ACFT MAINT DISCREPANCIES AND ACFT EQUIP PROBLEMS.

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I HAVE HAD 2 EVENTS INVOLVING MY PRIVATE FLYING OF MY SMA LOW WINGER THAT COULD HAVE LED TO A SERIOUS ACCIDENT. IN BOTH CASES I WAS LUCKY OR SKILLFUL AND AVOIDED AN INCIDENT. BOTH INVOLVED PROBLEMS AFTER AVIONICS MAINT. THE FIRST INCIDENT HAPPENED IN 1987. I HAD A NEW TRANSPONDER INSTALLED BY FACILITY MECH. A FEW FLTS LATER MAKING A XWIND LNDG; THE ELEVATOR JAMMED JUST PRIOR TO THE FLARE. BRUT FORCE OVERCAME IT AND ACFT WASN'T DAMAGED. TRANSPONDER WAS NOT STRAPPED IN PROPERLY AND BINDING ONLY OCCURRED AT REAR FULLY AFT WITH R AILERON IN. THE MECH WAS GLAD TO STRAP UNIT IN PROPERLY BUT OF COURSE WOULD MAKE NO ENTRY IN LOGBOOK. THIS YR MY ACFT HAD BEEN IN FOR A TRANSPONDER AND PITOT STATIC CERTIFICATION; 5 MONTHS BEING WERE REQUIRED FOR THIS JOB! I FINALLY GOT MY ACFT BACK FROM MAINT FACILITY TO FIND A NEW ALTIMETER WAS INSTALLED WITHOUT INFORMING ME. DUE TO THE AGE OF MY ALTIMETER I'M SURE THIS WAS NEEDED. AFTER CHKING THE LOGBOOK; AND PAYING THE $625 I DECIDED ON A QUICKIE TEST FLT. SURPRISE! THE KOLLSMAN KNOB COULD NOT BE TURNED; IT WOULD NOT BUDGE! THE KNURLED KNOB WAS JAMMED ROCK TIGHT AGAINST THE PLASTIC INST PANEL COVER. A SIMPLE FIX. BOTH THESE CASES EMPHASIZE THE NEED FOR A COMPLETE OPS CHK AFTER AVIONICS MAINT. LOOK; TURN; FEEL; OPERATE AND TURN AGAIN; LOOK AT ALL AREAS INVOLVED IN THE INSTALLATION AS WELL AS ALL SYS. THIS COULD SAVE YOUR ACFT AND LIFE.