PLT OF AN SMT PVT ACFT FAILED TO FOLLOW IFR CLRNC ROUTING TRACK DUE TO A FAULTY RNAV RADIO RECEIVER INDICATOR.

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IFR FLT PLAN. DEPARTED PNE TO HGR. GIVEN ROUTING MXE V474 SCAPE DIRECT. MODENA WAS NOTAMED UNUSABLE. USING RNAV KNS-80. 5 NM PRIOR TO MXE CLRED DIRECT DELRO. SET UP OFF LRP 248 DEG RADIAL 18 NM SINCE MXE UNUSABLE. 10 NM W OF MXE GIVEN VECTOR SW FOR SPACING AND THEN WAS TURNED DIRECT DRLRO. SET UP CORRECTLY AS NOTED ABOVE. ATC CALLED AND ADVISED 4 NM OFF COURSE; ALTHOUGH HSI SHOWED CTRED. RECHKED RNAV SETTING AND CONTINUED. ATC THEN ADVISED ACFT WAS OFF COURSE TO THE NW AND GAVE RADAR VECTORS. CONTINUED RADAR VECTORS TO SCAPE. RNAV WAS THEN SET TO LRP OR THS. POINTER 20 DEGS TO THE R OF COURSE COMPARED TO #2 VOR SET ON THS UP UNTIL NOENO AT WHICH POINT IT CORRECTED TO AGREE WITH #2 VOR. AT SCAPE SET RNAV TO HGR OFF THS. ACCURATE TO LNDG. THE THING THAT BOTHERED ME WAS THAT EVEN THOUGH THE RNAV WAS OBVIOUSLY OFF BY 20 DEGS; THERE WAS NO INDICATION WHATSOEVER. THE NEEDLES AND FLAGS SHOWED A CORRECT COURSE. TRYING TO RECONCILE COURSE AND EXACT POS OFF THE AIRWAY WAS VERY DIFFICULT GIVEN THE LOCATION. ONCE ON THE AIRWAY; IT BECAME CLR HOW FAR THE RNAV WAS OFF (OTHER ACTIONS). ONCE THE DISCREPANCY WAS FIRST NOTED BY ATC; I SHOULD HAVE ASKED FOR VECTORS; UNTIL I WORKED OUT THE DISCREPANCY.