CPR PLT WITH EQUIP PROB HAS FURTHER DIFFICULTIES RESOLVING DISPUTED ALT WITH ZTL.

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C441 DEPARTED TYS ON IFR FLT PLAN TO GSO. TYS DEP ASSIGNED 10000 FT. PASSING 8000 FT; HANDED OFF TO ZTL. CALLED ZTL; RPTED CLBING TO 10000 FT. THEY CLRED ACFT TO 15000 FT. READ BACK; CLBING TO 15000 FT; AND SELECTED 15000 FT ON AUTOPLT ALT PRESELECT AND ARMED IT. HAD CONVERSATION WITH ZTL REGARDING NEW RTE AND REQUESTED DIRECT HMV; WHICH WAS GRANTED. WHILE LISTENING ON #2 COM RADIO; I CALLED BNA FLT SVC ON 122.3 TO GIVE PIREP ON BASES AND TOPS; WHICH BRIEFER REQUESTED ON #1 COM RADIO. DURING CONVERSATION WITH BNA; ZTL HAD OTHER TFC; SO I REDUCED VOLUME ON #2 COM TO HEAR BNA. I STILL HEARD VOICES ON #2; BUT SOON IT QUIT. AFTER THE PIREP AND SOME WX UPDATES FOR GSO; I SWITCHED TO #1 COM TO GSO ATIS 128.55 TO GET CURRENT WX AT GSO WHEN IN RANGE. I MOVED APCH PLATES FROM COPLT YOKE TO PLT POS WHILE ACFT CONTINUED AUTOPLT CLB. I FELT AUTOPLT COUPLE AND NOSE BEGAN DOWN TO LEVEL OFF; AND RETURNED ATTN TO CHART TO PREPARE FOR 300 FT OVCST; 1 MI VISIBILITY APCH. THE ACFT WENT SLIGHTLY ABOVE 15000 FT IN THE COUPLED LEVELOFF; WHICH IS NORMAL IF ACFT CLBING MORE THAN 500 FPM. THE ALT PRESELECT HAS AUDIBLE TONE IF ACFT EXCEEDS 300 FT EITHER SIDE OF ALT AND NO TONE WENT OFF SO ACFT WITHIN ALT PARAMETERS TO PLT. I AM CAREFUL NOT TO EXCEED ASSIGNED ALT. DURING THIS TIME ZTL TRIED TO CALL BUT VOLUME DOWN ON #2 COM; AND SWITCH HAD ACTUALLY CUT OFF THE RECEIVER. ZTL TRIED TO CALL ME TO CONFIRM ALT BECAUSE THEIR COMPUTER WAS SHOWING LEVEL AT 15800 FT AND THEY NEEDED TO FIND OUT ACTUAL ALT BECAUSE THEY HAD 16000 FT COMMUTER TFC IN THE AREA. APPARENTLY; THE ENCODER WAS TELLING 15800 FT TO CTR WHEN IN ACTUALITY THE ACFT WAS AT 15100- 15200 FT; BUT THEY HAD NO WAY TO CONFIRM BECAUSE WE DID NOT HEAR THEIR CALLS. CTR APPARENTLY TURNED THE COMMUTER TO AVOID COMPROMISE OF SEPARATION. I RECOGNIZED IT WAS QUIET AND LOOKED AT #2 COM TO CHK VOLUME. AS SOON AS VOLUME WAS UP; ZTL SAID THEY WERE TRYING TO CALL AND TO VERIFY ALT; WHICH I DID AT 15100-15200 FT. THEY SAID RPTING 15800 FT; SO I SWITCHED #1 ENCODING SYS OFF AND SELECTED #2 SYS TO STRAIGHTEN OUT THE DISCREPANCY. ZTL ASKED THE COMMUTER FOR ACFT ALT BY OBSERVATION AND HE REPLIED '15500 FT; POSSIBLY 15300 FT;' WHICH CONFIRMED MY ACFT NOT AT 15800 FT. ALTHOUGH THE CTR COMPUTER PERCEIVED ELECTRONICALLY A COMPROMISE OF SEPARATION; I DON'T BELIEVE THERE WAS AN ACTUAL COMPROMISE. I CALLED THE CTR ADMINISTRATIVE OFFICER AND DISCUSSED THE MATTER. AFTER TALKING TO HIM; I HAD AVIONICS SHOP CHK THE ALTIMETERS FOR ACCURACY; AND THE GNDING SYS AND XPONDERS; WHICH THEY FOUND OUT OF CORRELATION. I HAD THE PLT'S ALTIMETER ENCODER REPLACED AND SWITCHED XPONDERS; SO IF IT WAS IN THE XPONDER; IT WOULD RESOLVE THE PROB. ALL THE NEW SYS WERE RECERTIFIED IFR PER FARS. ALSO; THE #2 COM HAD A NEW VOLUME CTL SWITCH INSTALLED. AS THEY WERE ABLE TO VERIFY IT CUT OUT AT REDUCED VOLUME. SUGGESTED REMEDIAL EFFORTS: 1) REPAIR AND REPLACE DEFECTIVE COMPONENTS IN AVIONICS SYS. 2) GET PERMISSION TO BE OFF FREQ FOR PIREPS AND OTHER ADMINISTRATIVE CALLS. 3) CHK VOLUME DURING XMISSIONS TO OTHER ACFT IF POSSIBLE TO INSURE CONTINUED RECEPTION. 4) ALTHOUGH THIS FLT HAD NO PAX AND WAS A FERRY FLT FOR ROUTINE MAINT ON THE ENGS AND AIRFRAME; THERE IS NO SUCH THING AS A 'MILK RUN' AND A HIGH DEG OF SITUATIONAL AWARENESS IS REQUIRED DURING ALL FLT ACTIVITIES. AS I LEARNED IN THE NAVY -- COMPLACENCY KILLS. 5) I'VE BEEN FLYING 34.5 YRS AND GO TO FLT SAFETY IN WICHITA ANNUALLY FOR CREW TRAINING. I RECOGNIZE THE EFFECT AGE HAS ON PLTS AND TRY TO COMPENSATE. I HOPE TO BE FLYING SAFELY AT 77.