First: Disconnect the cable connected to Chain#1 to test Chain#2 hash board.
Second: Disconnect cables connected to Chain#1 and Chain#2, turn over the control board, and then connect the Chain#1 to Connector#2 of the control board, to test Chain#1 hash board.
Note: Since there is no +12v in the Connector 1#, you have to turn over the control board to test Chain 1#