What was found
ON BLOCKOUT BETWEEN STARTING NR 1 AND NR 2 ENGINES SMOKE STARTED COMING IN TO THE COCKPIT FROM THE EQUIPMENT BAY. PERFORMED VISUAL INSPECTION OF AVIONICS COMPARTMENT, CHECKED ALL BOXES FOR EVIDENCE OF ODOR, REMOVED RELAY PANEL COVERS LOOKING FOR EVIDENCE OF DAMAGE AND LOOKED AT ALL VISABLE WIRING IN AVIONICS COMPARTMENT WITH NO VISIBLE DEFECTS NOTED. FOUND COOLING FAN FILTER VERY DIRTY/BLOCKED AND SMELLED OF SMOKE. REPLACED COOLING FAN FILTER IAW MM 21-21-16-4. ORDERED AVIOINCS COOLING FAN AND SMOKE DETECTORS AS PRECAUTIONARY. REMOVED AND REPLACED AVIONICS COOLING FAN REF MM 21-21-11-4 OPS.