A C550 FLT CREW DEVIATED FROM THEIR ASSIGNED ALT.

Narrative

WHILE IN LEVEL CRUISE AT FL390 AND DEVIATING AROUND TSTM; I (CAPT) NOTED A FLICKER ON MY EFIS DISPLAY AND THEN ALL WAS NORMAL. I HAD BEEN DEVIATING AROUND THE STORM USING THE HDG BUG. I FIRST NOTED THAT THE ACFT WAS NOT RESPONDING TO MY HDG COMMANDS. THEN I NOTICED THE ALT WAS NOT HOLDING AND DRIFTING UP. I ARRESTED THE ASCENT AND WAS ABLE TO LIMIT IT TO 260 FT PER MY ALTIMETER. ONCE BACK ON ALT; WE (THE CREW) LOOKED FOR THE CAUSE. IT SEEMS WHEN THE FO ADJUSTED THE DIM CTL ON THE RADAR HE RELOCATED THE SYMBOL GENERATOR SELECTOR AS WELL. (THE SYMBOL GENERATOR CTL KNOB IS CO-LOCATED WITH THE RADAR DIM CTL.) WHEN THE SYMBOL GENERATOR IS MOVED FROM THE 'BOTH' SELECTION (AS HE DID MOMENTARILY); IT CAUSES THE AUTOPLT TO FOLLOW THE SELECTION AND BECAUSE THE FO'S ALT HOLD WAS NOT SELECTED; THE ACFT HAD NO ALT HOLD COMMAND FROM THE AUTOPLT. I FEEL THAT IT IS A POOR DESIGN TO CO-LOCATE THESE CTLS ON THE MFD CTL PANEL. BUT GIVEN THAT; THE FLT DIRECTOR CTLS OF BOTH #1 AND #2 NEED TO BE OPERATED IN SYNCHRONIZED FASHION TO PREVENT DISCREPANCIES IN THE EVENT THE AUTOPLT REVERTS TO PNF SIDE. NO COMMENT WAS RECEIVED FROM ATC AND THE DISCREPANCY WAS LESS THAN 300 FT; THEREFORE; A HEIGHT KEEPING ERROR RPT HAS NOT BEEN FILED.