I've lost track of the exact sequence of events, but at various points in time the backup battery in the PK-232 was both a leaded coin cell and a holder for three AA cells. I think the original PK-232 had no battery at all, but I am not certain. Obviously the three AA cells give you a nominal 4.5V, but I don't think I have ever seen a coin cell above 3 volts. I am currently restoring a PK-232 that has the coin cell installed, but it is dead. It is wired through a diode into the 5 volt rail used to power the non-volatile memory. Was there a special coin cell for this purpose, or did they get away with using a standard 3V unit?