What was found
AIR TURN BACK, CLIMBING THROUGH FL 280, CABIN ALTITUDE WARNING HORN CAME ON. CABIN WAS CLIMBING THRU 10,000' & AUTO FAIL, PRESSURIZATION, LIGHT WAS ILLUMINATED. EMER DESCENT TO 9,000' WAS ACCOMPLISHED. RETURNED & CABIN NOISE NEAR L1 DOOR HEARD BY FLIGHT ATTENDANTS. FOUND S105 GND SAFETY SWITCH FAULTY. R & R S105 SWITCH IAW MM.