CESSNA 172 PLT DECLARED AN EMER AND RETURNED TO LAND AFTER EXPERIENCING INTERMITTENT 2-WAY RADIO COMS AND INST PANEL LIGHTING.

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I DEPARTED GAI AT XA00 HRS FOR A VFR FLT TO YTZ. VFR CONDITIONS PREVAILED WITH WINDS FROM 260 DEGS AT 10 KTS AND VISIBILITY 4-6 MI. AT ABOUT XA30 HRS; WHILE FLYING NEAR HAGERSTOWN; MD; I BEGAN EXPERIENCING RADIO PROBS. AT THAT TIME; I WAS IN CONTACT WITH; I BELIEVE; WASHINGTON APCH. SUDDENLY THE COM AND NAV LIGHTS ON THE PANEL BEGAN BLINKING AND REPEATED 'BZZZZ; BZZZZ; BZZZZ' SOUNDS (LIKE 2 HOT WIRES TOUCHING EACH OTHER) WERE COMING FROM THE PANEL. NO SMOKE OR SMELL WAS DETECTED. AT TIMES THE EQUIP WOULD BE DEAD SILENT; OTHER TIMES IT WOULD BE BUZZING AND BLINKING; AND OCCASIONALLY IT WOULD BE FUNCTIONING NORMAL. REVIEW OF PANEL INSTS; ELECTRICAL GAUGES; AND BREAKERS INDICATED NO OTHER VISIBLE PROBS. SINCE I WAS UNABLE TO IDENT ANY CORRECTABLE PROB; WITHIN MINS I DECIDED TO DECLARE AN EMER AND RETURN TO GAI. I NOTIFIED THE ATC; WITH WHOM I HAD LAST CONTACT; ABOUT MY FAILING RADIO EQUIP; DECLARED MY EMER; AND MADE A L U-TURN FOR GAI. I CONTINUED TO XMIT INTERMITTENTLY (VARIABLE INTERVAL OF 2 MINS) ABOUT MY FAILING RADIOS AND NAV INSTS; AND OCCASIONALLY WOULD HEAR FROM WASHINGTON APCH BUT WAS UNABLE TO MAINTAIN ANY SUSTAINED DIALOGUE. INITIALLY I RETAINED MY ASSIGNED SQUAWK FROM DULLES APCH; BUT LATER SWITCHED MY XPONDER TO 7700. EVENTUALLY I WAS HANDED OFF TO BALTIMORE APCH WHO WAS ABLE TO GIVE ME A HEADING TO GAI. 45 MINS LATER (FROM THE TIME THE PROB WAS IDENTED) I LANDED AT GAI WITHOUT AN INCIDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT IT WAS FOUND THAT THE ALTERNATOR BELT WAS LOOSE AND NOT TURNING THE ALTERNATOR TO PRODUCE ELECTRICAL NEEDS FOR THE ELECTRICAL EQUIP TURNED ON. THUS; RUNNING DOWN THE BATTERY UNTIL THERE WAS NO ELECTRICAL CURRENT LEFT FOR THE RADIO OR PANEL LIGHTS.