As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CRUISE #1 GENERATOR FREQUENCIES SHOWED 394. AFTER LANDING #1 GENERATOR FREQUENCIES DROPPED TO 392. GROUNDED - YES. REMOVED AND REPLACED #1 IDG REF B737-900ER AMM 24-11-11-400-801. REMOVED IDG FILTERS TO CHECK FOR CONTAMINATION ON REMOVED IDG, NO CONTAMINATION FOUND REF AMM 24-11-11-080-801. LEAK AND OPS CK GOOD REF AMM 24-11-11-400-801.