Going through my boards, I came across a second Dig Dug board.  This one boots to static garbage and no sound.    Watchdog is barking.

Removing one of the CPUs gets different garbage.  Tried replacing with a known good one, no change.

Checking the pins shows me that a couple of the data lines, from D3 to D6 (I cannot remember which ones right now) are completely floating!  Rarely see that.  On a hunch, I replace the CPU’s socket, but this changes nothing.

Board traces go two a few chips (a 7474 and some other multiplexers or demultiplexers), and the floating lines seem to follow them.  But following the traces is tricky, so I gotta dig up some schematics to see what the Hell is going on.