Retro Teardown: The Sinclair 300 Calculator

September 15, 2016 by Robin Mitchell

Calculators are always taken for granted, especially in the age of smartphones. Today we will look at a calculator from 1975 (40 years old) which was one of Clive Sinclair's fantastic, innovative products!

Calculators are always taken for granted, especially in the age of smartphones. Today we will look at a calculator from 1975 (40 years old) which was one of Clive Sinclair's fantastic, innovative products!

A Calculated History

Clive Sinclair is a famous British inventor who was responsible for some of the most iconic electronics during the 70’s and 80’s. These inventions include the Sinclair C5, the ZX Spectrum, portable TVs and calculators. Despite the fact that Sinclair did not invent the first pocket calculator he did invent the smallest slimline calculator by far, the Sinclair Executive.


The Sinclair Executive. Image courtesy of MaltaGC (CC-BY-SA-3.0), via Wikimedia Commons


The company, Sinclair Radionics, was responsible for many of the Sinclair calculators, including the calculator in this teardown.

While these calculators were invaluable in any number crunching environment, they did have their drawbacks. One notable pitfall was the power consumption and resulting weight. The instructions for the Sinclair Oxford 300 actually state that, because the power draw is so large (10’s of mA), it is advised to use a wall wart to power the calculator or use the large heavy 9V PP3 batteries.

But despite this, they represented innovative ideas, especially with the single chip design and the display which used incredibly tiny 7-segment displays to reduce production costs. To counteract the size of the display, a plastic lens mould is found over each display digit to make the display larger.

The Oxford 300 calculator was sold between 1975 and 1976, making this calculator 40 years old! The calculator sold for $46 ($206 today) and has the following function capabilities:

  • Add, subtract, multiply, and divide (floating point)
  • Percentages
  • Sin, Tan, Cosine, and their inversions
  • Pi
  • Square Root
  • 1 / x
  • Natural Log
  • Exponential
  • Scientific Notation (1 x 10 ^ n)
  • Degrees / Radians
  • Memory

A note from the author: It was really painful opening this calculator due to its age and rarity. It will be a scar on my memory for a long time.

The Outside

Despite its age, this calculator is not only well-constructed but it's also still working! The buttons have a tactile feel (unlike the stereotypical Sinclair computers) and are very responsive. But we are not here to admire the outside—we are here to see the innards!


Considering its age, this is in great nick!

The battery compartment is clearly very old and could do with a repair. The PP3 connector is rather worn down and appears to be made of potentially dangerous metals (such as lead). Again, despite its age, it is tough and in good shape.


So old, so old!

Removing the case was very strange; it slides off instead of using screws. The back of the case had four holes which I assumed where screw fittings, but it turns out that rubber feet used to be in those holes. After 15 minutes of carefully applying force in different directions to see which way the plastic wanted to move, it became evident that the back actually slides off. Personally, I like that (screws can become damaged and lost).

Strangely, the PCB only uses solder mask on traces and not on empty PCB.


I must admit, this PCB design is a first for me (the color mainly)

The Main PCB

The PCB is made of a paper-based epoxy material (FR2) and only has solder mask on copper. The Sinclair logo can be seen on the bottom of the main PCB that holds the processor.

Those black plastic circles are large plastic pins that push into slots to keep the PCB down. It is amazing to see that not a single screw is used in this construction, which makes it cheaper to manufacture (Sinclair is known for reducing cost where possible).


Close up of the PCB underside showing two of the three supporting pegs.

Again, that black circuit is a peg, it’s not an IC! The soldering is incredibly neat and there are no signs of corrosion anywhere! Hats off to Clive for this incredible construction.


Such a beautiful construction technique! Sinclair designs are usually pretty.

The top side of this PCB has the components and it was no surprise. There appears to be a resistor array connected to the switch matrix (pull-ups / -downs anyone?) and this is directly connected to the IC pins. This would imply that during production someone realised that the lack of pull resistors would cause intermittent operation. So, running down the hall to tell the production of the issue, they got a big box of these resistor packages and just stuck them straight to the IC!


Sinclair designs are also forgetful! Someone neglected the pull-up/down resistors!


The chip that powers the Oxford 300 is a C596L, but no datasheets turn up on Google. These were single-chip applications where the chip would contain the segment driving, key decoding, and data processing all in one. This helped to get products to shelves more quickly and cheaply than using generic parts (such as the Intel 4004 chipset calculator).


The single-chip solution to portable calculators

This shows how the PCB interconnected between the display and the keyboard. Again, the condition of this calculator is impeccable!


The processor board in all its glory!

The Keypad

The keypad is rather strange by Sinclair’s standards because normally Sinclair products use a membrane with carbon contacts as switches. The keys use a plastic button that sits inside the plastic mould which sit on the surface mounted metal tactile switches. This gives a really nice tactile feel when using the keypad and you get feedback from the switch to know that you have in fact pressed it.


Buttons that only fit in one direction prevent upside down keys


On top of really nice tactile switches, the PCB pads are gold-plated. The switch array also has a sheet of plastic on top. I imagine this is to keep dust / debris from getting inside the very low profile switches and permanently damaging them.


The quality of this PCB is incredible especially for the 70s

Sinclair is really obsessed with putting their logo on just about everything!


Ah yes, the Sinclair Logo!

The Display

The best has been saved for the end! Truly. This is what made these calculators so innovative.

To cut down production costs, Sinclair used these teeny tiny 7-segment displays. These are great for power consumption and compactness but they are also really difficult to see. So, to get around this, Sinclair had the brilliant idea of using small plastic lenses over each display to magnify the segments and make them appear larger than they really are. And what a difference they make!


Just look at that display, they don't make em like they used to!

This close up of the display shows the tiny wires that are connected to each segment of the digits. Chances are (due to their size) they are welded on via ultrasonic vibration (like normal ICs).


Close up of the tiny gold wires that bond the segments to IO pins


Close up of the display at work. Image courtesy of Nathan Zeldes.


Seeing how technology has changed over the past 40 years is truly shocking. Back in 1975, if you were lucky enough to be able to afford this calculator it would have been your most powerful tool. This calculator would be your spreadsheet, accountant, homework solver, and everyday calculation assistant.

Maybe I have the brain of an old man, but seeing how modern technology is so much more complex than this calculator yet seems to do less is rather sad. For me, however, if you saw my computer desk you would not see a smartphone, MP3 player, or tablet. Instead, you would see a tape deck, ZX Spectrum (with 32K RAM pack of course), a mechanical PS/2 keyboard and a Windows 7 machine that is still using DDR2 memory, and floppy disks.

Am I stuck in the past? Or do I prefer technology that is pure? Who knows! But stick around for the next teardown.


Next Retro Teardown: Commodore 64

  • F
    fela September 17, 2016

    Excellent article, thank you very much…

    Like. Reply
  • Stevepeters April 13, 2021

    Thanks for taking the time to do this I’m looking into getting one so it’s handy to know how to remove the case.  I remember these for sale ,way out of my price range, even the basic ones.

    Like. Reply