A B757-200 FLT CREW ON CLB OUT EXPERIENCED A SMOKE AND FUMES IN THE COCKPIT. THEY DECLARED AN EMER AND RETURNED TO LAND AT DEP ARPT.

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DURING CLB OUT; I NOTICED MY CDU DISPLAY WAS DISTORTED. I POINTED IT OUT TO THE CAPT. I CHKED THE CDU BOX TO SEE IF IT WAS HOT AND IT WAS NOT. AT ABOUT 14 TO 16 MINS INTO THE FLT; LEVEL AT AN INTERMEDIATE CLB OUT ALT OF FL260; BOTH THE CAPT AND MYSELF NOTICED A STRONG ELECTRICAL BURNING SMELL. THAT STARTED TO INTENSIFY. THE CAPT PUT HIS O2 MASK ON AND I GOT OUT OF MY SEAT AND PULLED THE CDU CIRCUIT BREAKERS ON THE OVERHEAD AND THEN GOT IN MY SEAT AND PUT ON MY O2 MASK. THE CAPT COORDINATING A TURN BACK TO THE FIELD. I STARTED TO FLY THE ACFT BACK TO THE FIELD AND THE CAPT DID THE 'SMOKE/FUME/ODOR' CHKLIST. I DECLARED AN EMER AND ADVISED ATC TO HAVE EMER EQUIPMENT MEET THE FLT. THE CAPT CALLED THE FIRST FLT ATTENDANT AND ADVISED HIM OF THE SIT. I FLEW THE ACFT TO THE INTXN AND TRANSFERRED THE CTLS TO THE CAPT AND HE MADE AN UNEVENTFUL OVERWEIGHT LNDG AT 208;000 LBS. WE TURNED OFF THE RWY AND THE FIRE TRUCKS DID AN EXTERIOR INSPECTION OF THE ACFT NOTING NOTHING. WE TAXIED TO THE GATE AND PARKED.