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Sharp Compet 20 Electronic Desktop Calculator

Updated 7/17/2022

The exhibited machine sadly suffered at the hands of careless shippers, receiving damage on its way to the museum. The damage to the machine was unfortunately functional as well as cosmetic. While the cosmetic damage was somewhat repairable, the real difficulty is in determining what damage was caused to the operability of the machine. When received, one Nixie tube was shattered, three others would not light, and the machine was not responsive to keyboard input. The shattered Nixie tube was replaced with an exact replacement, courtesy of a fellow calculator enthusiast, Brent Hilpert. Over time, it was found that a number of the circuit boards had small fractures in them (due to the rough handling in shipment), causing some circuit traces to be broken. Upon carefully inspecting each board with a back-light and repairing the fractures found, the three initially non-working Nixie tubes began working again. After quite a bit more electronic detective work, the machine is now almost completely operational, with incorrect display of multiplication problems on entry(though the correct answer is given), and problems with the error/overflow detection circuitry.

The Old Calculator Museum later added second Compet 20 to its collection. Unfortunately, this second example of the calculator, while in pretty good cosmetic condition, is completely non-functional and attempts thus far to resurrect it have proven unsuccessful.

Miller's of Columbia Tag on Museum's Second Compet 20

This second machine was initially sold by an office equipment/supply/furniture company in Columbia, South Carolina called Miller's of Columbia. Miller's of Columbia was founded in early 1967, just in time to be able to offer the Compet 20 to an eager market looking to replace old mechanical or electric adding machines or calculators with a vastly faster and completely silent electronic calculator. The Compet 20 was very attractively priced, making it stiff competition for US-made calculators from Friden, Monroe, Wang Laboratories, and SCM (Smith-Corona Marchant). Miller's of Columbia remained in business until sometime in 2019, when it appeared to run into financial issues and was closed down.

Profile View of Sharp Compet 20

In the process of working through the problems with the first Compet 20, some reverse-engineering has been performed that sheds some light on the basic architecture of the calculator, but without schematics or service documentation, it is difficult to identify the sources of the remaining issues, or make further process with restoring the second Compet 20. If anyone out there has service experience with these calculators in the day, or perhaps has old documentation (such as Service Manuals, Schematics, or Operator's Guides) for this or similar-appearing Sharp, Facit or Addo calculators, or if you have one of these machines in fully operational condition, we would greatly appreciate hearing from you. Click HERE to send an EMail message the the museum staff.

The ID Tags on exhibited Compet 20 and second Compet 20

The Compet 20 is the second electronic calculator that Hayakawa Electric Co., Ltd. produced for consumer use, and marked the beginning of the company's second generation of electronic calculators. At the time the Compet 20 was introduced, Sharp was a trade name for the company's products, though the Sharp name was featured prominently on the calculator, with Hayakawa Electric showing only on the model/serial number tag. Sharp's first electronic calculator, the Compet 10 (Model CS-10A), announced in March of 1964 and available for purchase in Japan in June of 1964, is claimed as the world's first all-transistor electronic calculator, a fact which is disputable, as there were other transistorized electronic calculators introduced at very nearly the same time. The Compet 10 was available for retail sale only in Japan, although it is known that a small number of the machines were modified for use on European power and sold through third parties in European countries.

The Compet 20's Predecessor, the Sharp CS-10A
Image Courtesy Nigel Tout
Photo Taken at Science Museum, London

While Sharp's Compet 10 was indeed among the earliest of the transistorized solid-state electronic calculators, there is some contention as to who was really first. IME in Italy, along with Mathatronics and Friden in the United States all made introductions of solid-state electronic calculators at around the same time in 1964. Any of these companies could lay claim to the title of "first", but with differences in definition as to the meaning of "formal introduction", "announcement", or "first customer shipment" and similar marketing-related terms, the claim of first really has no true significance. From a historical standpoint, it should suffice that Hayakawa Electric's Compet 10 was indeed one of the earliest transistorized electronic calculators.

