Unitra K-765, or Commodore in Polish guise

During the communist era, pocket calculators were produced by two companies – Wroclaw’s Elwro and Bydgoszcz’s Unitra Eltra. We will take a look at the constructions made in Upper Silesia at another time, today’s hero will be a somewhat strange and already forgotten construction from the Brda River.

Unitra K-765 calculator

The Unitra K-765 is a pocket calculator manufactured at the Bydgoszcz plant since 1977. The casing of the device was made of white/cream plastic, which, however, changed to a more yellow color over time. The display was placed under a translucent red plastic. Just below it is an aluminum insert with the Unitra Eltra logo and model designation printed on it. At the bottom of the calculator there is a keyboard, which is quite pleasant to use, although the keys themselves have a slight travel. In the keyboard area there was also a power switch, which in my copy had already started to interrupt, but I managed to eliminate this problem by cleaning its contacts.

The device, like other Unitra calculators, is powered by a single 9V battery or a dedicated power supply with a mono jack connector. A battery cover and nameplate were included on the bottom.

Commodore 887D (http://www.matthewmawson.net/collection.html)

When discussing the Unitra K-765 calculator, it must be said up front that it is not a Polish design. The device was designed by Commodore and originally had the designation 887D. The only difference here is the logo on the front of the calculator.

What does the interior hide?

Interior of the K-765

To get to the inside of the device, you need to unscrew two screws and release two latches located nicely below in rectangular holes. After removing the case, you can see the interior of the calculator, which is quite minimalistic.

At the very top there was a place for the battery and a jack socket for powering the calculator from an external voltage source. Slightly lower you can see the display, and just below it the main processor with a rather rudely soldered-in power filter capacitor. The lower area contained nothing but three stickers and screws holding the keyboard.

Motherboard and keyboard design

After removing the screws and releasing the board from the plastic latches, you can see the other side of the printed circuit board and the keyboard structure itself. The board on this side has been covered with soldermask, and the only free areas are formed by metal contacts, short-circuited by rubber buttons, most likely covered with graphite. Also noticeable here is an on/off switch made of metal plates that short-circuit or disconnect the main power line.

3D-98MT processor

The main processor in the K-765 calculator is a chip manufactured by Commodore with the designation 3D-98MT. It is a typical example of a calculator-on-chip. The design is powered by 9V and does not need any external modules to operate the LED display and keyboard. The chip supports basic mathematical operations and has a memory.

This chip, in addition to the K-765 calculator, or de facto Commodore 887D, was also placed in the model designated 797.

Display

The K-765 was equipped with a 9-digit LED display. This structure, like the CPU, was designed by Commodore. This display is almost identical to the NSA 1198, manufactured in Poland, used in the Brda 11U. Each digit consists of 7 segments, while a single segment itself is three separate luminous structures. Despite such a large number of segments, the display has relatively few leads. This is due to the fact that a single bus is used for control, so the display must be multiplexed.

Is the K-765 a failed project?

Calculators produced at Unitra Eltra plants over the years

In preparing this material, I was looking for information on where this calculator really came from, because its release is quite puzzling, but let’s start at the beginning.

Unitra’s Bydgoszcz plant was founded in 1923, although at the time it was not yet Unitra. The company specialized in the production of radios until the mid-1970s, but at some point some party official decided that calculators should be produced in Poland. Office designs were handled by Wroclaw’s Elwro, while pocket designs were to be made at Bydgoszcz’s Eltra. It all started with a model with the working designation K-741 and its successor K-741M, in which the keyboard layout was slightly changed. These calculators were Japanese designs based on a VFD display and transferred to our home market. Later that year, another K-764 device appeared, which, renamed later to the Brda 10U model, became the beginning of a rather cult series of calculators that borrowed their name from the river flowing through Bydgoszcz.

The Brda 10U, 12U and 14S together form a very successful family of pocket-sized devices. Here we have a basic model, a slightly more advanced model and a typical scientific model. The calculators were produced until 1977, when the Brda 10U is replaced by the 11U model, gaining some useful features in the process. The same year also sees the appearance of our present-day hero, the K-765, whose purpose is shrouded in mystery.

Thinking about the reason for the introduction of the K-765 model, two reasons really come to mind. The K-765 can be compared to the Brda 10U in terms of functionality, so since the 10U model is being replaced by the 11U, perhaps Unitra wanted something simpler than the 11U to remain on the market. But thinking more about it, one can come to the conclusion that such a move is not entirely reasonable, after all, one could have simply left the Brda 10U on the market. It is much more likely that the K-765 little to be the beginning of a new series of pocket calculators, which was ultimately intended to replace the Brda models or function in parallel with them. Well, what exactly was the purpose of the K-765 model we will probably never know again. The only fact is that the line was not developed any further thereafter.

An interesting fact, apropos of Unitra Eltra calculators may be the Brda 13U model. Sometimes you can come across opinions that such a device did not exist at all, but this is not true. The Brda 13U was created in 1979 and was essentially a National Semiconductor 835 calculator. The 13U model was quite simple and can be compared to the 10U in terms of functionality. Today, this calculator is an extremely rare white rarity.

Sources:

  • http://www.matthewmawson.net/collection.html
  • https://pl.wikipedia.org/wiki/Eltra
  • http://www.calcuseum.com/SCRAPBOOK/BONUS/40137/1.htm
  • https://unitraklub.pl/unitra-eltra

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