What was found
AFTER TEN MINUTES INTO FLIGHT, THE LEFT GENERATOR, LEFT AC BUS, LEFT DC BUS AND AC CROSSTIE LOCKOUT LIGHTS ILLUMINATED. RESET GENERATOR AND VOLTS AND FREQUENCIES APPEARED NORMAL. ATTEMPTING TO TURN GENERATOR ON AGAIN CAUSED IT TO RETRIP. FLIGHT RETURNED TO EWR AND LANDED WITHOUT INCIDENT. REPLACED GENERATOR CONTROL PANEL, OPERATIONAL CHECK OK.