A LOW TIME PA28 PLT ASSUMED THE TWR AT ATW WAS CLOSED; WHICH WAS NOT THE CASE; AND TAXIED WITHOUT PERMISSION.

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PLANNING TO DEPART VFR AT NIGHT FROM A CLASS D ARPT; ATTEMPTED TO CONTACT GND CTL FOR PERMISSION TO TAXI. RECEIVED NO RESPONSE. ATTEMPTED; SUBSEQUENTLY; TO CONTACT THE TWR. AGAIN; RECEIVED NO RESPONSE. ASSUMING THAT THE TWR WAS ALREADY CLOSED; XMITTED INTENTIONS ON THE TWR FREQ AND BEGAN TO TAXI TO THE ACTIVE RWY. WHILE I WAS HOLDING SHORT OF THE RWY; AN ARPT SECURITY VEHICLE CAUGHT UP WITH ME; DISPATCHED BY THE TWR AS IT TURNED OUT. THE TWR WAS NOT CLOSED; AND ALL THEY COULD HEAR WAS BLANK XMISSIONS FROM ME. ARPT SECURITY PERSONNEL CONFIRMED FIRST-HAND MY RADIO PROBS. HE ALSO VERIFIED I WAS ON THE CORRECT FREQ. CONTINUING TO TROUBLESHOOT MY RADIO; WHILE THE ARPT SECURITY OFFICER WAS HELPING WIT HIS FLASHLIGHT; I HAD UNPLUGGED MY HEADSET AND PLUGGED IT INTO ONE OF THE PAX'S RECEPTACLES; THUS RE-ESTABLISHING CONTACT WITH THE TWR. TWR SUBSEQUENTLY CLRED ME FOR A VFR DEP. I HAD ENDANGERED NO ONE; AND DID NOT CAUSE ANY DISRUPTIONS; MERELY TAXIED WITHOUT ACTUALLY BEING IN RADIO CONTACT WITH GND CTL. IF I WERE IN A SIMILAR SIT AGAIN; I WOULD ATTEMPT TO FURTHER TROUBLESHOOT THE RADIO BEFORE TAXIING.