What was found
(CAN) IN CRUISE, CABIN PRESSURIZATION BEGAN AN UNCOMMANDED DESCENT IN MANUAL DC MODE. CREW WAS ABLE TO MAINTAIN CABIN PRESSURIZATION IN MANUAL AC MODE BUT OPTED TO RETURN FOR MAINTENANCE. FOUND CHAFED WIRES NEAR OUTFLOW VALVE WHICH CAUSED FAILURES IN BOTH THE OUTFLOW VALVES AND THE PRESSURE CONTROLLER. WIRING REPAIRED AND VALVES AND CONTROLLER REPLACED.