What was found
MAIN BATTERY TEMP AT 55 DEG AND VOLTAGE AT 15 VOLTS WITH MAIN AND AUX BATTERY CAUTION LIGHTS ILLUMINATED, AIRCRAFT RETURNED TO GATE AND SHUTDOWN POWER. WHEN EXTERNAL POWER APPLIED, FIRE DETECTION WARNING LIGHTS ILLUMINATED AND APU FIRE PANEL WAS BLANK, CREW NOTED SMOKE COMING FROM APU COMPARTMENT. THE CREW WAS ADVISED TO SHUT OFF POWER AND CONTACT THE FIRE DEPARTMENT. THE FIRE DEPARTMENT RESPONDED, NO FIRE WAS NOTED, BUT A LOT OF SMOKE WAS COMING FROM THE APU COMPARTMENT. APU START CONTACTOR APPEARS TO BE WELDED AND CIRCUIT BREAKER N-8 (APU AUX) WAS POPPED CAUSING THE STARTER TO OVERHEAT AND GENERATE THE SMOKE. REMOVED AND REPLACED APU START CONTACTOR AND STARTER/GENERATOR, OPS CHECK GOOD. PERFORMED HIDDEN DAMAGE INSPECTION, NO FURTHER DAMAGE FOUND.