What was found
(CAN) MFG DESIGN FLAW, INSTRUMENT LIGHTING SYS WAS DISCOVERED WHEN NEW AIR DATA DISPLAY UNIT WAS INSTALLED FOR RVSM UPGRADE. SYMPTOM WAS ADDU LIGHT WOULD GO INTO NIGHT MODE (VERY DIM) WHEN EMER BATT PACK WAS TURNED ON. PACK IS KEPT ON IN FLIGHT TO RUN STANDBY GYRO. PROBLEM WAS THAT IN NON-EMER MODE, EMERGENCY PACK WAS POWERING WHOLE 5 VOLT INSTRUMENT LIGHTING BUS VIA RELAY K2 AND CR4 ON TB1 IN CIRCUIT BOARD UR-1. 2.2 AMP DRAW THROUGH CR4 WOULD CAUSE IT TO OVERHEAT BUT IN TURN DROP VOLTAGE LOW ENOUGH THAT LAMP LIGHTING WAS TOO DIM TO BE NOTICED. PROBLEM WENT UN-NOTICED UNTIL RVSM MOD WAS DONE. MFG WAS NOTIFIED OF PROBLEM, SENT WIRING MOD TO SYS.