Got an old flash I had pulled down from our shelves so that I could have a test platform for early Williams boards. It came with a driver board that has the weirdest switch behavior. It acts as if it's having shorted rows;pressing one switch makes the whole row register. It's clearly the boards, because it does it without the switch plugs in. Amazingly I can even remove U15 and U16, and it still thinks row 3 is all being pressed. I've replaced both chips and the 6821, and it's still doing it. I'd think something was putting garbage on the data line, but all other features work fine. Lamps attract, solenoids fire, displays show numbers. So what exactly is going on?