A C340 ON CLBOUT AT 1500 FT IS ADVISED OF A HDG DEV AND INCURSION INTO CTLED AIRSPACE CAUSED BY A FAILED HORIZ SIT INDICATOR GYRO SLAVING SWITCH.

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RECEIVED CLRNC OF R TURN AFTER TKOF TO 220 DEGS AT OR BELOW 2500 FT. PROCEEDED WITH TKOF AND BEGAN R TURN TO 220 DEGS. WHEN HANDED OFF TO ADVISORIES; THE CTLR ASKED WHY WE WERE DOING A 360 DEG TURN TO GET TO 220 DEGS. HE WAS QUITE UPSET. HE ASSIGNED A VECTOR IMMEDIATELY; WHICH WE COMPLIED WITH. AGAIN; THE CTLR WAS UPSET THAT WE WERE 40 DEGS OFF COURSE ASSIGNED. WE WERE HANDED OFF TO ANOTHER CTLR WHO WAS VECTORING US SEVERAL TIMES IN RAPID FIRE AND GETTING NO RESULTS. HE ANNOUNCED 'YOU ARE HDG OUT OF MY APCH PATH.' THE CAUSE WAS DETERMINED TO BE A FAILURE TO PROPERLY AUTO SLAVE THE HSI TO THE COMPASS. A SWITCH WHICH ENABLES OR DISABLES SLAVING WAS EVIDENTLY DEFECTIVE SINCE CYCLING IT SEVERAL TIMES CORRECTED THE PROB. WHEN THE GYRO PROB WAS FOUND; I NOTIFIED THE CTLR AND DECLINED AN OFFER TO RETURN TO ARPT SINCE WE WERE VFR AND NO EMER EXISTED. WE RESUMED A SAFE COURSE TO AVX THEN N. NO FURTHER PROBS WERE NOTICED WITH THE EQUIP. I AM HAVING IT CHKED OUT. I HAVE NEVER CYCLED THIS SWITCH BEFORE AND THE PREVIOUS OWNER HAS NOT EITHER. ALSO THE CONSTANT VECTORING BY ATC MADE IT ALMOST IMPOSSIBLE TO CHK THE HSI AGAINST THE MAGNETIC COMPASS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE SWITCH WAS INITIALLY INTERMITTENT THEN WAS COMPLETELY OPEN AND AFTER FREQUENT OP OF THE SWITCH IT BECAME OPERATIONAL. THE RPTR SAID DURING THE 10 YRS OF OWNERSHIP OF THE AIRPLANE THE FUNCTION OF THE SWITCH WAS UNKNOWN. THE RPTR SAID THE SWITCH HAS BEEN REPLACED AND LABELED.