AN APPARENT ALTDEV ALT EXCURSION WHEN SMA LOSES PRIMARY ELECTRICAL PWR SOURCE FOR A SHORT PERIOD.

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ON THE MORNING OF FEB/92; I WAS IFR FROM STEPHENVILLE; TX; TO LAREDO INTL ARPT. MOST OF THE FLT WAS COMPLETED IN VFR CONDITIONS ON TOP OF SOLID OVCST. DURING DSCNT INTO LAREDO AN AREA OF VFR CONDITION WAS ENCOUNTERED THAT REACHED ALL OF THE WAY TO THE SURFACE. I WAS CLRED TO 2500 FT AND GIVEN VECTORS FOR ILS RWY 17R. SHORTLY AFTER REACHING 2500 FT AND ASSIGNED HDG HOUSTON CTR INFORMED ME THE ALT HE WAS RECEIVING WAS 200 FT LOW. HE RECONFIRMED THE ALTIMETER SETTING; AND ACCORDING TO MY ALTIMETER I WAS EXACTLY 2500 FT. RADIO COMS WERE ALSO GETTING POOR. HAVING NEVER FLOWN IN THIS AREA I DID NOT KNOW IF THIS WAS USUAL. THESE WERE THE FIRST TWO INDICATIONS THAT SOMETHING WAS NOT RIGHT. SHORTLY AFTER THIS ALL COM WAS LOST AND MY RADIOS; AND D.G. HAD FLAGGED. I DID NOT FEEL THERE WAS SUFFICIENT INSTRUMENTATION TO COMPLETE A SAFE APCH. MY LAST ASSIGNED HDG WOULD HAVE TAKEN ME INTO MEXICO. I FELT THE MOST LOGICAL SOLUTION WAS TO MAKE THE SHORT CLB TO VFR CONDITIONS GO BACK TO THE CLRING AND MAKE A VFR DSCNT. AFTER STARTING THE CLB I DISCOVERED; AND CORRECTED THE SOURCE OF MY ELECTRICAL FAILURE. I ESTABLISHED CONTACT WITH HOUSTON CTR. HE GAVE ME NEW VECTORS AND THE APCH WAS COMPLETED WITHOUT INCIDENT.