It takes four AA-size 500ma nicads for a total of 6V.
A clever trick if you can do it. NiCds have a terminal voltage of around
1.2-1.25V per cell.
4 NiCds are often used as a 5V supply (with no other regulator). This
machine might depend on this. If it really needs 6V, then you need 5
cells (my Panasonic HHC uses 5 cells followed by a discrete-component
low-drop-out regualtor circuit).
And don't you mean 500mAh NiCds?
-tony