A B737-300 IN CRUISE AT FL350 DECLARED AN EMER AND RETURNED TO THE DEP ARPT DUE TO FUMES IN CABIN. CAUSED BY A FAILED EQUIP COOLING FAN.

Narrative

WE WERE AT CRUISE JUST SW OF ZZZ WHEN THE PURSER CALLED TO RPT AN ACRID SULFUR SMELL NEAR ROW 10. NO FLAMES OR SMOKE ASSOCIATED WITH SMELL. THE ACFT HAD CEILING LIGHTS INOP AND DEFERRED AT ROWS 9-12. WE SUSPECTED A PROB WITH LIGHTING IN THE AREA OR A BALLAST UNIT. ASKED THE PURSER TO RPT BACK IN A FEW MINS WITH AN UPDATE. IN THE MEANTIME; WE RAN THE CABIN LIGHTING FIRE/SMOKE CHKLIST AS A PRECAUTION. PURSER CALLED BACK TO RPT ODOR DISSIPATING A SHORT TIME LATER. WE TOOK A FEW MINS TO REVIEW OTHER CHKLISTS THAT MAY BECOME APPROPRIATE IF INDEED WE DO HAVE A SERIOUS PROB. (ELECTRICAL FIRE; SMOKE FUMES ODOR; ETC). WE HAD BEGUN THE DSCNT TO ZZZ1 WHEN THE PURSER CALLED BACK SAYING THE ODOR HAD RETURNED AND NOW IT WAS STRONGER. AT THIS POINT WE RAN THE ELECTRICAL FIRE CHKLIST; MAKING SURE ALL ASSOCIATED BREAKERS WITH CABIN LIGHTING HAD BEEN PULLED; SHUT OFF THE RECIRCULATION FAN; GALLEY PWR AND SPLIT THE BUSSES WITH THE BUS XFER SWITCH. WE DECLARED AN EMER WITH ATC WHO GAVE US DIRECT ZZZ1; REQUESTED FIRE TRUCKS; NOTIFIED DISPATCH; BRIEFED THE FLT ATTENDANTS FOR A CABIN ADVISORY THAT MAY ESCALATE TO AN EVAC; AND BRIEFED THE PAX. COM WAS EXCELLENT AT ALL TIMES WITH THE FLT ATTENDANTS. AS WE APCHED ZZZ1 THE PURSER CALLED TO RPT ODOR DISSIPATED. WE REITERATED THAT THIS WOULD BE A CABIN ADVISORY UNLESS THEY NOTICED SMOKE OR FLAMES AND TO CALL IMMEDIATELY UPON LNDG WITH AN UPDATE. WE LANDED UNEVENTFULLY. THE PURSER CALLED STATING NO SMOKE OR FIRE SO WE MADE A 'REMAIN SEATED' PA. THE FIRE TRUCKS FOLLOWED US TO THE GATE AND DEBRIEFED. WE THEN DEBRIEFED THE FLT ATTENDANTS AFTER THE PAX DEPLANED. WE ALL FELT THE SIT WENT VERY WELL WITH EXCELLENT COM. IN TALKING TO THE MECHS; THEY FOUND THE EQUIP COOLING EXHAUST FAN HAD BURNED ITSELF OUT AND POPPED THE BREAKER WHILE WE WERE AT THE GATE. THEY REPLACED THE FAN AND SHOWED US THE DEFECTIVE FAN THAT HAD A STRONG ACRID ODOR AND LOOSE BEARINGS. THE FAN BLADES HAD CUT INTO THE HOUSING UNIT. WITH A NEW FAN INSTALLED WE PROCEEDED ON TO ZZZ2 UNEVENTFULLY.