What was found
ACFT RETURNED TO DEPARTURE WITH NR1 ENGINE GEN FAULTED AND NR 1 AC BUSS FAILURE. INSPECTED AND PERFORMED GPCU BITE. TROUBLE SHOOTING INDICATED NO RESPONSE FROM NR 1 GCU. PULLED NR 1 GCU FROM RACK SMELLED STRONG BURN ODOR. SWAPPED APU GCU INTO NR 1 POSITION. NR1 AC BUSS NOW FUNCTIONING RESET NR 1 ENG IDG REF AMM 24-20-00-09-0 AND OPERATED NR 1 ENG AND NR 1 AC BUSS GROUND CHECKED GOOD. REF AMM 24-22-12-04-0 CANNED APU GCU PN 740120C/ SN 3132 FROM A/C 815 AND INSTALLED IN APU POSITION. APU ELECTRICAL GRD CHECKS GOOD REF AMM 24-23-12-04-0 NR 1 ENG GEN AND AC BUSS GROUND CHECK GOOD, OK TO CONTINUE.