As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FLIGHT RETURNED TO SFO DUE TO A INOPERATIVE NR 1 SPEED DRIVE. REPLACED NR 1 CONSTANT SPEED DRIVE. CHARGE PRESSURE SWITCH BLOWN OUT ON CONSTANT SPEED DRIVE. REPLACED CSD. CSD PRE-COOLER REPLACED DUE TO REPEAT FAILURE OF THE CSD. OVERHAULED CONSTANT SPEED DRIVE. NORMAL WEAR FOUND ON PRE-COOLER.