STUCK MICROPHONE INTERFERES WITH SDF GND CTLR FREQ; BLOCKED FREQ. ACFT RADIO COM PROB. GND CTLR UNABLE TO ESTABLISH RADIO CONTACT.

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ON RAMP IN SDF; AWAITING CARGO LOADING TO BE COMPLETED. AT ONE POINT; SOMEONE CAME ON BOARD AND ASKED IF WE HAD A STUCK MICROPHONE. ALL 3 CREW MEMBERS CHKED HIS OWN MICROPHONE ON COM #1 (THE SAME RADIO THAT WAS TUNED TO SDF GND CTL FREQ). AS IT TURNS OUT; THE SECOND OBSERVER PANEL WAS AT FAULT AND WAS INTERMITTENTLY XMITTING ON #1 RADIO; WHICH WAS TUNED TO THE GND CTL FREQ MOST OF THE TIME. WHEN THE RADIO CHKS WERE PERFORMED; WE CHKED ON ONE OF OUR COMPANY FREQS AND ALSO; THE SECOND OBSERVER PANEL WAS NOT FAULTY AT THAT PARTICULAR TIME; THEREFORE; THE RADIO CHK WAS NOT ONLY SUCCESSFUL; BUT THE GND CTLR (WHOSE FREQ WAS BEING BLOCKED FROM TIME-TO-TIME) HAD NO OPPORTUNITY TO INFORM US OF THE POSSIBILITY THAT OUR EQUIP WAS AT FAULT. WHEN WE ATTEMPTED TO CALL GND CTL FOR TAXI INSTRUCTIONS; WE DISCOVERED THAT #1 RADIO WAS INOP. WHEN CONTACTING GND CTL ON #2 RADIO; WE WERE INFORMED THAT WE HAD BEEN BLOCKING THAT FREQ FOR THE LAST ONE AND ONE HALF HRS. WE RETURNED TO OUR RAMP FOR REPAIRS. THERE IS SOME POSSIBILITY THAT SOME OF OUR COCKPIT CONVERSATION WAS INTELLIGIBLE ON THE FREQ; AND MAY HAVE BEEN OFFENSIVE TO SOME LISTENERS. IN THE FUTURE; I WILL MAKE CERTAIN TO PLACE ALL 'XMIT SWITCHES' IN THE COCKPIT TO INTERPHONE DURING PREFLT AND ALSO AFTER BLOCKING IN TO PRECLUDE THIS FROM HAPPENING AGAIN.