PRECAUTIONARY LNDG MADE WHEN ALL ELECTRICAL EQUIPMENT FAILED DURING NIGHT FLT.

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ON SAT MORNING; AT APPROX XA50 LCL; SMA X WAS ENRTE FROM EWR TO ISP VFR IN NYC TCA. UPON DEP FROM EWR; NOTICED A SMALL DISCHARGE ON AMMETER. NOTIFIED ATC AND RESET CB'S. ATC ASKED IF WE WANTED TO RETURN TO EWR. INFORMED ATC THAT SITUATION WAS NOW NORMAL (DISCHARGE STOPPED) AND WOULD CONTINUE FLT TO ISP. A SHORT WHILE LATER (APPROX 4 MINS) AMMETER DISCHARGED AGAIN. STARTED TO TURN OFF UNNECESSARY ELECTRICAL LOADS DUE ELECTRICAL FAILURE. LEFT ONE NAV/COM TRANSPONDER AND NAV LIGHTS ON. ATC STARTED TO SAY SOMETHING WHEN THE COM (ONLY ONE LEFT ON) GOT DISTORTED. TOLD ATC OF PROB RECEIVING AND SET TRANSPONDER TO 7600. PLANNED TO CONTINUE BUT LIGHTS FADED RAPIDLY AND A STRANGE SOUND ('WHOMP; WHOMP;' ETC) WAS HEARD. ASSUMED THAT ALTERNATOR BELT BROKE AND WAS HITTING PROP. NOTICED JFK IN FRONT OF ACFT; APPROX 3 MI. DECIDED TO MAKE PRECAUTIONARY LNDG RATHER THAN FLY AT NIGHT W/O LIGHTS OR RADIO. CIRCLED THE FIELD 3 TIMES LOOKING FOR TFC AND LIGHT GUN SIGNALS. FINALLY; SEEING NO TFC; LANDED RWY 13R AND TAXIED OFF. WAITED 10 MINS; THEN STOPPED ENG. LEFT ACFT AND WENT TO BLDG APPROX 100 YDS AND CALLED AUTHORITIES (SEE ATTACHED RPT OF ARPT SUPVR). CALLED JFK TWR ON TELEPHONE. TWR INFORMED US THAT TRACON 'LOST YOU ON RADAR;' SO HE PUT LIGHTS (RWY) ON LEVEL 5 AND STOPPED ALL TFC UNTIL WE COULD BE LOCATED. HE ASSUMED WE WOULD LAND AT JFK. NOTE: BEFORE LEAVING EWR DID A COMPLETE PREFLT CHK AND RUN UP. CHKED ALTERNATOR BELT AND ALTERNATOR PWR (SWITCH ON/OFF)--ALL WAS WORKING OK.