What was found
SUPPLEMENTAL MRR - PILOT REPORTED, ENGINE MAIN DECK CARGO FIRE AND SMOKE LIGHT NR 3 ILLUMINATED, WHILE AT FL 390. INITIATED UPS EMER PROCEDURE. WITH PACKS OFF AND A/C COMPLETELY PRESSURIZED AND PASSING THROUGH 10,000 FT, LIGHT EXTINGUISHED. NO SMOKE OR FIRE WAS EVER VISIBLE TO CREW. REPLACED BOTH 3A AND 3B SMOKE DETECTORS. ALSO, REPLACED AMPLIFIER AS PRECAUTIONARY, OPS CK GOOD PER MM 26-14-00.