A CHARTER B727 ENTERS A TSTM WHEN THEIR RADAR FAILED TO SHOW THE HAZARDS AHEAD.

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IN CRUISE/DSCNT INTO KNOXVILLE; TN; THE FLT ENCOUNTERED UNEXPECTED AND UNRPTED SEVERE TURB; HVY PRECIPITATION; AND HAIL. THE ACFT SUFFERED AT LEAST TWO OBSERVED LIGHTNING STRIKES TO THE NOSE. AIRBORNE WX RADAR (HONEYWELL PRIMUS-90) SHOWED ONLY LIGHT TO MODERATE PRECIPITATION IN THE GENERAL VICINITY. THE FLT WAS UNABLE TO HOLD ASSIGNED ALT; AND IMMEDIATELY CONTACTED ATC FOR A BLOCK ALT AND RIGHT-OF-COURSE DEV. AFTER 30-60 SECONDS; THE ACFT WAS IN THE CLR. THERE WERE NO RPTED INJURIES TO CREW OR PAX; AND NO IMMEDIATE SIGN OF DAMAGE TO THE ACFT OR ITS SYS. THE ACFT'S ONBOARD WX RADAR HAS A HISTORY OF MAINT PROBS; INCLUDING FAILURE TO SHOW AREAS OF KNOWN PRECIPITATION. THE ACFT WAS FLOWN TO A MAINT FACILITY FOR INSPECTION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR SAID THAT HE HAS HAD SOME EXPERIENCES WITH OTHER ACFT WITH THIS TYPE RADAR BUT FEELS THAT IT IS A 'COMPANY MAINT' PROB. HE SAYS THAT THEIR MAINT DEPT DOESN'T HAVE THE EXPERTISE TO FIX THE RADAR RETURN PROBS. OTHER ACR'S HAVE THIS SYS WITHOUT SHOWING ANY PROBS WITH IT. THE OWNER OF THIS PART 129 CARRIER HAS ISSUED A 'BLANK CHK' TO MAINT IN ORDER FOR THEM TO FIX THE RADARS.