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what do I need for the "vintage series" HP35,45,55,65
AC or DC output ? Voltage? max current?

(Of course I could buy an original charger on eBay such as the 82002A I guess, but I have almost 20 different chargers for old dead cellphones, so I want to save me that expense.)

The charger for these machines has 2 outputs (with a common -ve connection on the centre pin).

One output is 4.2V regulated, DC. This is used to run the calculator logic when the charger is connected.

The other output is constant _current_, 55mA with an open-circuit voltage of about 15V. Note this is not the same as a 15V adapter, it must be constant current since it's directly connected to the NiCd battery when the charger is connected. This is used, of course, to charge the battery.

On the 65 and 67, the card reader sense amplifier chip is connected to the battery connections. You need to take care not to damage that if you use a non-standard charger. This chip is HP-custom, and the one in the 65 is supposed to be more fragile (electrically) than the one in the 67 and later machines.

There's an ASCII-art schematic of the HP charger over in the Articles Forum. And a hand-drawn one on the HPCC schematics CD-ROM (at the 'back' of every classic-series schematic IIRC). If you want to build something, that would be a good starting point. The transistors are all pretty generic types, nothing critical.

But I doubt any other charger will work without damage!