What was found
ON TAKEOFF ROLL WITH UPPER AND LOWER YAW DAMPERS AND MACH TRIM IN A-CONFIGURATION, THE LOWER YAW DAMPER FAILED BETWEEN 50 AND 70 KTS, AMBER "YD OFF LOWER" EICAS MESSAGE APPEARED. REPLACED PITOT LINE TO NR 1 ADC PER 750 MM CHAPTER 34-11-01 LEAK CHECK GOOD. COMPLIED WITH PITOT / STATIC CHECK PER FAR 91.411, NO LEAKS NOTED, TIGHTENED LOOSE TERMINAL ON STANDBY POWER SWITCH, SYSTEM OP CHECK GOOD.