As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER TAKEOFF CABIN PRESSURE DUMPED SUDDENLY AND EMERGENCY PRESSURE CAME ON. RETURNED TO FIELD BECAUSE OF BURNING SMELL. TROUBLESHOT FOUND ACM 450 DEG SWITCH FAILED IN OPEN POSITION. REMOVED AND REPLACED SWITCH WIT A NEW SWITCH IAW CE 560 MM 21-51-00. OPS CHECK GOOD.