket for next time).
On the subject of this MPU we are redesigning it here at Homepin and adding circuitry to prevent this common issue. We will probably get rid of U20 altogether.
