What was found
DURING PRE-START CHECKS WITH ACFT BATTERIES ON AND APU RUNNING AND ALL SYSTEMS (AFIS, AVIONICS) OPERATING, DISTINCT ACRID FUMES AND FAINT GREY SMOKE OBSERVED IN CABIN. ALL SYSTEMS OPERATED NORMALLY AND NO CIRCUIT BREAKERS TRIPPED. ACFT POWERED DOWN AND MX CONTACTED FOR FURTHER INVESTIGATION. MX OBSERVED NR2 ATC RECEIVER SHOWS FAIL LIGHT. UPON INSP, DISCOVERED BURNING ELECTRICAL SMELL COMING FROM INSIDE RECEIVER. PULLED NR 2 ATC CB AND COLLARED NR 2 AFIS ATC.