C172 LOSES PRIMARY ELECTRICAL PWR IN IMC OP; GETS A LAST MIN VECTOR FROM PCT CTLR TO DIVERT TO HEF; VA.

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WE DID A VOR APCH INTO W66 CIRCLING TO RWY 32 WITHOUT A PROB. ON THE PRACTICE MISSED APCH; WE WENT DIRECT TO CASANOVA VOR AND CONTACTED POTOMAC APCH ON 124.65. WE WERE CLRED TO CLB TO 3000 FT AND WERE TOLD TO EXPECT RADAR VECTORS FOR THE ILS RWY 16L CIRCLING RWY 34R INTO MANASSAS. FIRST POTOMAC APCH STARTED TELLING US THAT OUR XPONDER WENT BAD. AFTER THAT OUR DME WENT BAD TOO. THE AMMETER SHOWED A DISCHARGE; AND AT THAT MOMENT WE EXPERIENCED A RADIO FAILURE ON AN ASSIGNED HDG OF 360 DEGS IN 3000 FT IN IMC. WE FOLLOWED THE CHKLIST FOR ELECTRICAL FAILURE PER THE C172 BY SHUTTING THE AVIONICS SWITCH OFF; NON-ESSENTIAL ELECTRICAL EQUIP DOWN; AND THE MASTER SWITCH OFF. THEN WE SWITCHED THE MASTER ON AGAIN. WE HAD; FOR A FEW SECONDS; PWR TO TELL APCH CTL THAT WE NEEDED IMMEDIATE VECTORS TO MANASSAS DUE TO OUR PROB. THEY TOLD US THAT THEY WERE TRYING TO GIVE US VECTORS FOR THE VISUAL AT MANASSAS. ON AN ASSIGNED HDG OF 090 DEGS; WE EXPERIENCED LOST COM AGAIN. WE SQUAWKED AT ALL TIMES 7600. OVER LAKE MANASSAS; WE COULD SEE SOME BREAKS IN CLOUDS. WE CIRCLED AROUND AND IN THE END DSNDED BELOW THE CEILING WHEN WE NOTICED WE WERE OVER THE NOKESVILLE AREA IN 1000 FT MSL. WE FLEW VISUALLY TO MANASSAS; GOT A GREEN LIGHT GUN SIGNAL AND LANDED ON RWY 34L. OUR FLAPS WERE INOP; TOO. AFTER LNDG; WE GOT A GREEN LIGHT AND TAXIED TO THE RAMP. THE AIRPLANE WAS GNDED PENDING INSPECTION.