M20 PILOT REPORTS VOR RECEIVER MALFUNCTION AND DESCENT BELOW MDA DURING VOR DME RWY 9 APPROACH TO RYY.

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I WAS BEING VECTORED FOR THE VOR/DME RWY 9 APCH INTO RYY; WHEN I WAS VECTORED THROUGH THE FINAL APCH COURSE; WHICH WAS INDICATED ON MY VOR COURSE NEEDLE. ATL APCH ASKED; WHICH APCH I WAS DOING; AND I RESPONDED THAT I WANTED THE VOR APCH; AND THEY RESPONDED BY SAYING; 'SORRY; I JUST SENT YOU THROUGH FINAL; BUT I GUESS YOU SEE THAT.' I REPLIED THAT I DID SEE IT. THEY TURNED ME TO A HDG OF 090 DEGS; AND INTERCEPTED THE FINAL APCH COURSE. THEN I WAS CLRED FOR THE VOR/DME RWY 9 APCH; MAINTAIN 3000 FT UNTIL ESTABLISHED. APCH CAME BACK ON; AND SAID THAT I APPEARED TO BE L OF COURSE; AND ASKED IF I CONCURRED. MY COURSE NEEDLE WAS CTRED; SO I DID NOT CONCUR. THEY ASKED ME TO TURN R; WHICH I DID; AND MY NEEDLE DIDN'T CHANGE; EVEN THOUGH APCH TOLD ME I WAS ON COURSE. I HAD A VFR-ONLY GPS ON BOARD; SO I BEGAN TO LOOK TO THAT FOR BACKUP GUIDANCE; SINCE I NO LONGER FULLY TRUSTED MY VOR. I WAS PAST THE FAF; ON COURSE; SO I BEGAN MY DSCNT TO THE MDA. I BEGAN TO BREAK OUT AT AROUND 1900 FT MSL; WHICH IN HINDSIGHT WAS MY MDA; BUT I HAD LOOKED OVER THE ILS OR LOC APCH INTO RWY 27 ALSO; AND I HAD 1540 FT MSL IN MY HEAD AS THE MDA. SINCE I THOUGHT I HAD PLENTY OF ALT ROOM TO GO; I CONTINUED TO DSND FROM 1900 FT TO 1540 FT; SO I COULD BE COMPLETELY CLR OF CLOUDS. AT THAT POINT; I HAD BEEN SWITCHED TO RYY TWR; AND THE CTLR SAID; 'MOONEY X; ALT ALERT; DO YOU HAVE THE RWY IN SIGHT?' I STATED THAT I DID NOT; BUT THAT I HAD VERT CONTACT. HE ASKED ME IF I MEANT THAT I HAD THE GND IN SIGHT; AND I REPLIED THAT I WAS IN THE CLR; WITH THE GND IN SIGHT; AND THAT I COULD SEE THE ARPT. I COULDN'T YET MAKE OUT THE RWY; BUT I DID SEE THE BEACON; AND THE ARPT ENVIRONMENT. I QUICKLY DID ACQUIRE THE RWY; AND I RPTED IT IN SIGHT. I WAS THEN CLRED TO LAND. I CONTINUED TO LAND WITHOUT INCIDENT. MY MAJOR MISTAKE WAS TO CONTINUE THE APCH WHEN I WAS HAVING AN INDICATION PROB WITH MY VOR/OBS. I WAS CONFIDENT IN MY POS GIVEN THE GPS; BUT SINCE IT WAS NOT IFR CERTIFIED; I SHOULD NOT HAVE TRUSTED IT. UNFORTUNATELY; THE CLOUDS AT MY POS WERE LOWER THAN ADVERTISED; AND I WENT BELOW THE PUBLISHED MDA ATTEMPTING TO STAY CLR OF CLOUDS ONCE I COULD SEE THE GND. CONTRIBUTING TO THE INCIDENT WAS THE FACT THAT I HAD LOOKED OVER THE RWY 27 APCH AS WELL; AND HAD GOTTEN THE RWY 27 MDA IN MY MIND; WHICH WAS OVER 300 FT LOWER THAN THE MDA FOR THE APCH I WAS ON. FACTORS AFFECTING MY PERFORMANCE WERE A NERVOUSNESS ABOUT BEING IFR ON 'ROUND DIALS' GIVEN THAT I WORK AT A REGIONAL AIRLINE FLYING IN A GLASS COCKPIT. I HAVEN'T SHOT A TRUE VOR APCH WITHOUT AN FMS OR MFD IN 2 YRS; AND I SHOULD GET MORE PRACTICE DOING SO BEFORE I GO OUT AND SHOOT APCHS IN ACTUAL DOWN TO MINIMUMS. MY THEORY ON THE VOR IS THAT THE VOR RECEIVER INSTALLED ON THIS ACFT IS OLD; AND ISN'T SENSITIVE ENOUGH TO PICK UP THE VOR FROM WHERE THE RMG VOR IS. SINCE IT HAD BEEN WORKING INFLT; I BELIEVE THAT IF I HAD COME AROUND FOR THE LOC RWY 27 APCH; IT WOULD HAVE WORKED FINE GIVEN THAT THE SIGNAL WOULD HAVE BEEN STRONG AND MORE PRECISE. I SHOULD HAVE GONE MISSED; AND GOTTEN VECTORS FOR THE LOC RWY 27; CIRCLE TO LAND RWY 9; WHICH THE ARPT HAD THE MINIMUMS FOR. EVEN A TAILWIND LNDG WOULD HAVE BEEN SAFER THAN WHAT I DID. I LEARNED A LOT FROM THIS FLT; AND I APPRECIATE THE OPPORTUNITY TO CONTRIBUTE MY SCENARIO TO HELP AVIATION SAFETY.