SMA ON IFR FLT PLAN TOLD ATC UNABLE TO PICK UP MODE C.

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IFR RACINE TO MADISON AND RETURN. MKE DEP PICKED UP MODE C. TRANSFERRED TO MADISON APCH. MADISON SAID THEY WEREN'T RECEIVING MODE C. THIS HAS HAPPENED BEFORE; ABOUT A YR AGO AND EVENTUALLY MADISON PICKED IT UP. CONTINUED APCH AND LANDED. DEPARTED MADISON ABOUT XX PM. MADISON STILL DIDN'T PICK UP MODE C. MKE SAID THEY DIDN'T EITHER BUT I DON'T BELIEVE FLT PATH TOOK ME THROUGH OR ABOVE MKE ARSA. CHKED CONNECTIONS TO ENCODER AFTER LNDG BUT COULD FIND NOTHING WRONG. FLT CHKED AIRPLANE ON 9/AB/91 AND MKE APCH RPTED ENCODER WAS WORKING PROPERLY. IFR FLT TO MADISON 9/AD/91. VERIFIED ENCODER OP AFTER TKOF WITH MKE. MADISON MADE NO COMMENT SO PRESUMABLY THEY WERE PICKING IT UP. TOOK OFF A FEW HRS LATER AND MADISON SAID THEY COULDN'T PICK UP MY MODE C. VERIFIED THIS JUST BEFORE SWITCHING TO MKE APCH. MKE WAS PICKING UP MY MODE C LOUD AND CLR. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES UNABLE TO DETERMINE PROBLEM. HAS TRANSPONDER IN FOR MAINT CHK AGAIN. REALLY HOPES SOMETHING CAN BE FOUND TO RESOLVE THIS TROUBLE. WILL LET US KNOW IF THIS OCCURS. FEELS HIS EQUIP IS THE PROBLEM SINCE MSN HAD NOT COMMENTED REF OTHER COMPLAINTS.