What was found
AT FL400FL GEN 1 SOURCE LIGHT OFF BUS BUS AND DRIVE ILLUMINATED. RAN SOURCE OFF THEN DRIVE OFF QRH, DISCONNECTED GEN 1 VIA SWITCH, STARTED APU AND PUT IT ON BUS. GROUNDED - YES. FOUND #I ENG IDG DPI EXTENDED. WHILE PERFORMING DPI CHECK REF 12-13-21-200-802. FOUND FERROUS PARTICLES MEASURING ABOUT 1/4 X 3/32ND IN SCAVENGE/CHARGE FILTER. #1 ENG IDG NEEDS TO BE REPLACED PER B737-800 AMM TASK 12-13-21-200-802 (G) TABLE 301.ACCOMPLISHED ALT DEACTIVATION PROCEDURES PER CH5 TABLE 3. PARTS VERIFICATION. REMOVED & REPLACED #1 ENG IDG.