ALT BUST.

Narrative

AFTER CLRNC TO TRANSITION LAS VEGAS CLASS B AIRSPACE; I WAS CLRED TO MAINTAIN 4000 MSL WHILE XING THE APCH END OF MCCARRAN RWY 25. A COMBINATION OF EVENTS CAUSED ME TO DSND TO 3500 MSL. I NOTICED THE ERROR JUST AS I WAS NOTIFIED BY THE ALERT CTLR THAT I WAS TOO LOW. I WAS ABLE TO INITIATE A CLB AND RETURN TO MY ASSIGNED ALT BEFORE EFFECTING OTHER TFC. THIS INCIDENT WAS CAUSED BY 2 FACTORS WITH ANOTHER CONTRIBUTING FACTOR. 1ST; THE PLT'S PUSH-TO-TALK SWITCH HAD BEEN INOP FOR SEVERAL WKS. THIS NECESSITATED USING THE HAND MIKE. AFTER LEVELING OFF AT 4000 MSL (I THOUGHT) I DROPPED THE MIKE. AT THAT MOMENT; THE ELECTRIC TRIM SLIPPED AND THE PITCH DID NOT RESPOND. BECAUSE I WAS REACHING FOR THE MIKE; I MISSED THE CONTINUED DSCNT. THE CONTRIBUTING FACTOR IS THE PART 135 REQUIREMENT THAT ONLY THE OPERATOR IS RESPONSIBLE FOR MAINT. MY OPERATOR'S MECH AND AVIONICS SHOP WILL NOT EVEN TALK TO ME; THE OWNER. I HAD MENTIONED TO THE OPERATOR SEVERAL TIMES ABOUT THE PUSH-TO-TALK SWITCH; AND HAD THOUGHT IT WAS CORRECTED. HE HAD BEEN TOO BUSY; HOWEVER; AND HAD NOT TAKEN CARE OF THE PROB. THIS IS NOT THE 1ST TIME I HAVE HAD TO LITERALLY HOUND THE OPERATOR TO CORRECT SOMETHING HIS EXPERIENCED; PROFESSIONAL PLTS FIND ONLY A MINOR ANNOYANCE; BUT THAT I; A LOW-TIME PVT PLT FLYING FOR MY OWN ENJOYMENT; CONSIDER A HAZARD.