What was found
DURING CLIMB, THE LT CSD RED, DISCONNECTED CSD IAW QRH, VOLT FREQUENCY 0 PRIOR TO DISCONNECT. AN ATTEMPT WAS MADE TO START THE APU, BUT APU DID NOT START IN FLIGHT, SEVERAL ATTEMPTS WERE MADE. RETURNED TO DEPARTURE, THE AIRCRAFT WAS GROUNDED. APU STARTED FINE ON THE GROUND. MAINTENANCE REMOVED AND REPLACED THE LT CSD & GENERATOR IAW AMM 24-10-01-401, OPS CHECKED NORMAL. REMOVED & REPLACED THE APU START RELAY R2-63 IAW WDM 49-41-00. ACCOMPLISHED ALL TESTS, APU OPS CHECKED OK.