Mr. T. Hayakawa, Founder of Hayakawa Electric proudly displaying his company's Compet 20 electronic calculator

The Compet 20 was a redesign and major improvement over the Compet 10, relying on advances in transistor technology as well as miniaturization, to reduce the size and increase the reliability of the machine. Some of the basic architectural concepts behind the Compet 10 were carried over to the Compet 20, but Sharp's engineers had learned a lot from the development of the Compet 10, and made a lot of improvements to the architecture that made the Compet 20 a solid platform as a basis for a number of future models based on the same design concepts. The Compet 20 was introduced in Japan in September of 1965, about a year and a half after the introduction of the Compet 10. In November of the same year, Sharp introduced a higher-end version of the Compet 20, designated the Compet 21 (Model CS-21A) that added automatic calculation of square roots. The Compet 20 and 21 are visually identical from the outside. The difference between them is additional circuitry in the Compet 21 to provide the square root function. The square root operation calculates the square root of a given number to six digits behind the decimal point. To calculate a square root on the Compet 21, the number to have its square root extracted is entered on the keyboard, and then the [÷] key is pressed, immediately followed by the [=] key. Pressing the [=] key immediately after the [÷] key without entering a divisor triggers the square root operation. This is a trick that Sharp continued to use on a number of future calculators in order to save the cost of an additional key-switch and key cap for the square root function.

Near the end of the Sharp Compet assembly line, circa 1967.
Note finished Sharp Compet 30 calculators in foreground

The Compet 10 used early Japanese-made Germanium transistors that suffered from reliability and longevity issues because the Japanese manufacturers (Nippon Electric Co. (NEC) and Hitachi) were very new to the art of manufacturing transistors, and Germanium was a tricky element to use to make transistors. Early on, the Japanese-made transistors had issues with humidity from the atmosphere getting inside the transistor causing the transistor to fail. Germanium was also quite heat-sensitive. This was apparent with the Compet 10 as it had a tendency to malfunction when it had been running for a significant length of time. Heat buildup inside the cabinet would cause the characteristics of the transistors to change, resulting in malfunction. Turning the calculator off and allowing it to cool down would return the machine to operation, but this was certainly an inconvenience. At the time, US semiconductor companies had perfected the manufacture of transistors that were made from Silicon, a much more stable element that made transistors faster, use less power (and therefore generated less heat), and were vastly more reliable than Germanium transistors. Texas Instruments perfected the first Silicon transistors, and was reluctant to license the technology to the Japanese because of the speed at which the Japanese manufacturers had grown their own Germanium transistor manufacturing capacity, and with Japanese labor rates much lower than those in the US, the US manufacturers feared that the Japanese would overrun the market with lower-cost Silicon transistors. The US companies eventually secured lucrative licensing agreements with NEC and Hitachi, as well as Toshiba., and these companies promptly ramped up their production lines (all with the help of large amounts of money from the Japanese government) to produce Silicon transistors. Because of this, the Compet 20 was able to utilize Japanese-made Silicon transistors rather than having to stick with the fussy Germanium transistors, especially in the calculating logic of the calculator. Germanium transistors, which were considerably less-expensive, were still used in less critical aspects of the Compet 20, such as for driving the Nixie tube display. The use of Silicon transistors in the Compet 20 made the calculator faster, less power hungry, much less prone to thermal issues, and tremendously more reliable than the Compet 10. Along with the benefits of Silicon transistors, Japanese transistor manufacturers had gained a great deal of experience in semiconductor manufacturing during the time between the introduction of the Compet 10 and the Compet 20, which certainly also contributed to improved reliability. The Compet 20 also improved upon the Compet 10 with a ten-key keyboard versus the full keyboard arrangement (a column of 1 through 9 keys for each digit entered) that was used on the Compet 10.

The Compet 10 was designed, mainly in the interest of time, for sale in Japan only. It's power supply circuitry and electrical characteristics were specific to the Japanese 100 Volt AC electrical grid, making the machines difficult to to export outside Japan. Sharp's marketing management insisted that the successor to the Compet 10 must be designed so that its power supply and circuitry could easily be adapted to power grids in Asia, Europe and the Americas so that the calculator could be exported. The power supply circuitry in the Compet 20 had a multi-tap power transformer that could be configured to operate on different AC mains power ranging from 100 Volts to 240 Volts AC at 50 or 60Hz.

