As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: PILOT REPORTS DURING CRUISE "IDG 1 OFF BUS" FORCED ELEC EMERGENCY CONDITION. APU STARTED SUCCESSFULLY TO RESTORE POWER, IDG 1 OFF BUS REMAINED. MAINTENANCE REPLACED #1 GCU IAW AMM 24-21-19. TEST OK IAW AMM 24-21-19. #1 IDG TEST OK IAW AMM 24-21-01.