Categorized | repair

My calculator was afraid of the dark

For me calculators are like pens… many have passed through my hands, but my home is like an airport to them – they arrive, spend some time, then vanish during the middle of the night to destinations unknown. Especially that beautiful Hewlett-Packard 48G… come back! Yesterday after rummaging around for some graph paper my old accounting calculator appeared, blinking his/her display as it was exposed to the light for the first time in who knows how long.


It was alive! Finally something useful has returned home to the world of the useful. Well sort of… in the dark it shut down. *sigh* But in the spirit of repairing instead of replacing, this wouldn’t beat me. Armed with some miniature screwdrivers and a small box to hold all those screws in, I leapt in and started removing every screw in sight.

First I removed the rubber feet, as they are well-known hiding spots for crucial screws and rivets… but nothing there.


At this stage the screws looked like heading for the departure gates, so they went into the screw zoo…


Finally the cover came off, revealing some nice PCB work. Curiosity got the better of me and I kept unscrewing..


But at this stage, perhaps my enthusiasm had taken me too far… this looks easy to break and difficult to put back correctly!


Whoops… looks like things have gone a little out of hand. If this doesn’t go back together properly, the whole exercise and calculator has been a waste.

But there is something that intrigues me, two diodes, and something that looks like a cell or capacitor.



I was hoping it would be a capacitor that was charged by the solar cell, that would give me an excuse to use the SMD 0.2 Farad capacitors that I ordered by accident from element14 last month. (Never order components at three in the morning)

But no, it looked like a typical button cell. Time to give it a health check:


(Yes, I know. Cheap multimeter) So the cell measures 0.66 volts DC. But what should it be? Only one way to find out…


Sigh. So much trouble caused by such a small thing. (Note to Casio: I know you want to sell more calculators to lazy people who cannot disassemble them to replace the battery – but really, you couldn’t have done a better job in hiding this one). The cell is a GR927, and not listed in the Panasonic Australia or US web site. Argh. It is probably an OEM product, as it isn’t listed in element14 or RS. So back to Google. Aha! It’s a “399 silver oxide battery” – 1.55V @ 60 mAh. Off to the centre of retail madness…

Ok, I’m back! That was quick. :)

And the saga is over. The calculator can now return to my desktop, adding up those material costs and how much money that isn’t around. I keep the cell in my dead battery box, one day we will have a disposal centre for them…


So there you have it people – don’t throw something out for a simple fault. Spend a few moments to investigate and repair. You might have lost half an hour, but you saved money, a tiny portion of the environment, and gained the satisfaction of doing it yourself.

In the meanwhile have fun and keep checking into Why not follow things on twitterGoogle+, subscribe  for email updates or RSS using the links on the right-hand column? And join our friendly Google Group – dedicated to the projects and related items on this website. Sign up – it’s free, helpful to each other –  and we can all learn something.

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John Boxall

Founder, owner and managing editor of

One Response to “My calculator was afraid of the dark”

  1. Wes says:

    Nice story John! Good lesson in this age of “Toss it!”.

    AE6ZM & VE7ELE


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