C172 PLT HAD ELECTRICAL FAILURE.

Narrative

DURING CONTACT WITH LIT APCH CTL ON 119.5; THE CTLR NOTIFIED ME THAT HE WAS NOT RECEIVING XPONDER; PLEASE RESET. I NOTICED AS I REACHED OVER TO DO ACTUAL RESET THAT THE LIGHTS ON THE XPONDER WERE DIM; INCLUDING PWR AND INTERROGATION. IT WAS THEN I NOTICED ELECTRICAL DISCHARGE ONLY ON MY AMMETER. I HAD ALREADY BEEN GIVEN 'EXPECT RWY 18' CLRNC AND I HAD BEEN LISTENING TO TFC ON THE FREQ. ALTHOUGH I COULD NOT RAISE APCH ON RADIO #1; OR TWR ON #2; I WAS IN A POS TO FINISH THE APCH AND LNDG WITHOUT ADVERSELY EFFECTING EITHER APCHING OR DEPARTING TFC ON RWY 18. I TURNED OFF ALL PWR USAGE EXCEPT #1 RADIO; BUT STILL COULD NOT REACH ATC. I RPTED SHORT FINAL; THEN CLR OF RWY 18; THEN I TAXIED TO PARKING. I'M NOT CERTAIN IF I EVER RECEIVED A LIGHT FROM THE TWR. CORRECTIVE ACTIONS: I AM MAKING A SCAN OF VOLTMETER AS A STANDARD PERIODIC PART OF MONITORING ACFT SYS AND I AM HAVING THE BATTERY CHARGING SYS REVIEWED BY A MECH. THE INCIPIENT LOSS OF ELECTRICAL PWR IS NOT OBVIOUS TO A PLT WHO IS NOT LOOKING FOR SPECIFIC SIGNS. AN ADDITIONAL REMEDY WOULD BE TO CARRY A BACKUP HAND-HELD XMITTER. I TRIED AFTER LNDG TO PHONE THE TWR TO EXPLAIN WHAT OCCURRED BUT ALL I COULD GET WAS AN ANSWERING MACHINE. I DID NOT LEAVE A MESSAGE. I HAVE HAD MY FAVORITE MECH CHK OUT THE AIRPLANE AND HE FOUND THE PROB TO BE A BROKEN BRACKET THAT HOLDS THE ALTERNATOR IN PLACE.