A PA28 PLT ENTERED IMC NEAR ATL AND ASKED FOR RADAR VECTORS ON 121.5 ONLY TO DISCOVER HIS MODE C FAILED.

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WHILE IN CRUISE TO HOME ARPT D73 INADVERTENTLY ENTERED INTO IMC. I IMMEDIATELY ATTEMPTED A 180 DEG TURN BUT CONDITIONS HAD WORSENED BEHIND. BEGAN CLB AND CONTACTED ATLANTA APCH ON 121.5. ADVISED OF MY SIT AND THAT I NEEDED VECTORS AS I WOULD SOON BE IN ATLANTA'S AIRSPACE. DURING VECTOR PROCESS TO GRIFFIN; GA; IT BECAME APPARENT THAT MY MODE C WAS NOT OPERATING CORRECTLY AND I HAD TO GIVE REGULAR ALTITUDE READINGS. I WAS UNAWARE OF PROBS WITH MY XPONDER'S MODE 'C.' TO AVOID SUCH AN OCCURRENCE I SHOULD AVOID FLYING DURING PERIODS OF MIXED IFR/VFR. THE MODE C PROB IS SOMETHING THAT WOULD HAVE TO BE CAUGHT AT ANNUAL OR BROUGHT TO ATTENTION BY ATC. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT TRACON PROVIDED AN IFR RATED PLT FAMILIAR WITH HIS ACFT TO TALK HIM DOWN. THE WX DEVELOPED IN ABOUT 3 MINS HE BELIEVES; AND SURROUNDED HIM; ALTHOUGH HE WAS ABLE TO CLB SPIRALING UP TO GET VMC. HE SAID THAT HE WAS TERRIFIED AND DID NOT WANT TO EVER FLY AGAIN BUT SINCE THEN HAS DECIDED TO GET AN IFR RATING. HE TALKED WITH ATC AFTER LNDG AND THEY DID NOT SAY ANYTHING ABOUT AN INVESTIGATION; THEY WERE JUST GLAD THAT HE WAS OK.