Jim Z
Customer's unit runs great, but gives no output.
The power regulator board had suffered an obvious meltdown.
By applying 12vdc to the brushes, get good AC output.

Installing a new regulator board still gives no output.
System Control Board (#83970GS) is NLA....Is there a way to bypass this board?
Not sure what signals it makes to the regulator board to create voltage to the brushes.
My guess is your not getting field flash and the new voltage regulator won't start. The board on many units applied battery voltage to the brushes momentarily to flash the field. Try touching battery voltage with the unit running to the 4 wire and see if the voltage regulator takes over.