What was found
(CAN) ON TAKEOFF THE TWO ALTERNATOR LIGHTS CAME ON, THE PILOT RETURNED TO THE AIRPORT. THE LIGHTS EXTINGUISHED AND THEN HE CARRIED OUT A GROUND RUN, EVERYTHING WAS BACK TO NORMAL. A SECOND TAKEOFF WAS CARRIED OUT, THE PROBLEM RETURNED. THE AUXILIARY CONTROL BOX WAS GROUND CHECKED SERVICEABLE, HOWEVER, WE NOTICED THAT THE VOLTMETER WAS HIGH. WE CARRIED OUT A GROUND RUN WITH AN INDEPENDENT VOLTMETER, WE DETERMINED THAT THE LT ALTERNATOR WAS CHARGING AT FULL CAPACITY (30VDC), AND IT WAS ENGAGING THE OVER VOLTAGE RELAY. WE DISCOVERED THAT THE NEGATIVE FIELD GROUND WAS WORN AND IT WAS TOUCHING THE CHARGING GROUND, CAUSING THE ALTERNATOR TO CHARGE AT ITS MAXIMUM.