What was found
(CAN) DURING FIRST DEPARTURE OF THE DAY THE NR 1 PFD DISPLAY UNIT (DU) WOULD NOT DISPLAY (BLANK). THE DU WAS REPLACED. THE A/C FLEW WITH NIL PROBLEMS.ON CLIMB OUT ON THE NEXT FLIGHT THE CREW REPORTED SMOKE IN THE FLIGHT DECK. THE AIRCRAFT RETURNED. THE NR 2 PFD WAS BLANK ON ARRIVAL AND CB WAS OUT. BURNING SMELL WAS CONFIRMED TO BE FROM THE NR 2 PFD. THE PFD WAS REPLACED. TECH RECORDS SHOW THE NR 2 PFD WAS PREVIOUSLY REPLACED ON MAY 7/02 DUE TO BLUE LINE ON SCREEN.