As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING PDC FOUND NR 1 TRANSFER BUS OFF LIGHT ON. NO POWER TO NR 1 BUSS. ACFT GROUNDED. PERFORMED TRANSFER BUSS OFF LIGHT ILLUMINATES FAULT ISOLATION IAW FIM 24-21 TASK 822. FOUND BTB FAULT ON NR 1 GEN CONTROL UNIT. PERFORMED BTB FAULT ISOLATION IAW FIM 24-21 TASK 810. REMOVED AND REPLACED BTB1, C804 IAW AMM 24-21-41. OPS CHECKS GOOD. GROUND VERIFICATION SATISFACTORY.