CTR WX RADAR SITE ONLY PICKING UP HEAVIER WX AND DISPLAYING IT AS LIGHT WX.

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ON THE MORNING OF DEC/XX/96; I WAS CTLING THE IKK LOW SECTOR AT ZAU. THERE WERE RAIN SHOWERS AND TSTMS S AND E OF THE EON VOR. OUR RADAR WAS PAINTING A LINE OF WX FROM DNJ VOR TO ABOUT 10 MI N OF GUS VOR. I HAD A PA28 AT 6000 FT NEAR THE BJT VOR AND HE REQUESTED A VECTOR CLR OF THE WX. I INFORMED HIM THAT FROM WHAT I WAS PAINTING ON MY RADAR A W HEADING WOULD GET HIM CLR OF THE WX THE FASTEST. AFTER THE PA28 HAD GONE ABOUT 25 MI HE INFORMED ME THAT HE WAS TURNING N TO GET OUT OF A TSTM. AT THIS POINT I WAS SHOWING HIM AT LEAST 3 MI CLR OF ALL THE WX. AFTER HE HAD TURNED N; I ASKED HIM HIS FLT CONDITIONS AND HE INFORMED ME THAT HE WAS IN MODERATE RAIN; MODERATE TURB WITH LIGHTNING. I HAD INFORMED THE SUPVR TO HAVE THE SMCL CHK OUT THE JOT WX RADAR. AFTER I WAS RELEASED FROM THE POS; I WENT UP TO THE WX STATION WITH THE SUPVR AND WE ATTAINED A PRINTOUT OF THE WX. I ALSO WENT TO THE SMCL TO EXPLAIN WHAT HAD HAPPENED. THEY INFORMED ME THAT THE JOT RADAR WAS OPERATING NORMALLY. WHEN THEY PUNCHED UP THE BOUNDARIES OF COVERAGE FOR THE RADAR SITE ON THEIR SCOPE YOU COULD SEE THAT THE IND RADAR WAS PICKING UP THE WX BUT THE JOT RADAR WAS ONLY PICKING UP THE HEAVIER WX AND DISPLAYING IT AS LIGHT WX. I HAVE NOW FILED 2 UNSATISFACTORY CONDITION RPTS WITH THE FAA. THE FIRST ONE THEY SAID THE RADAR WAS OPERATING NORMALLY AND COULD NOT FIND ANYTHING WRONG; THE LATEST ONE I MAILED YESTERDAY.