What was found
LEFT PACK "OFF" LIGHT WITH ASSOCIATED "L PACK OFF" DISPLAYED IN CRUISE WITH PACK SELECTED TO "STBY" (REF DF583) DUCT PRESSURE AND PRESSURIZATION NORMAL. RECURRED ON FLT 9834-13. REPLACED LT PACK FLOW CONTROL AND SHUTOFF VALVE. OPS CHECK GOOD. LEFT PACK DOES NOT SUPPLY SUFFICIENT AIR FOR CABIN PRESSURE WHEN LEFT WING ANTI-ICE IS SWITCHED ON. PACK AIR SUPPLY IS SUFFICIENT WITH ANTI-ICE OFF. SAME CONDITION WITH RIGHT BLEED SUPPLYING LEFT SIDE. FOR TROUBLESHOOTING LT AND RT FLOW CONTROL VALVE WERE SWAPPED. FOUND PROBLEM FOLLOWED TO RIGHT FLOW CONTROL VALVE. REPLACED RT FLOW CONTROL VALVE. BOTH PACKS OPS CHECK NORMAL.