As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
LEFT CONSTANT SPEED DRIVE LIGHT CAME ON AND GENERATOR DROPPED OFF LINE. AIRCRAFT RETURNED TO FIELD. REPLACED LEFT INTEGRATED DRIVE GENERATOR (IDG) AND LEFT GENERATOR CONTROL UNIT (GCU). IDG HAD INTERNAL GEAR FAILURE AND GCU (MFG P/N 734284E) HAD ZENER DIODE VR6 FAILURE.