AN L1011 AT FL310 DECLARED AN EMER AND DIVERTED DUE TO LOSS OF ALL HEADING REFERENCE ON L AND R INSTRUMENTS CAUSED BY AN ELECTRICAL FIRE IN THE ALTERNATE SOURCE SELECT PANEL.

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DURING CLB ENRTE ZZZ-XYZ; WITH AUTOPLT ENGAGED; ACFT ABRUPTLY PITCHED OVER AT ABOUT FL240. DISCONNECTED A AUTOPLT AND RESUMED CLB IN HAND FLOWN MANNER. ENGAGED B AUTOPLT AND CONTINUED CLB TO FL310. SHORTLY AFTER LEVEL OFF AT FL 310; ACFT ABRUPTLY PITCHED OVER AGAIN. DISCONNECTED AUTOPLT AND RESUMED HAND FLT; DECIDED TO RETURN TO ZZZ. AT THIS POINT BOTH FLT DIRECTORS FROZE IN LINED-UP POS (STRAIGHT AND LEVEL). ACFT BEGAN TO PORPOISE AND DEVIATION (+ 500 FPM MAX) FROM FL310 OCCURRED. HEADING AT 070; NO MOON; ONLY BLACK OUTSIDE. RECEIVED CLRNC TO RETURN TO ZZZ AND BEGAN L TURN. ALL HEADING (BOTH SIDES) INFO FROZE DURING TURN AT 360. VERTICAL SPEED CTL ON MODE SELECT PANEL BECAME INOP. SO ADVISED OF HEADING BY REFERENCE TO WHISKY COMPASS. DUMPED FUEL WHILE ON WAY BACK TO ZZZ. PERFORMED ALL CHKLISTS. SHOT VISUAL APCH TO RWY 8L ZZZ. ALL INST EITHER INOP (EXCEPT AIRSPEED; AND ALTIMETER) OR GIVING ERRONEOUS DIRECTION. LANDED AND TAXIED TO GATE WITHOUT INCIDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED IN CLB AT FL240 THE ACFT PITCHED OVER WITH 'A' AUTOPLT CONNECTED. THE RPTR SAID 'B' AUTOPLT WAS ENGAGED AND CLB CONTINUED NORMALLY TO FL310 AND LEVEL OFF WHERE THE ACFT PITCHED OVER AGAIN. THE RPTR SAID THE 'B' AUTOPLT WAS DISCONNECTED AND THE ACFT WAS HAND FLOWN. THE RPTR STATED AT THIS TIME ALL HEADING REFERENCE INDICATIONS WERE LOST; FLT DIRECTOR BARS FROZE AND ACFT BEGAN TO PORPOISE 500 FT UP AND DOWN. THE RPTR SAID AN EMER WAS DECLARED AND A TURN WAS MADE USING THE STANDBY COMPASS. THE RPTR SAID MAINT FOUND THE CAPTAIN'S ALTERNATE SOURCE SELECT PANEL HAD AN INTERNAL ELECTRICAL FIRE WHICH RENDERED THE HEADING REFERENCE DATA INOPERATIVE. THE RPTR STATED AT NO TIME WAS SMOKE; FLAMES OR FUMES DETECTED; AND NO CIRCUIT BREAKERS TRIPPED.