I have a question.
Did you - or the owner - recently do some work on this game involving soldering? Maybe a coil or switch wire came off that needed to be resoldered?
The reason I ask is I have seen errant solder fall off a soldering iron, etc, and land on a PCB or other place that can cause an intermittent short.
IMHO, if U20 is blowing violently - as in blowing the top off the chip, etc, then I would strongly suspect coil power is making it's way to the switch matrix. I don't think a short from the 12v unregulated circuit would cause a violent failure of U20.
I think one thing I would try is to use the burn in cycle and let the game run a burn in for a few hours to see what happens. If vibration somewhere is causing a coil power short to the switch matrix a burn in might help to narrow it down.
Michael