What was found
PHX - FLT 45 PHX TO KOA. UPON ROTATION, THE LEFT AC BUS TRIPPED OFF AND COULD NOT BE RESET, OR POWERED BY ANOTHER GENERATOR. THE FLIGHT RETURNED TO PHX AND LANDED WITHOUT FURTHER INCIDENT. MAINTENANCE BITE CHECKED THE BUS PROTECTION CONTROL UNIT (BPCU) AND FOUND "NO RESPONSE FROM CONTROL" MESSAGE. THE NUMBER ONE GENERATOR CONTROL UNIT (GCU) WAS INSPECTED AND FOUND LOOSE IN ITS RACK. THE NUMBER ONE GCU WAS REPLACED. OPERATIONAL CHECKS WERE FOUND OK.