RPTR TAKES OFF WITHOUT IFR CLRNC INTO IMC CONDITIONS CONTRARY TO ATC INSTRUCTIONS.

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SIT: IFR TO VFR ON TOP DEPARTING CMA IN DARK; FSS INDICATED IFR CONDITION AT DEP ARPT; CMA WITH TOPS 2500 FT CLOUD COVER. CALLED CLRNC DELIVERY RECEIVED INSTRUCTION ON 120.75 (MUGU). INSTRUCTED TO CONTACT MUGU APCH ON 124.7 AND SQUAWK. RUN-UP COMPLETED. INSTEAD OF CONTACTING MUGU I FOR SOME UNKNOWN REASON DIALED CTAF FREQ OF 128.2 AND BROADCAST IN THE BLIND. SQUAWK WAS CORRECT. I HAD AN EERIE SENSATION SOMETHING WAS AMISS BUT COULD NOT PUT MY MIND TO IT. IT WAS TOO QUIET! UPON REACHING 2500 FT MSL I CALLED APCH ON 120.75 AND DECLARED 'VFR ON TOP'. AT THAT POINT I REALIZED MY ERROR AND WAS ALSO INSTRUCTED TO DIAL MUGU APCH 124.7; WHICH I DID. CTLR ACKNOWLEDGED MY XMISSION AND THEN INFORMED ME OF CLRNC VIOLATION AND IMPENDING NOTIFICATION OF FAA. IN RETROSPECT I'M NOT SURE WHY THIS HAPPENED AS I FREQUENTLY FILE IFR. I DID MISS THE TURN OFF TO THE RUN-UP AREA TO RWY 26 AND PERHAPS THIS DIVERTED MY ATTN. IT WAS EARLY AM AND DARK; FOGGY; BUT APPREHENSION WAS NOT A PROB. I SOMEHOW LOCKED ONTO THE INSTRUCTIONS; FLEW THEM JUST FINE; BUT DID IT AS IF IT WERE A VFR-TWR CLOSED SIT. MY ANSWER TO PREVENT THIS IS TO REVIEW MY COCKPIT AWARENESS; READ AND PERCEIVE MY INSTRUCTIONS; NOT JUST CASUALLY FILE THEM AWAY AS A ROUTINE FLT. THE INSTRUCTIONS ARE ALWAYS THE SAME POSSIBLY LULLING ME INTO COMPLACENCY RATHER THAN REALLY LISTENING!