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Old 2nd Aug 2021, 2:22 pm   #11
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 4,297
Default Re: Old calculators.

There was a chap in the States who made replacement battery packs for these calculators. I have no idea if he still does, but looking on E-bay may be worthwhile.

Alternatively you can put new cells into an old pack. It's just 3 NiCd cells in series. I probably have a spare housing and contact strips somewhere, the problem would be finding them.

Originally (HP35) the 3 pin power connector was wired so that the middle pin was logic ground, one outside pin was +4V or so to the logic, the other went to the +ve side of the battery. A spring contact connected the outside 2 pins when the charger wasn't plugged in so that the battery powered the calculator. The charger contained a voltage regulator to run the logic and a constant current supply to charge the battery.

In the models with card readers -- HP65 and HP67, it's much the same but the card reader IC runs directly off the battery. As a result these machines will not handle cards if there isn't a good battery fitted. In fact, in the case of the HP65 the card reader chip can be damaged if you plug the charger in without a battery (or with an open-circuit battery) in the calculator, it can't stand the 15V or so open-circuit voltage of the constant current side of the charger. Note that this applies even if the calculator is switched off! The card reader chip was modified to withstand this voltage, and HP67 machines should be safe, but it's not a good idea to risk it. Some people like to charge the battery outside the calculator as a result, there was even a little HP battery holder unit ('Reserve Power Pack') that connected to the normal charger and took a battery pack.

It is not clear what you are missing. The 3 pins on the calculator? The socket on the end of the charger lead? Or the complete charger unit?

Mains chargers do turn up from time to time, they are not that rare. The 12V charger to power the machine in a car is a lot rarer. So is that battery holder thing I mentioned.
TonyDuell is offline   Reply With Quote