Teardown With A Twist: 1975 Sinclair Scientific Calculator

When writing a recent piece about Reverse Polish Notation, or RPN, as a hook for my writing I retrieved my Sinclair Scientific calculator from storage. This was an important model in the genesis of the scientific calculator, not for being either a trailblazer or even for being especially good, but for the interesting manner of its operation and that it was one of the first scientific calculators at an affordable price.

I bought the calculator in a 1980s rummage sale, bodged its broken battery clip to bring it to life, and had it on my bench for a few years. Even in the early 1990s (and even if you didn’t use it), having a retro calculator on your bench gave you a bit of street cred. But then as life moved around me it went into that storage box, and until the RPN article that’s where it stayed. Finding it was a significant task, to locate something about the size of a candy bar in the storage box it had inhabited for two decades, among a slightly chaotic brace of shelves full of similar boxes.

The Sinclair's clean design still looks good four decades later.
The Sinclair’s clean design still looks good four decades later.

Looking at it though as an adult, it becomes obvious that this is an interesting machine in its own right, and one that deserves a closer examination. What follows will not be the only teardown of a Sinclair Scientific on the web, after all nobody could match [Ken Shirriff]’s examination of the internals of its chip, but it should provide an insight into the calculator’s construction, and plenty of satisfying pictures for lovers of 1970s consumer electronics.

The Sinclair is protected by a rigid black plastic case, meaning that it has survived the decades well. On the inside of the case is a crib sheet for its RPN syntax and scientific functions, an invaluable aid when it comes to performing any calculations.

It shares the same external design as the earlier Sinclair Cambridge, a more humble arithmetic calculator, but where the Cambridge’s plastic is black, on the Scientific it is white. The LED display sits behind a purple-tinted window, and the blue-and-black keyboard occupies the lower two-thirds of the front panel. At 50 x 111 x 16 mm it is a true pocket calculator, with an elegance many of its contemporaries failed to achieve and which is certainly not matched by most recent calculators. Good industrial design does not age, and while the Sinclair’s design makes it visibly a product of the early 1970s space-age aesthetic it is nevertheless an attractive item in its own right.

The Sinclair Scientific circuit board, component side.
The Sinclair Scientific circuit board, component side.

On the back there is only the clip-on cover for the battery compartment, which lies under the keyboard. Opening it reveals the underside of the keyboard PCB with a label showing battery orientation  and the springy battery clips for a set of 4 AAA cells. One of the clips had been corroded by a leaking cell and had snapped when I got the calculator, necessitating a piece of copper foil between cell and clip for operation. The label recommends Duracell AAAs, which in the mid 1970s were not cheap. At rest the calculator draws 35 mA from them, so an owner without deep pockets would have had to ensure to turn it off immediately after calculations.

The upper and lower halves of the case are held together with moulded clips, meaning that with care they can be pried apart to reveal the circuitry. On this example one of the clips has been broken, sadly I can’t remember if this was caused by an enthusiastic but inept younger me. Once the rear panel has been removed, the 28-pin dual-in-line package of the TMS0805 calculator chip with its March 1975 date code comes into view, plus the two driver chips and the Bowmar LED display module. This last component is mounted flush with the PCB in a recess milled into it. There are a handful of discrete components including an inductor and a set of diodes, probably a simple inverter to generate a power rail. Owners of later Sinclair computers may find these inverter circuits to be familiar.

The Sinclair Scientific, keyboard side.
The Sinclair Scientific, keyboard side.

The TMS0805 is at the root of what makes this calculator an interesting piece, being a chip designed for simpler arithmetic calculators on which Sinclair famously managed to place code for a scientific one thanks to a brilliant member of staff. Its 28-pin dual-in-line package seems huge by the standards of 2017, but Sinclair have done a very good job of packing it and its ancillaries into as small a space as possible. Judging by its flowing lines this is clearly a hand-laid-out board.

The rest of the photographs of this calculator teardown are in the gallery below, and should provide plenty of fodder for the vintage calculator enthusiast. However this is not the end of the story of my Sinclair Scientific, because it comes with one non-standard feature, the twist mentioned in the article title. Its first owner personalised it by scratching his signature on its back, so I was able to track him down and ask someone who used one of these machines what it was like.

Turn it over, and there's the signature.
Turn it over, and there’s the signature.

It must be an odd experience to have a random stranger email you out of the blue about a calculator you owned over four decades ago, but Emeritus Professor [John Stradling] of Oxford University’s Nuffield Department of Medicine took it in his stride. No mystery then how it found its way to me as an Oxford-based 1980s teenager.

Professor [Stradling] has now retired from his medical career, but is still an active medical researcher and scientist. He bought the Sinclair as a junior doctor, and told me that it was very much the ‘must have’ accessory at the time as there were a lot of calculations to be made with respect to drugs, or fluids. He revealed that maths is not necessarily the strong point of many doctors through the education system selecting students who study biology instead, and the advent of calculators like the Sinclair was something of a boon to them.

We take a calculator for granted today, it’s an app on a smartphone or if it’s a physical device it’s a lightweight and slim machine with functions way in excess of those on the Sinclair, that runs on next-to-no power. It’s therefore interesting to have a glimpse into where scientific calculators originated, and to have a first-hand account of what it meant to own one in an age of slide rules. The Sinclair Scientific wasn’t the first pocket scientific calculator, nor was it the best of its era. But the story of its development using a chip never intended for the job is a fascinating one, a true hack if ever we saw one. And while today it wouldn’t be the machine you’d reach for from choice, it’s still one with a pleasing aesthetic in an extremely compact form factor. You’d probably be lucky to find one in a rummage sale today, but if one comes your way, snap it up. Meanwhile, enjoy our gallery of the Sinclair Scientific’s internals.

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