A FALCON 50 FLT CREW DEVIATED FROM ASSIGNED ALT WHEN THE AUTOPLT DISCONNECTED AND THE ACFT BEGAN TO CLB.

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WE WERE IN NORMAL CRUISE AT FL370 ON A NE COURSE OPERATING IN THE AML VOR AREA ON OUR WAY TO SYR. THE AFTERNOON WINTER SUN WAS SHINNING THROUGH THE CAPT'S L SIDE WINDOW ON THE ANNUNCIATOR PANEL. I NOTICED WE HAD STARTED CLBING AND POINTED IT OUT TO THE CAPT. BY THE TIME HE RECOVERED WE HAD GAINED APPROX 400 FT AND RECEIVED A TA ADVISING THERE WAS TFC 1200 FT ABOVE US. WE DSNDED BACK DOWN TO FL370 WITH NO COMMENT FROM ZDC. THE AUTOPLT DISCONNECTING CAUSED THE ALTDEV. THIS FALCON 50 HAS AN OLD STYLE AUTOPLT THAT DOES NOT HAVE AN AUDIBLE WARNING. IN ADDITION; THE AUTOPLT OFF LIGHT ON THE ANNUNCIATOR PANEL ILLUMINATES FOR ABOUT 1 SECOND WHEN THE AUTOPLT DISCONNECTS. ALSO; THE SUN WAS SHINING DIRECTLY ON THE ANNUNCIATOR PANEL SO WE COULD NOT SEE THE AUTOPLT ANNUNCIATOR LIGHT ILLUMINATE. IF THIS ACFT HAD AN AUDIBLE WARNING FOR AUTOPLT DISCONNECT THIS DEV WOULD NOT HAVE HAPPENED. ALSO IT IS THE CREW'S JOB TO ALWAYS KNOW THE STATUS OF THE ACFT SYS. IF I WAS FOCUSING ON BASIC FLT INFO CORRECTIVE ACTION COULD HAVE BEEN TAKEN QUICKER. THE CAPT COULD HAVE USED A MORE AGGRESSIVE CORRECTION ACTION AS WELL.