PA32 PLT WANDERS OFF AIRWAY OVER WATER WHEN 1 OF 2 VOR RECEIVERS MALFUNCTIONS. CTLR SAVES THE DAY.

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I WAS ON AN IFR FLT PLAN FROM NASSAU TO FT LAUDERDALE EXECUTIVE ARPT. MY CLRNC RTE WAS: CLRED TO FXE BR 57V RPT DECAL INTXN; MAINTAIN 8000 FT. NASSAU DEP VECTORED ME TO INTERCEPT 57V WHICH IS THE 314 DEG RADIAL FROM NASSAU VOR TO CAREY INTXN. I IDENTED THE INTXN WITH THE DME AND TURNING TO A HDG OF 280 DEGS TO TRACK 57V TO BIMINI. I WAS NAVING USING THE #1 VOR. IT INDICATED THAT I WAS DRIFTING RIGHT AWAY FROM THE CTRLINE OF THE AIRWAY. I WAS CORRECTING FOR THE DRIFT WHEN MIAMI APCH ADVISED ME THAT I WAS OFF THE AIRWAY AND; IN FACT; FLYING AWAY FROM IT. APCH GAVE ME A HEADING TO FLY AND TOLD ME TO PROCEED DIRECT BIMINI. I TUNED THE #2 VOR TO BIMINI; IDENTED; AND USED IT TO FLY DIRECT BIMINI. WITHIN 30 NM OF BIMINI THE #1 VOR NEEDLE CTRED INDICATING THAT I WAS ON THE AIRWAY. THUS; I BECAME AWARE THAT I HAD A PROB WITH THE #1 VOR. BOTH VORS HAD BEEN CHKED PRIOR THE FLT; BOTH WERE IN TOLERANCE. INASMUCH AS I WAS VECTORED MOSTLY DURING THE FLT TO NASSAU; I DIDN'T DETECT A PROB WITH THE #1 VOR. SUBSEQUENTLY; INFLT TESTS REVEALED THAT IT WAS NOT RELIABLE WHEN GREATER THAN APPROX 30-35 NM DISTANCE FROM A STATION; WHETHER TRACKING TO; OR FROM THE STATION. THE #2 VOR FUNCTIONED WELL; HOWEVER BOTH ARE BEING SCHEDULED FOR SHOP TESTING AND CALIBRATION.