A B747-100 IN CRUISE AT FL290 DECLARED AN EMER AND DIVERTED DUE TO SMOKE IN THE COCKPIT CAUSED BY AN OVERHEATED MASTER DIM AND TEST CIRCUIT BOARD.

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FIRST WE SMELLED AN UNUSUAL ODOR; THEN WE SAW SMOKE IN THE COCKPIT. WE STARTED SMOKE SOURCE IDENT CHKLIST. WE DONNED OXYGEN MASKS AND GOGGLES. WE TOLD ATC OUR PROB AND ASKED FOR A HDG TO DTW AND DSCNT CLRNC. I TOLD ATC WE INTENDED TO LAND AT DTW AND DUMP FUEL ON THE WAY THERE. THEY ASKED WHAT ASSISTANCE WE WANTED. I TOLD THEM TO ROLL EVERYTHING. WE DECLARED AN EMER. WE TOLD THEM WE WOULD EXCEED 250 KIAS BELOW 10000 FT DUE TO THE EMER. WE LANDED OVERWT 648000 LBS (MAX LNDG GROSS WT 585000 LBS); TURNED OFF THE RWY AT RWY 13; STOPPED AND SHUT DOWN. COM IN THE COCKPIT WERE AWKWARD; SINCE WE NEVER USE MASKS IN THE SIMULATOR. TIME FROM THE DISCOVERY OF SMOKE TO LNDG WAS ABOUT 10 MINS. THE PROB TURNED OUT TO HAVE BEEN CAUSED BY AN ELECTRICAL COMPONENT IN THE COCKPIT. A MECH TOLD ME THAT THIS PARTICULAR CARD IN THE MASTER DIM AND TEST BOX IS ALWAYS HOT AND ALL OF THESE CARDS ARE BLACK DUE TO THE CONSTANT HEAT. SOUNDS TO ME LIKE THIS MIGHT RECUR. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE CIRCUIT BOARD REMOVED WAS DISCOLORED DUE TO HEATING OVER A LONG PERIOD OF TIME. THE RPTR SAID THE CIRCUIT BOARD WAS REPLACED AND CHKED OK.