Mira lo que he encontrado, lo mismo te da una pista.

Hi,

No batteries inside; there shouldn't be. Instead there is a 'Gold-Cap' which is the normal way of retaining the computer-memory at power-off.
Next to the microprocessor IC701 (PCB3, Front-Panel) and connected to it's pins 72/73 (Vdd) is a capacitor C712 (0.047F/5.5V). Notice: This is Farad, NOT MICROFarad.
Replace it with anything available like 0.047F, 0.1F, 0.22F etc. The higher capacity the longer time memory is retained.
In parallel with C712 is a 22nF cap. C720 and a 47uF/10V cap. C713.
I suppose leakage in either would drastically lower the back-up time. Experience still comes down to C712 being the culprit.

rgds,

Saludos