What was found
(CAN) AFTER TAKEOFF, FLAPS FAILED AT 6 DEGREES. RAN QRH, RETURNED TO ORIGIN. EXTENSIVE TROUBLESHOOTING PERFORMED. MAINTENANCE COMPLIED WITH RIGGING CHECK OF BOTH FLAP BPSUS AND FOUND FECU POTS 5, 6, 7 AND 8 OUT OF RIG. RE-RIGGED BPSUS IN REF TO MM 27-51-10. RAN FLAPS AND SYS OPERATED NORMAL.