Finally acquired a Woodstock

[Chasfield]

I have adapted the HP21's battery compartment to use a pair of AAAs. The original battery carrier would take AAs through its partially detached base plate but the latter was left flapping in the breeze (the PO cured this with a strip of tape) and battery swapping was a bit fiddly. The other issue is whether or not the 3.2 volts delivered by fresh alkalines is too hot for the 2.5 volt calculator circuit. I could find no certain answer to this question, so I installed a cut down, three-cell AAA battery carrier and used the spare space to add 2.8 ohms of series resistance that can be shunted out with a slide switch when the batteries have lost their edge. This resistor arrangement shaves ~ 0.3 to 0.5 volts off a pair of brand new cells, depending on current draw. Once the the calculator shows a low battery condition the resistor can be switched out to let the batteries run down to their normal low voltage state. This may be a solution looking for a problem but it doesn't hurt anything and will marginally extend total battery life.

Beware of very cheap battery holders. Mine has crimped connectors which lost crimp pressure when I got too near their supporting plastic with the soldering iron. I had to jumper an unexpected open circuit with copper wire.

With the exception of a 3 mm hole in the battery compartment cover, the mod is reversible - as long as I can lever off the epoxied in retaining screw boss. The battery carrier is located by a couple of foam sticky tabs.