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Joined: Apr 2006
The battery charger for the HP-65 is the same as that used for the HP-35,45,55,65,67,70, and 80. They are fairly common.
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Joined: Aug 2005
They're not that rare (at least the one for the 'classic' battery pack is not rare) -- I've managed to get 3 of them without really trying (I am _NOT_ a collector).
One interesting thing, though. Take it apart (4 screws on the bottom and carefully separate the case). Inside are 4 components -- 2 resistors, a transistor and an LED. It works like this : The classic series mains adapter has a constant-current output. This is connected to the battery pack (to charge it) with one of the resistors in series. The base-emitter junction of the transistor is connected across that resistor, so that when the battery is in place and charging, the transistor turns on. The LED and other resistor are in series between the collector of the transistor and the
constant voltage output of the adapter (the line that would supply the calculator logic normally). So that when the transistor turns on, the LED lights up.
So far, so good, it's all what you'd expect. The odd part, though, is the construction. There is no PCB. Nor even a tagstrip. The transistor is placed in a cylindrical post moulded in the base of the housing. The LED fits over a plastic riddge with one lead each side. And the whole thing is assembled by just soldering the component wires together. It's a 'birds nest' of components -- the sort of thing you'd find
in an experimental setup but not in a production device, normally. These HP 'reserve power packs' must have been hand-built -- I can't think of any machine available at the time
that could wire like that. I would have thought it would have been cheaper to use a PCB, though.