What was found
NR 1 DRIVE LIGHT ILLUMINATED IN FLIGHT FL 350 WITH NR 1 SOURCE OFF CAUTION ALSO ILLUMINATED. GROUNDED - YES. AFTER MEL 24-1B CONFIGURATION CHANGES REVERSED AND WITH ENGINE RUN, IDG DRIVE LIGHT, REMAINS ILLUMINATED AT ENG IDLE AND NO VOLTS OR FREQS ON P5-13 OVERHEAD PANEL. NR 1 GCU SHOWS "IDG" FAULT. REF AMM 24-21-81, PRECAUTIONARY, REMOVED/REPLACED NR 1 GCU, OPS NML PER SAME AMM, BUT WITH ENG RUN NO CHANGE OF INOP IDG. RETURN A/C TO NML- ORIGINAL GCU REINSTALLED REF SAME AMM. OPS NML REF SAME AMM. SUSPECT FAILED IDG. REMOVED AND REPLACED THE IDG REF AMM 24-11-11. OPS CHECKED NORMAL.