Try measure the VDD pin of the RTC chip when the machine is on (need to open the 600 case - be carefull to not break the back grips) and measure the VDD in the battery + pin and in the RTC chip ones.
And pull out the diode 912 (the echivalent one of A501) and measure it both ways with contuinity test. In one hand will be measure 0.3 or 0.7 (ok) in other they will looks like open.
Is not hard this diode fails (open or shorts) with some short-circuit like battery leaks does.
