What was found
ALTERNATOR LOW CHARGE WARNING LIGHT CAME ON DURING GROUND RUN-UP. RECYCLED FIELD SWITCH AND IT CAME BACK ONLINE. WARNING LIGHT CAME BACK ON DURING TAKEOFF. ACFT RETURNED. DURING TROUBLESHOOTING, FOUND HIGH RESISTANCE IN FIELD CIRCUIT. DISASSEMBLED ALTERNATOR AND FOUND BLACK BANDS/DEPOSIT ON SLIP RINGS. CLEANED SLIP RINGS AND ASSEMBLED ALTERNATOR. INSTALLED AND PERFORMED GROUND RUN. OBTAINED PROPER CHARGING VOLTAGE. ACFT IS RETURNED TO SERVICE. THERE HAVE BEEN FAILURES IN SIMILAR MODE ON THIS MODEL ALTERNATORS IN THE PAST. SUSPECT INFERIOR MATERIAL USED FOR MFG BRUSHES CAUSING HIGH RESISTANCE BETWEEN SLIP RINGS AND BRUSHES. FAILURE IN LESS THAN 30 TIS IS TOO SOON.