As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AT THE GATE AFTER THE ACFT WAS BOARDED AND BEFORE DEPARTURE, THERE WAS A BURNING ODOR IN THE MID CABIN AREA. THE ODOR MOVED FWD TO THE COCKPIT. THE CREW CARRIED OUT A RAPID DEPLANEMENT. CONTRACTED MX, DETERMINED THAT THE ODOR HAD COME FROM A CABIN LIGHT BALLAST AT ROW 17AC. SIDEWALL LIGHT BALLAST PS101 WAS FOUND AT FAULT. THE BALLAST WAS REMOVED AND THE SIDEWALL LIGHTS DEFERRED IAW MEL 33-21-01.