Date: 201301
Local time of day: 0601-1200
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Light: Daylight
Aircraft: B737-500
Operator: Air Carrier
Operating under: Part 121
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Time Pressure; Workload
Second reporter function: First Officer; Pilot Flying
Anomaly: Aircraft Equipment Problem Critical; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Detector: Person Flight Attendant; Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed in Emergency Condition; Flight Crew Diverted; General Maintenance Action; General Declared Emergency
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Cruising at FL360; we noticed a burning smell or fumes in the cockpit. Flight attendants called and smelled the same fumes in the cabin. I asked if they had any visible smoke in the cabin and they reported no visible smoke. We had none in the cockpit. The First Officer and I donned our oxygen masks; declared an emergency; executed the Smoke/Fire/Fumes Checklist; and diverted to ZZZ with emergency equipment standing by. Landing was uneventful emergency equipment saw nothing abnormal and we taxied to the gate and deplaned. Maintenance logbook entry made. I was the Pilot Monitoring. From the time of the smell of smoke and the donning of the oxygen masks to the time we landed; time becomes a precious commodity. Working the descent; declaring an emergency; and navigating a divert to a destination 35 miles away from FL360; executing a Smoke/Fumes/Fire Checklist to control/contain the fumes; programming the FMC; getting weather; choosing a runway and getting landing data; talking to the flight attendants and passengers; there becomes a priority in the order these things happen and the order of communications. All this happens in less than 20 minutes to the ground; which feels like 5 minutes. Dispatch felt left out but I had no time to TYPE an ACARS message. Instead I choose to call Ops and have them call Dispatch and tell them we were diverting for fumes in the cockpit and to expect us in less than 10 minutes. 'This killed 2 birds with one stone.'


Second narrative: Cruising at FL360 we smelled something burning in cockpit. No other indications. Flight attendants called on interphone to alert pilots of strong burning smell. Donned O2 masks; readied goggles; ran QRH; and told Center we needed lower and then declared emergency. Since ZZZ was only 35 miles away; we elected to land there. I flew the aircraft while the Captain ran checklists and computed landing data; and communicated with flight attendants and passengers. Had equipment waiting on us. Came to a complete stop on runway so emergency personnel could look over exterior of aircraft. As soon as we determined the plane was safe to taxi; we proceeded uneventfully to the gate where all Passengers disembarked. Mechanics later found a burned up TR 3 [Transformer Rectifier] on the plane.


Synopsis: B737-500 flight crew detects a burning smell at FL360 and elects to divert to the nearest suitable airport while descending and performing QRH procedures.  Maintenance discovers a burned up TR3.