A BELL 206 HELI PLT; ON A TFC RPT ASSIGNMENT; DEPARTED MYF WITHOUT PERMISSION.

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TKOF; FOR THE EARLY MORNING TFC RPTS; USUALLY OCCURRED BEFORE THE TWR OPENS AT XA00. HOWEVER; DUE TO SOME CONFUSION AS TO WHO THE ONBOARD CAMERA OPERATOR/TALENT WOULD BE; TKOF WAS DELAYED A FEW MINS WHILE THE CORRECT PERSON GOT ON BOARD AND SETTLED IN. I HEARD THE ATIS FAINTLY COMING IN ON THE #2 RADIO; BUT FOR SOME REASON IT DIDN'T REGISTER THAT THIS MEANT ATC SVCS WERE UP AND RUNNING FOR THE ARPT. THINKING THE TWR STILL NOT IN OP; I MADE A STANDARD CTAF CALL ON THE PROPER FREQ USING THE #1 RADIO. UNFORTUNATELY; I FAILED TO CHK THE VOLUME OF THE #1 RADIO -- IT WAS TURNED ALL THE WAY DOWN. BECAUSE IT WAS A HELI OP; I DIDN'T USE THE RWY; SO NO CONFLICT OF LNDG OR TKOF OF AIRPLANES OCCURRED. AFTER TKOF; STILL THINKING THE TWR CLOSED; I MADE ANOTHER BROADCAST ON CTAF STATING MY INTENTIONS OF TURNING S. IT WASN'T UNTIL I CHANGED FREQS TO SAN THAT I REALIZED THE VOLUME WAS TURNED DOWN. FORTUNATELY; NO CLASS B AIRSPACE VIOLATION OCCURRED. MYF TWR NEVER CALLED MY OFFICE WANTING TO CHEW ME OUT; AND I HOPE I LEARNED MY LESSONS OF AT LEAST 3 THINGS NOT TO DO AGAIN.