Given that the Compet 20 was designed to be exported, Sharp began looking for distributors all around the world. Some enterprising folks in Australia had purchased a number of Sharp Compet 10 calculators and modified the power supply along with a few other changes to allow the calculators to operate in Australia. This company was called Olims Electronics, a company that marketed various imported electronics such as radios, television sets, Hi-Fi systems, and other appliances. The company would modify the devices as needed to meet Australian requirements and sold them through their network of retail outlets. Sharp was aware of Olims Electronics, and figured that Australia might be a good place to start the export of the Compet 20 to, as there was a great market in the country, and a well-established network of retail stores and service outlets that could sell and support the calculators. Olims Electronics also had their own electronics manufacturing capabilities, and it the ability for Olims Electronics to actually manufacture the calculators was negotiated as part of the agreement. At first, completed calculators were shipped to Olims Electronics' main plant in Australia, there they would put their tag on it, test the machine out, and them and ship them out to their retail outlets for sale. The machines sold like hotcakes. In time, Olims Electronics was able to put together a manufacturing line to build the Compet 20 themselves, with the company paying a royalty to Sharp for every machine they produced (whether it sold or not).

After the success of the export of the Compet 20 to Australia, sometime in the mid-1960's, the Swedish company Facit, a powerful force in the European mechanical calculator market, forged an OEM relationship with Sharp such that it would import electronic calculators made by Sharp, and market, sell, and support the calculators under the Facit brand in the European marketplace. Facit's calculators had minor variances in features and cabinet/keyboard color schemes. For example, the Facit 1121 is Facit's version of the Compet 20. The 1121 has 16 digits of capacity versus the 14 digits of the Compet 20, and lacks the [000] key of the Compet 20. Functionally, the two machines operate the same, and from a circuitry perspective, the machines share a common electronic architecture.

Sharp Compet 20 with "Manufactured by Olims Electronics" tag.
Photo Courtesy of Mikail Yussim

Along with Sharp's OEM relationship with Facit, Sharp also had a licensing agreement with the Australian electronics firm Olims Electronics, headquartered in Sydney. Olims Electronics was the leading manufacturer of consumer electronics for the Australian market. In 1963, Olims formed a relationship with Sharp to manufacture small transistorized television sets designed by Sharp for distribution and sale within Australia. Expanding on this successful relationship, in 1966, Olims became an OEM customer of Sharp for it's electronic calculators, officially beginning with the Compet 20. However, unofficially, there were also a limited number of earlier Compet 10's (shipped to Olims Electronics in completed form, and re-worked by Olims Electronics for use on the Australian power grid) that made it to Olims for sale in Australia once the agreement was signed. The major assemblies for the Compet 20 (circuit boards, keyboard, chassis components, cabinet electronic components, backplane, and power supply) were shipped to Olims Electronics from Japan, and Olims Electronics would assemble the machines in their factory, placing a tag on the finished product stating "Manufactured by Olims Electronics, PTY, Ltd.". Olims Electronics was responsible for all marketing, distribution and support of the Sharp calculators in Australia. The company performed all maintenance and repair services for the calculators in their shops. The Sharp calculators were extremely successful in Australia, and both companies profited well as a result of this arrangement.

The exhibited Compet 20, serial number 711901, was manufactured in January of 1967 by Sharp in Japan. The museum also has a second, non-functional Compet 20 that was produced in September of 1967. It appears that there were no significant changes in design during that time-span, as all aspects of the machines from the circuit boards to the mechanical components are for all intents and purposes identical.

The Compet 20 is a fairly basic four-function electronic calculator. It has a 14-digit (plus a 15th sign digit) Nixie-tube display. The digit Nixie tubes indicate 0 through 9, and also include a right-hand decimal point. There is no leading zero suppression, but the machine does have floating decimal point (and a 'fixed' point mode determined by the setting of the [M] key, explained later). Add and subtract operations work adding-machine style, with the [+] key actually performing a "+=", and the [-] key performing a "-=" operation, even though the key cap nomenclature doesn't indicate this. The [=] key is only used to generate results for multiplication and division operations.

