What was found
(CAN) PRIOR TO DEPARTURE, CREW NOTED THE BUSS TIE CB WAS POPPED. THE CB WAS RESET AND A NORMAL ENGINE START WAS CARRIED OUT. ON CLIMB OUT THE AMBER BUSS T CASS MESSAGE DISPLAYED AND THE BUSS TIE S/B POPPED. CREW RETURNED TO BASE. DURING THE TROUBLESHOOTING PROCEDURE IT WAS NOTED THE LT PWR JCT BOX WAS DEFECTIVE AND REPLACED WITH A SERVICEABLE SPARE. FUNCTION TEST WAS CARRIED OUT IAW CH 25-52 AND REPLACEMENT WAS DEEMED SERVICABLE. A NORMAL GROUND RUN WAS CONDUCTED AND ACFT RETURNED TO SERVICE. NOTE: THIS IS THE SECOND FAILURE OF THIS ASSY SINCE NEW TT 870 HRS THE DEFECTIVE UNIT IS BEING RETURNED TO MFG FOR INVESTIGATION.