What was found
GUM - FLT 0861 - L/P: 7938849 - NR 1 TR CIRCUIT BREAKER TRIPPED ON TAKEOFF, AIR RETURN TO GUM. REF WDW 24-32-01 AND 24-55-12. CHECK 3-PHASE POWER TO NR 1 TR UNIT AND FOUND B-PHASE MISSING. RESET TR UNIT CIRCUIT BREAKER C810, NO HELP. RESET DIST NR 1 SECT Y AND POWER RESTORED TO NORMAL. REPLACED DIST NR 1 SECT Y CIRCUIT BREAKER AND NR 1 TR UNIT CIRCUIT BREAKER PRECAUTIONARY. OPERATED WITH BOTH APU AND ENGINE GENERATOR. OPS CHECK NORMAL. REF: OPERATOR CONTROL NR: 97ZZZX4366. (X)