AN MD80 ON LEVELOFF AT 9000 FT. THE ACFT YAWED R AND YAW DAMPER MACH TRIM ANNUNCIATORS ILLUMINATED; CAUSED BY A FAILED DFGS 'A' SYS.

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CLRED TO 9000 FT MSL; FO FLYING AND CAPT ON THE RADIOS. JUST AS THE ACFT CAPTURED 9000 FT AND PF LEVELED OFF THE ACFT YAWED APPROX 7 DEG NOSE R. AS THE YAW OCCURRED THE AUTOTHROTTLES QUIT AND THE 'MACH TRIM COMPARATOR' AND 'YAW DAMPER OFF' ANNUNCIATORS ILLUMINATED. ADDITIONALLY THE DIGITAL FLT GUIDANCE PANEL ALT WINDOW BLANKED OUT. BOTH PLTS QUICKLY SCANNED THE ENGS AS IT FELT LIKE AN ENG FAILURE. DURING THIS TIME THE ACFT WENT UP TO APPROX 9150 FT ON THE ALTIMETER OF THE PF. AT VIRTUALLY THE SAME EXACT MOMENT; RIC DEP CALLED AND SAID THEY WANTED US AT 9000 FT. THE CAPT RESPONDED 'WE ARE DSNDING TO 9000 FT.' RIC DEP CALLED BACK AND ASKED WHY WE WERE NOT AT 9000 FT. THE CAPT RESPONDED 'WE HAVE AN EQUIP FAILURE AND ARE DSNDING. WE WILL GET BACK TO YOU.' THE ALT WAS NOTED AS WAS THE FACT THAT THE YAW STOPPED. THE CAPT THEN CALLED BACK AND RPTED THE ALTDEV NOTED ABOVE. SUBSEQUENT INVESTIGATION REVEALED CIRCUIT BREAKER C17 (28V AC DIGITAL FLT GUIDANCE SWITCH A) HAD POPPED. APPROPRIATE ENRTE MAINT ADVICE WAS RECEIVED VIA PHONE PATCH WITH MAINT TECHNICIAN AND A MAINT LOG ENTRY WAS MADE. ON ARR IN ZZZ; THE MECH WHO MET THE PLANE FOUND THE ONBOARD DFGS TEST PANEL SHOWED THE SYS HAD FAULTS.