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Citizen SLD-721

Now here's a funky little calculator from goodness-knows-when. The Citizen SLD-721 was made in Malaysia and it has some tricks, as well as mysteries, up its sleeve.

top view of the Citizen SLD-721

To begin with, it has what it calls "2 power": it is powered by a photovoltaic cell as well as a single coin cell battery. Cool - let's crack this thing open and insert a battery!

the back of the Citizen SLD-721 with the back panel removed

We have removed the back panel and along with an unlabeled chip (a sure mark of quality) we see an unhelpfully unmarked battery well. After some measuring I find that it takes an LR43 battery.

Now, before we put that battery in, let's have a look at what's under the hood. We're still looking for the production date, after all.

I was hoping (though not hopeful) that we'd find a date under the back cover - no such luck. We'll have to take a look at the underside of the motherboard, which is being held into place with two screws and seven melted posts. Thanks Citizen, very cool!

After cutting the posts and unscrewing the screws we can flip over the board and find... nothing conclusive.

the obverse side of the main board of the Citizen SLD-721

There's some text along the top of the board: s2 10SB806DMB 02( 330. Along the bottom, the board reads 9040. The 40th week of 1990? That would match the slim form factor, the gray LCD and the solar cell, all of which were cheaply available by 1990.

We're feeling confident, but to close the book on this thing, let's have a closer look at the sticker on the back cover: closeup of the back sticker of the Citizen-SLD-721. It reads CITIZEN LDF DISPLAY CALCULATOR / TYPE: SLD-721 / Japan SBM Corporation / T.M. LICENSED BY CITIZEN WATCH CO., LTD. TOKYO, JAPAN / MADE IN MALAYSIA FABRIQUE AU MALAISIE / s/no 901003

That serial number - 901003. Serial numbers very rarely represent a pure count of produced items - they're coded, like electrical components. I'd wager that the 90 at the start of this serial number represents the year it was built. The other numbers are a toss-up; however, if we keep in mind that the 40th week of 1990 started on 1 October, we may become tempted to declare that serial number a date sequence and assign a date of birth of 3 October 1990, the day of German reunification. Now, that may be a bit too bold - however, our more conservative determination stands. That 90 matching the 90 on the inside of the device is too much of a coincidence, so we're calling it. 1990!

Phew, that was a bit of a journey. Let's get on with the features of this little box of mysteries.

bottom view of the Citizen SLD-721

Screws are back in, the thing is back together at last.

The Citizen SLD-721, made by Citizen's parent company Japan CBM (Citizen Business Machines) Corporation, is 125mm long, 70mm wide, and it's 7mm thick, a fact it tries to hide by scalloping the back panels. It has that "2 power" feature, which may have you thinking you can run the device in the dark; not so. The device only runs on the photovoltaic cell, but the battery powers its memory. So if a cloud passes over and you lose functionality, the device will remember what you'd been doing, and any values committed to memory will remain. Snazzy, and something I've not seen in any other calculators.

As is common with solar-powered calculators, the Citizen SLD-721 does not have an off switch, only an on switch. It would likely have come with a case, or it would just have relied on set periods of idleness to turn itself off. I've made a little belt from electrical tape that I stretch over the solar cell to keep it from turning itself on.

But the double power feature is only one of two marvels that this thing has introduced in our lives:

animation of the Citizen SLD-721 flipping its screen up and down

Flipping screens are a fairly common feature in larger calculators, like the ones used in shops, but I've not seen any in pocket-sized specimens such as this one. A very supple hinge that has just enough resistance to hold the screen up in any angle you like. It bends to about 54° for those of you playing along at home.

It's cheap and cheerful, this Citizen pocket calculator, but it gets the job done and has some fun features that make it stand out. A good start to the 1990s.

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