What was found
CREW REPORTED SMOKE IN THE COCKPIT. SMOKE FROM CB PANEL BEHIND F/O. SLIDING WINDOW OPENED TO VENT SMOKE. RETURNED TO GATE. REMOVED AND REPLACED THE APU GCU. DID NOT CURE PROBLEM. FOUND B-LEG ON APU GENERATOR DEAD. DEFER PER MEL 24-2. PLACARDS INSTALLED. AIRCRAFT RETURNED TO SERVICE. NOISE COMING FROM BEHIND F/O CIRCUIT BREAKER PANEL. REPLACED APU GCU & APU GEN BREAKER. OPS CHECK OK WITH EXTERNAL POWER APPLIED & APU SUPPLYING PNEUMATIC. ALSO OPS CHECK OK WITH NR 1 & NR 2 ENGINE GEN ON LINE. APU NOT ACCEPTING GROUND POWER. REMOVED AND REPLACED NR 1 BT BREAKER. OPS CHECKED GOOD. AIRCRAFT RETURNED TO SERVICE.