What was found
FLT 1304, SJU-JFK. RETURNED TO FIELD. SMOKE IN COCKPIT AFTER AUDIBLE POP IN AVIONICS COMPT. SMOKE CLEARED AFTER SEVERAL MOMENTS; ACCOMPLISHED CH 5 INSP PER TSM 05-50-00-810-831. CHECKED ALL 3 AVIONICS COMPTS. CK'D ALL C/B'S FOR POPPING; CK'D BLOWER & EXTRACT FANS FOR SMELL OR VIBRATION. REMOVED AND INSP'D BLOWER FAN PER AMM 21-26-51. NO IDENTIFIABLE FAULTS OR SOURCE OF SMELL NOTED. NO PRIOR HISTORY. ACCOMPLISHED AEVC TEST. A/C OK FOR SERVICE. NOTIFIED MCC. AS PRECAUTIONARY, PLACED BLOWER FAN ON MEL 21-26-01-A CAT C MX4 UPDATED. PLACARD INSTALLED PER MCC.