SMOKE IN COCKPIT AT 400 FT AGL MANDATES A RETURN LAND BY AN HS-125-800 FLT CREW AFTER A NIGHT OP DEP FROM SAN; CA.

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UPON TKOF OUT OF SAN AT NIGHT; MY FO AND I HEARD A LOUD BANG ABOUT 400 FT AGL. AT FIRST WE THOUGHT WE HIT A BIRD. WHEN I LOOKED OVER AT HIM I SAW SMOKE POURING OUT FROM BEHIND THE CIRCUIT BREAKER PANEL. AT THIS TIME I LEVELED THE PLANE OFF AT 1000 FT MSL AND TOLD THE FO TO DECLARE AN EMER; WE WERE GOING BACK. THE FO TOLD SAN TWR WE WERE DECLARING AN EMER AND MAKING L TFC FOR RWY 27. WE HAD 2 PAX ON BOARD AND WERE ACCIDENTALLY CLOSED OFF FROM THE COCKPIT BY THE CABIN DOOR. THEY WERE NOT SURE WHAT WAS GOING ON. THE TWR CLRED US TO LAND AND WE DID SO WITHOUT INCIDENT. THE CABIN WAS FILLED WITH SMOKE BUT NOT YET TO THE POINT FOR THE NEED OF GOGGLES. THE FIRE TRUCKS MET US AT THE TXWY AND WE EVACED THE ACFT. THE ONLY THING I WOULD HAVE DONE DIFFERENTLY IS EVAC FASTER. I THOUGHT THE SMOKE WAS SUBSIDING AND CONSIDERED TO TAXI THE ACFT BACK; WHICH WAS STUPID. IF THERE IS A NEXT TIME; I WILL DEFINITELY GET MY CREW AND THE PAX OUT AS FAST AND SAFELY AS POSSIBLE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: PIC STATED THAT HE HAD HIS HANDS FULL DURING THIS INCIDENT. TIME CONSTRAINTS WERE SUCH THAT HE AND THE FO BARELY HAD TIME TO COMPLETE THE MEMORY ITEMS ON THE CHKLIST PRIOR TO GETTING TOTALLY CONCENTRATED ON THE APCH AND LNDG. FURTHER CONVERSATION REVEALED THAT THE CAUSE OF THE EVENT WAS THE SOLID STATE MODULE THAT GOVERNS THE AMOUNT OF HEAT TO THE HEATED WINDSHIELDS -- LOW HEAT ON THE GND AND A HIGHER LEVEL IN THE AIR. THIS EVENT OCCURRED AT 400 FT AGL; JUST AFTER THE MODULE HAD SHIFTED GEARS SO TO SPEAK. LATER INSPECTION REVEALED THAT THE SOLDERED SHIELD (SEAL) HAD BEEN BLOWN AWAY BY A PRIMARY INTERNAL ELECTRICAL FAILURE OR SHORT. THE INSIDE '...WAS COOKED....' THE COCKPIT DOOR IS NORMALLY KEPT OPEN PER OPS PROCS AND THE MANUAL. THE LATCH SLIPPED ON TKOF; THEREBY ISOLATING THE PAX. THE PAX HAD BEEN BRIEFED ON NO SMOKING PROCS; BUT NOT ON ACTUAL ACFT SMOKE PROCS SINCE THERE IS NO WAY FOR A MANUAL DEPLOYMENT BY THE PAX. THE PIC FELT THAT THE CABIN CONDITION WAS NOT THREATENED SINCE THE ACFT WAS NOT YET PRESSURIZED; THE SMOKE WAS EMANATING FROM THE COCKPIT AND THE AIR FLOW EXITING THE ACFT WAS A POSITIVE ONE. HE HAD NOT WANTED TO INTRODUCE ANY UNNECESSARY OXYGEN FLOW TO THE REAR BY A MASK DEPLOYMENT OF 8 MASKS FOR 2 PERSONS. ON THE GND; THE PAX APPEARED TO BE FAIRLY TRAUMATIZED. RPTR INFORMED THE COMPANY THROUGH A COMPANY GENERATED FORM; THEY HAVE JUST STARTED A SAFETY INFO PROGRAM. NO FEEDBACK FROM COMPANY. RPTR SAID THAT HIS CONCERN FOR AN IMMEDIATE EVAC WAS GOVERNED BY THE TFC ON FINAL AND HE WANTED TO GET OFF THE RWY ASAP. AS HE WAS TURNING AND STOPPING ON THE TXWY; 2 OTHER LNDG ACFT ON FINAL WERE SENT AROUND. THERE WAS 1 OTHER CASE OF SMOKE ON AN HS-125; BUT THE SOURCE WASN'T KNOWN BY THIS PIC. PARTING ADVICE WAS: DON'T JUST THINK ABOUT ENG FAILURES ON TKOF.