As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING INITIAL CLIMB AC BUS NR 3 AND ASSOCIATED LIGHTS RT EMERGENCY, AC AND RT EMERGENCY DC BUS LIGHTS CAME ON. COULD NOT RESTORE THE RT EMERGENCY BUS. FLIGHT RETURNED TO ORD. REPLACED THE GENERATOR CONTROL UNIT AND THE BUS POWER CONTROL AND RECONNECTED THE CSD. BOTH UNITS TESTED GOOD IN THE SHOP. CAUSE OF PROBLEM WAS NOT DETERMINED.