That's what I'm asking. You say you need ALL 3 plugged in to make the fuse blow?
So any two combinations of connectors or a single connector plugged in does not blow the fuse?
So if power supply (A2J3) to displays and power supply(A2J2) to CPU Board (A1J1) are connected
the fuse doesn't blow? If you connect A1J3 connecting CPU board and displays does the fuse then blow?
BTW , do all your connecting and disconnecting of connectors with the Power OFF.
You may have a short somewhere also. Have you measured the +60 and +42 VDC for the displays to ground for a short? You also mentioned that you think the connectors may be mispinned. For that you could do a continuity check of A1J3 to the display connector.
If the displays are NOT Connected, does the fuse still blow when all three of those connectors are connected?
BTW, what System 1 pin is it?