Posts: 883
Threads: 17
Joined: Feb 2006
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.
