The different of 2 kind 82120A battery pac

[Chiu Yiu Wai]

I have 2 nos. 82120A battery pac , one have 2 plug feet , one have 3 plug feet . Do anyone can tell me other than the number of feet , do they have any different?

[Randy]

While they appear much the same, there are many internal differences.

The original 3 pin 82120A, when attached to the charger, isolates the calculator from the battery and supplies a separately regulated +5 volt supply to the calculator. The later 2 pin version has the battery connected at all times and uses a different charging supply circuit.

The 3 pin version can, in rare cases, cause a memory loss in the calculator when the charger is plugged in and the supply to the calculator is interrupted.

Whenever I rebuild a 3 pin version with new cells, I convert it to the 2 pin style by eliminating the flex circuit which is needed for the +5 connection, remove the +5 78L05 regulator and add a 6.2 volt zener across the battery. The charging current is a bit lower but in the scheme of things, it doesn't matter.