Raytheon Chartplotter 420 - No FIX
I had this problem and fixed it by replacing the small 'watch' battery (CR2032) in the unit.
Remove the two screws to remove the threaded base. Remove the six small screws and pry the unit apart. The battery is 'stuck' to its electrodes so use a blade and luck to get the old one out without causing damage.Hold the electrode with a needle-nose so that they don't break off.