The exhibited Compet 20's Faulty Multiplication Display (1234 X 5678 entered)

The exhibited machine has a fault with the multiply operation, though it still gives correct answers. As yet, I have not been able to determine the cause of this fault. Due to this fault, this Compet 20 displays multiplication problems in a strange way. The multiplicand is first entered, then the [X] key is pressed. A red indicator lights up on the [X] key to indicate a multiplication is in progress, and the Nixie display after the last digit of the multiplicand lights up both the 7 and 9 digits at the same time (see photo above). Then, the multiplier is entered, and the display shifts all of the digits to the left for each digit of the multiplier entered. The resulting display looks rather odd, with the multiplicand and multiplier all on the display at the same time, separated by this odd looking digit with both 7 and 9 lit simultaneously. The [=] key is then pressed, and the product is correctly calculated and displayed. A correctly-operating Compet 20 would simply reset the display to all zeroes after the [X] key is pressed, with the multiplier showing up on the display as it is entered, and when the [=] key is pressed, the multiplication is performed, and the result displayed.

Division on the Compet 20 operates as expected, also with an indicator in the [÷] key cap like on the [X] key that lights up when the dividend has been entered and the machine is waiting for the divisor to be entered, followed by the [=] key to obtain the result. The Nixie tubes are left active during calculations, and during some of the longer calculations (though the machine is quite fast) the numbers in the display dance quite a jig while such operations are in progress.

The [M] key is a "push to latch, push again to unlatch" switch that changes the way that decimal point positioning is handled. When the [M] key is not activated, the calculator operates in full floating decimal mode. When the [M] key is locked down, the decimal point position is determined by where the user enters the decimal point in the first number of a calculation. For example, with the [M] key depressed, entering 1 divided by 2 will result in 0. However, performing 1.00 divided by 2 will result in 0.50; and performing 1.00000 divided by 2 will result in 0.50000. The [X-] key (with the - located under the X on the key cap) appears to work the same as the [X] key, except that the multiplier is negated. The [RC] key recalls the last number entered, for example, If you perform a 12 [+] 13 [RC], the 12 will return to the display, though if the [+] key is pressed after the [RC] key, the answer of 25 will be displayed. The Compet 20 also has a [→] key that deletes the last digit entered; a thoughtful addition that many early calculators (and many even today) lack. Lastly, the unusual [000] key causes entry of three zeros into the currently entered number. For example, pressing [2] [.] [000] [5] would result in "+0000000002.0005" in the display. It's not entirely clear why this key was included. If anyone knows why, I'd love to hear from you.

The Compet 20 with Case Removed

As part of the effort to resurrect the machine, I have been working on instrumenting the electronics as well as reverse engineering partial schematics of the machine to try to better understand how it works. As a result of this work, I've come to some assumptions about the general technology and operating principles of the machine. All of the technical information included here is based on these observations and assumptions.

The Compet 20 is built entirely with discrete transistor technology. There are no integrated circuits anywhere in the machine. Transistors are used for all active circuitry, including flip-flops, level shifters, inverters, and display drivers. An assortment of different types of transistors are used, including miniaturized devices made from white ceramic-cased "pancakes". These show up in the photos as white dots, some with green or yellow markings on them. These tiny transistors are part of the reason that the Compet 20 is a significantly more compact than many of the competing machines on the market at the time. These tiny transistors were manufactured by Nippon Electric Co. (NEC) in Japan, and were called "Micro Disk" transistors.

Diode circuitry is used for logic gating functions. The machine appears to have been designed using standardized circuits, with the same basic core of parts for flip-flops and gates, with component variances occurring in cases where different drive or input requirements exist. The machine appears to use two different sets of logic levels...one set of levels for use on-board, and another set of voltages used for signaling on the backplane. For on-board logic, +10V represents a 1, and near ground represents a 0. For backplane levels, +4.5 to +5 volts represents a 1, and ground represents a 0. It isn't clear to why the backplane logic levels are different than the on-board logic levels.

