ATC ADVISED PLT OF AN SMT TWIN OF ALTDEVS INDICATED BY THE ACFT'S XPONDER RETURN.

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INST FLT PLAN FILED FROM MADISON; WI; TO DULUTH; MN; WITH SECOND LEG OF TRIP FROM DULUTH; MN TO KENORA; ONTARIO; CANADA. PRIMARY XPONDER HAD BEEN GIVING ERRONEOUS ALT RPTS PERIODICALLY DURING THE FLT. MADISON APCH AND ZAU RECEIVED ACCURATE RPTS. ZMP AND DULUTH APCH RECEIVED INACCURATE RPTS; CHANGED TO STANDBY XPONDER WHEN THIS EVENT OCCURRED. I EMPLOYED THE USE OF THE PRIMARY XPONDER WHEN ESTABLISHING CONTACT WITH WINNIPEG CTR. UPON DSCNT INTO DEST ARPT (YOK); WINNIPEG ADVISED THAT ALTDEV EXISTED; IMMEDIATELY CHANGED TO STANDBY XPONDER FOR BALANCE OF TRIP AND THE RETURN TRIP 4 DAYS LATER. PRIMARY PRIMARY XPONDER HAS BEEN REPAIRED. '2 DECODERS FOR MODE C WERE INOPERABLE.' CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING: RPTR STATED THAT HE WAS NOT FURTHER CONTACTED BY EITHER THE FOREIGN GOV OR THE FAA IN REGARD TO THE POSSIBLE ALTDEV. AS HE INDICATED; THE ENCODING XPONDER WAS MALFUNCTIONING AND NOT ALWAYS GIVING A CORRECT ALT READOUT. HE WAS OPERATING A PIPER PA23-250 AZTEC ACFT WITH AN OLDER KING XPONDER WITH ENCODING ALTIMETER READOUT.