HP 67: charging of battery pack

[BobVA]

In this interesting thread Tony Duell states:

Quote:It is _NOT_ safe to operate a 65 or 67 from the AC adapter without a good battery installed.

On these machines, the chips on the CPU board run off the regulated voltage output of the charger (as in all other classics) but the card reader sense amplifier chip runs off the battery pack directly (in order to supply enough current to run the motor) and if the battery pack is open-circuit or missing, this chip can be damaged, since the voltage across this chip then rises to about 16V

The 65 user manual contains an explicit warning about this. The chip was re-designed for the 67 (the new version is used in the 97 and 41 card readers too), and the new version is claimed to be able to withstand the charging voltage, but I don't like to risk it. It's an HP custom chip, and spares are not trivial to obtain. I charge the batteries either in a classic without a card reader, or in a 'reserve power pack'

(That thread has some other cool details, too.)

The HP-67 manual I've got (later printing, maybe?) also has a warning about not operating the calculator without a battery pack.