What was found
(CAN) ON DAY OF INCID. A/C SAT ON GROUND, PILOTS RPORT APPROX. 10 MIN WAITING FOR TAKEOFF CLEARANCE, ENGINE AT 1000-1200 RPM, RADIOS ON, ALL LIGHTS ON TAKEOFF, EVERYTHING NORMAL, VOLTAGE REGULATOR INDICATING IN GREEN ARC 24-28 VO LTS TEN MINS OUT PILOTS EXPERIENCED RADIO COMMUNICATION PROBLEMS&FAILURE OF AUTO PILOT WHILE TRYING TO ISOLATE PROBLEM, VOLTAGE GAGE ANNUNCIATOR PANEL THAT ONE OF ALTNR INOPERATIVE. THEY CONTACTED CNTL TOWER BYCELL PHONE&RETURNED TO AIRPORT. COCKPIT LIGHTS WERE DIM. UPON INSP L/H ENGINE ALTNR BELT FOUND BROKEN OFF&R/H BELT SHOWED SIGNS OF SLIPPING. L/H BELT&ALTNR REPL. R/H BELT TIGHTENED. A/C GROUN D RUN, ALL SYSS CHECKED OUT OK, NO PROBLEM WAS