Build A Calculator, 1974 Style

Last month we touched upon the world of 1970s calculators with a teardown of a vintage Sinclair, and in the follow-up were sent an interesting link: a review of a classic Sinclair calculator kit from [John Boxall]. It’s a few years old now, from 2013, but since it passed us by at the time and there was clearly some interest in our recent teardown, it’s presented here for your interest.

It seems odd in 2017 that a calculator might be sold as a kit, but when you consider that in the early 1970s it would have represented an extremely expensive luxury purchase it makes some sense that electronics enthusiasts who were handy with a soldering iron might consider the cost saving of self-assembly to be worthwhile. The £24.95 price tag sounds pretty reasonable but translates to nearly £245 ($320) in today’s terms so was hardly cheap. The calculator in question is a Sinclair Cambridge, the arithmetic-only predecessor to the Sinclair Scientific we tore down, and judging by the date code on its display driver chip it dates from September 1974.

As a rare eBay find that had sat in storage for so long it was clear that some of the parts had suffered a little during the intervening years. The discrete components were replaced with modern equivalents, including a missing 1N914 diode, and the display was secured in its flush-fitting well in the board with wire links. The General Instrument calculator chip differs from the Texas Instruments part used in the Scientific, but otherwise the two calculators share many similarities. A full set of the notoriously fragile Sinclair battery clips are in place, with luck they’ll resist the urge to snap. A particularly neat touch is the inclusion of a length of solder and some solder wick, what seems straightforward to eyes used to surface-mount must have been impossibly fiddly to those brought up soldering tube bases.

The build raises an interesting question: is it sacrilege to take a rare survivor like this kit, and assemble it? Would you do it? We’d hesitate, maybe. But having done so it makes for a fascinating extra look at a Sinclair Cambridge, so is definitely worth a read. If you want to see the calculator in action he’s posted a video which we’ve put below the break, and if you need more detail including full-resolution pictures of the kit manual, he’s put up a Flickr gallery.

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