SMA MODE C GIVES FALSE ALT INFO.

Narrative

WHILE ON A COURSE TO AND FROM NW OF THE ST. LOUIS AREA FROM AND RETURNING TO ST. LOUIS DOWNTOWN PARKS ARPT; SMA WAS FLOWN AT AN ALT BTWN 1500-1800 FT MSL AS SHOWN ON THE ALTIMETER. BOTH RTES THE ALTIMETER WAS SET TO THE ST. LOUIS DOWNTOWN PARKS ATIS. THIS FLT PATH WAS UNDER THE 2000 FT FLOOR OF THE ST. LOUIS CLASS B AIRSPACE ALONG THE MISSISSIPPI RIVER; W OF THE CHAIN OF ROCKS BRIDGES; JUST N OF THE HWY 67 FORT BELLEFONTAIN MISSOURI RIVER BRIDGE; JUST S OF THE PWRPLANT NEAR PORTAGE DE SIOUX; OVER THE BLUFF AND NW. ON THE RETURN TRIP; WHILE APCHING THE 270 DEG RADIAL OFF THE TROY VOR; AN ORANGE LARGE BODY JET AIRLINER WAS NOTICED AS TFC HIGH AND PASSING ABOVE; L TO R. HAVING HAD THE ENCODER BOX OF MY XPONDER ADJUSTED BY RADIO SHOP ONLY 10 DAYS PRIOR; I HAVE REASONS TO BELIEVE THAT I MAY HAVE BEEN INDICATING AS MUCH AS 1200 FT HIGHER THAN ACTUAL ALTIMETER ALT. DURING THE CHK; THE TECHNICIAN WAS HAVING TROUBLE WITH THE EQUIP HE WAS USING. THE BATTERIES DIED AND HE CONNECTED TO AC POWER. HIS READING SHOWED MINUS 1000 FT WHEN PRESSURE ALT AT THAT TIME WAS PLUS 200 FT. HE ADJUSTED THE ENCODER SCREW TO OBTAIN A READING OF PLUS 200 FT WHILE CONTINUING TO HAVE TROUBLE WITH HIS EQUIP THROUGHOUT THE CHK. IF THE ENCODER WAS READING HIGH; AS MUCH AS 1200 FT OR MORE; THAT COULD HAVE POSSIBLE TRIGGERED A PROX WARNING AT THE ST. LOUIS TRACON WITH THE AIRLINE TFC OFF THE TROY VOR OR OTHER TFC IN THE AREA. ENCLOSED IS A COPY OF THE RPT FROM ANOTHER SHOP WHICH VERIFIES THE INCORRECT SETTING WHICH WAS SUSPECTED; AND NOTE OF CORRECTIONS MADE.