FLT INSTRUCTOR AND A STUDENT AT I69 WITH AN IFR FLT PLAN ARE UNABLE TO CONTACT CVG APCH ON THE GND FOR A RELEASE DECIDE TO DEPART AND CONTACT ATC.

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I WAS DEPARTING I69 (CLASS E AIRSPACE) WITH A STUDENT PLT ON AN IFR FLT PLAN. WE RECEIVED OUR CLRNC ON THE GND THROUGH CVG APCH CTL. CVG APCH CTL HAS A FREQ THAT IS SUPPOSED TO WORK ON THE GND AT I69 ARPT. THE CTLR TOLD US TO CONTACT HIM WHEN WE WERE READY FOR TKOF. WE TOLD HIM THAT WE WOULD BE USING RWY 22. WE TAXIED TO THE END OF THE RWY AND TRIED TO CONTACT CVG APCH SEVERAL TIMES. THE FREQ WAS DEAD. SO WE DECIDED TO DEPART VFR; BUT FORGOT TO PUT 1200 BACK INTO THE XPONDER. IT WAS XA00 AND CLR. WE WERE ABLE TO CONTACT CVG APCH WITHIN 2-3 MI AFTER TKOF. THE CTLR GAVE US AN ON-COURSE HDG; THEN GAVE US A PHONE NUMBER TO CALL WHEN WE REACHED OUR DEST. THE CTLR THAT I TALKED WITH ON THE PHONE TOLD ME THAT THEY KNOW THAT THERE IS A PROB AT I69 WITH DEAD SPOTS ON THE GND. SINCE I TOOK OFF VFR WITH A XPONDER CODE OTHER THAN 1200; THAT WAS A POSSIBLE VIOLATION AND HE WOULD HAVE TO FILE A RPT. THE RADIO RECEPTION PROB ON THE GND NEEDS TO BE FIXED. PLTS SHOULD BE MADE AWARE OF THIS PROB AND INFORMED BY ATC OF A PROC TO FOLLOW IN CASE OF SUCH AN EVENT.