What was found
HOOKED UP APU AND STARTER IMMEDIATELY CRANKED ENGINE. INVESTIGATION FOUND BOARD CONTAINING DIODES IN CABIN NEXT TO POWER RELAY TO HAVE BOTH DIODES INOPERATIVE. PART IS IN DIFFICULT LOCATION TO TEST INDIVIDUAL DIODES. REPLACED DIODE BOARD P/N 1570043-1 AND TESTED GOOD. RECOMMEND TESTING APU AT EACH ANNUAL. NOTE: PARTS MANUAL DOES NOT SHOW PART ON THIS SERIAL NUMBERED ACFT; HOWEVER, MAINTENANCE SCHEMATIC IS NOT SERIAL SPECIFIC.