You have to replace a chip that contains a battery. But, it is socketed, so you can do it yourself.
Getting the part is a little harder – since they are obsolete, we do not carry them. (Note that the 4000 is 20 years old at this time).
Happily, they are available on the surplus market, from some distributors, and Amazon lists them as well. You will need a BQ4013YMA-120. These are normally around $40 each (avoid cheap ones). Digikey has DS1245Y-120+ at $40, which will also work.
To install, disconnect the power and remove the top of the unit. It should be obvious what you have to do (carefully). Avoid static damage and be careful not to bend the pins.
If you don't want to do it yourself, I'll get hold of some and we can change it for the normal service charge. I would not be keen to put them on the Store, cos they would be expensive and easily damaged.
