What was found
SUSPECT AIR-GRD PROBLEM. FOUND 200V AT PITOT HEAD ON GRD ENG'S OFF. REPLACED SYS 1 AIR-GROUND RELAY K552, LT PROBE HEAT TEST RELAY K643 & GEAR TILT RT MLG RELAY K10446. ACCOMPLISHED CONTINUITY CHECKS. REMOVED RELAYS K514, 515 & 516 AIR GROUND SYS RELAY ESTABLISHED CONTINUITY AT EACH RELAY. WITH RELAYS INSTALLED UNABLE TO MAKE GROUND TO GET CORRECT COCKPIT INDICATION. REMOVING ANY ONE SYS 1 AIR GROUND RELAY ALLOWS CORRECT INDICATION FROM INSTALLED RELAYS. SUSPECT 28V DC POWER DROP. CHECKED D5664P PIN 33, NO 28V. RESET AIR GROUND SYS 1 POWER C/B U15, 28V NOW RESTORED. DISCONNECTED AUTO SPEEDBRAKE TRUCK TILT SENSOR, ALL INDICATIONS NORMAL. CONNECTED NEW S10598 & CONFIRMED INDICATIONS STATIC AND RUNNING.