As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: IN FLIGHT JUST AFTER T/O, FA AT 3R HEARD LOUD BANG AND SMELLED BURNING ELECTRICAL SMELL. QRH FOR SMOKE AND FUME COMPLETED. DECESION MADE TO RETURN TO JFK. EMERGENCY DECLARE AND AIRCRAFT LANDED OVERWEIGHT WITHOUT INCIDENT. MAITNENANCE REPLACED THE AIR CYCLE MACHINE IAW AMM 21-52-02.