ACR DC8 PIC MISINTERPS IFR DEP PROC AND BEGINS A TURN 1 MI OFF DEP END OF RWY INSTEAD OF AT 5 PT 5 MI.

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LEAVING DAL; OUR DEP INSTRUCTIONS WERE THE DALL5 DEP; WHICH IS RWY HDG UNTIL 5.5 DME; R TURN TO 360 DEGS FOR VECTORS; CLB AND MAINTAIN 5000 FT. IT WAS MY LEG (CAPT) BUT AFTER ENG START; WE HAD TROUBLE GETTING PUSHBACK CLRNC. AFTER TROUBLESHOOTING; WE DISCOVERED THE FO HAND MIKE WAS STUCK. WE WERE LATE; SO INSTEAD OF HAVING THE FE CHANGE OUT THE MIKE; I DECIDED TO GO AHEAD AND PUSH AND START TAXIING -- SINCE WE WERE LATE. SINCE MY RADIOS WORKED; I TOLD THE FO TO BRIEF AND DO THE TKOF. DURING TAXI OUT; THE FE HAD TIME AND CHANGED OUT THE FO'S MIKE. IT CHKED GOOD; SO I CHANGED BACK TO MY DEP AND I BRIEFED THE PERTINENT ITEMS; INCLUDING THE DEP PROC. BUT; WHEN IT CAME TIME FOR TKOF; I INITIATED A R TURN TO 360 DEGS AT 500 FT AGL. TWR WAS WATCHING AND ASKED IF WE WERE FLYING RWY HDG. UPON HEARING THEIR XMISSION; I IMMEDIATELY CORRECTED BACK TO RWY HDG. TWR'S VIGILANCE KEPT US FROM EXCEEDING MORE THAN A 15 DEG HDG DEV. CONTRIBUTING FACTORS WERE: 1) BEING LATE; NOT CHANGING THE MIKE OUT IN THE BLOCKS PRIOR TO TAXI; AND 2) THE CONFUSION OF THE CHANGE FROM 'MY TKOF' TO 'YOUR TKOF' BACK TO 'MY TKOF.'