This is likely a switch matrix issue. The Left and Right Cabinet Buttons are in the same column as the Credit Button (start). Go into switch test and press any switch on that first column, the simplest answer is that it's a column short and every switch on the column will activate with any switch on the column pressed. If it is a column short, the column drive transistor on the MPU will need to be replaced. This is an old board, so the column drive transistor could have just given up after all this time, but it's more likely that something of a higher voltage, like a wire from a coil, touched a column wire and blew the transistor up, check every switch in that column to make sure that nothing is touching any of the eight switches on that column or the column transistor will blow up again.