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Amongst other lockdown-induced quirks, I've discovered a soft-spot for the early-to-mid 1990s Casio fx calculators -- in particular the thin, light dual-source 'C-power' models. There's something so clean and taut about their aesthetics and layout. My latest acquisition is an fx-115N; I got this one in pretty good condition, together with its manual and vinyl pouch.

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A previous owner had name-tagged it on the reverse with a marker so long ago that no amount of isopropyl would get rid of it, though the back being black it doesn't show much. The red stain on the keypad bezel similarly won't shift, likely because the dye has stuck deep into the soft plastic laminate on top.

It wouldn't work in the dark, so a dead battery was suspected. Time for a service. There are 6 cross-head screws on the back to undo, and the calculator can be disassembled into four main parts.

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I love the elegant, minimal design of these fxs. From the left is the front cover with the keypad bezel and display window. I cleaned this with a wet-wipe. The window is made of a soft plastic and is often scratched and scuffed by now; normally I'd neaten it up with some plastic watch lens polish, but this example wasn't too bad. Next we have the membrane keypad, which I just cleaned in cold water with a brush and left to air-dry. The circuit 'board', such as it is, is a transparent plastic sheet printed with graphite paint. There's the solar cell, a few diodes, the SoC, the LCD, and that's all she wrote. Finally we have the back cover with some dead space at the top to incline the calculator on a desk, and an array of recessed conductive pads. Hence the flexible circuitry: key-presses from the membrane distort the clear plastic onto the pads.

Now the flexi-graphite approach has a small flaw, and I've seen this all the fxs of this construction that I've had apart. The battery contact on the back cover wears out the graphite on the corresponding pad.

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This is easily fixed by touching in the affected area with some graphite paint and a toothpick. I also did the round adjacent battery contact. The paint I'm using needs to be cured under a moderate heat to reach good conductivity; a couple of hours close under a halogen bulb does the trick.

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Here are the cured pads.

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With the cured paint checked with a multimeter, I cleaned the conductive pads on the back cover with isopropyl, put in a new battery, and re-assembled the calculator. Correct operation was verified, including with the solar cell obstructed. Now it's ready to take its place in the collection.
The nearly-identical fx-115V was my first scientific calculator. I still have it, and I put in a fresh battery last year.
I am amazed at how long Casio has been using the fx-115 (and fix-991) name. I recently bought a fx-991EX Classwiz as an upgrade for my fx-115ES Plus.
This is a nice one!
I have the fx-3600pv which i think is almost the same machine but with some programming
capabilities.
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