One of the 'glue' circuit boards

The brains of the machine are made up of twenty (perhaps this number is reflected in the model number of the machine?) circuit boards that plug into a hand-wired backplane. Two large boards are located at the rear of the machine, spanning the width of the cabinet. These boards, based on nomenclature found on one of them, form the sequencing logic for the machine. One of these boards, labeled "PROGRAM" is populated primarily with a large number of diodes, while the other has a board, labeled "A.P.", consists of a number of flip flops, combinatorial logic, and decoders. It is assumed from this arrangement that this machine is microcoded to a degree, with the PROGRAM board serving as a ROM (Read Only Memory) containing "subroutines" (Sharp's language) that are micro-operations that are stepped through to carry out the function of the calculator, and the "A.P." board is the sequencer that steps the machine through the micro-operations. If this assumption is correct, Sharp probably can lay claim to the first all-transistor microcoded electronic calculator. A microcoded architecture provides a flexible foundation for developing a range of calculators with differing features, such as number of digits, fixed or floating decimal, memory functionality, etc. If the basic set of micro instructions and its execution engine are properly designed, it becomes a relatively simple matter to make modifications to the behavior or functions of the machine without having the re-design the whole thing. The flexibility of this design is what made it a relatively simple operation to modify the Compet 20 to perform the square root operation, which ended up becoming the Compet 21, as well as providing some flexibility in feature sets, allowing the Facit calculators to use the same architecture (and in many cases, the same circuit boards), but have some minor differences in function for brand uniqueness.

A fully populated digit board (left) versus a digit board with only register, decode and driver logic (right)

The remaining 18 boards are smaller than the program boards, and are plugged into the backplane perpendicular to the program boards. Fourteen of these boards are what I refer to as "digit" boards. Each digit board contains four flip-flops connected in FIFO (First In, First Out) shift register fashion which serve to hold the BCD (binary-coded decimal) digit to be displayed at that digit position; an array of diode/resistor gates that serve to decode the BCD number stored in the 4-bit register into a 1 of 10 selector; and finally, driver transistors (2SC287) which switch on the appropriate digit in the Nixie tube based on the outputs of the decoder logic. The Nixie tube is mounted directly to the board, situated so that it is properly positioned to shine through the display window in the keyboard bezel. This parallel digit design, where each Nixie tube has its own display decoding and driving circuitry, is much more straightforward to design and troubleshoot than multiplexed displays, but costs significantly more to manufacture because many more parts are required. The remainder of the real-estate on the digit boards is dedicated to what I call a breadboard area, where up to 8 flip flops and various gating functions can be built up. This breadboard area is where the working registers, data path, and switching circuitry resides, distributed throughout all of the digit boards, with interconnections through the backplane. A 15th digit board has a special Nixie tube (NEC CD-59) that contains only a "+" and a "-", used for indicating the sign of the number in the display.

The hand-wired backplane

The rightmost card in in the cage is the keyboard interface. This card has an edge connector at its top edge that provides the connection for the wiring harness from the keyboard. This card provides keyboard encoding and signal conditioning from the switch contacts that make up the keyboard. The two left-most cards in the cage are pretty tightly packed with parts, and appear to be glue logic boards that contain numerous functions, including decimal point logic and drivers and error/overflow detection and lockout.

Compet 20 factory assembly line power supply adjustment, circa 1967

The calculator uses a conventional linear power supply. Voltages produced are +12, -12, and +5.25 volts, along with +185V (used for driving the Nixie tubes). The +5.25 Volt and +12 Volt supplies are transistor regulated.

For the year 1965, the Sharp Compet 20 was awarded the Japan Institute of Design Promotion's (JDP) annual Good Design award, a prestigious recognition of products designed and manufactured in Japan that exhibit a high level of design expertise in all aspects of a product's manifestation, including functionality, aesthetics, safety, technology and materials.

The Compet 20 is actually a fairly fast calculator for its time. Compared to the Friden 130, the Compet 20 is a speed-demon. Addition and subtraction are return virtually instantaneous results. The worst-case multiplication takes about 1/4 second (999999 X 999999), and thirteen 9's divided by 1 takes about 1/3 second. Note that in the "all nines" calculations, the full capacity of the calculator is not exercised. As with many early electronic calculators, the Compet 20 gives incorrect answers when the most-significant (14th) digit of an operand is non-zero. Typically this behavior is related to the fact that one of the digits in the accumulator is used as a single-digit counter for counting up or down (up for division and down for multiplication) as each digit of the multiplier or quotient are used/generated during multiplication and division operations.

Thanks to Mr. Iwase and Mr. Kamiya of Sharp Corporation for information on introduction dates of early Sharp calculators.

Sincere thanks to Brent Hilpert for providing a replacement NEC CD-65 Nixie tube to replace the original tube damaged in shipping.

Text and images Copyright ©1997-2023, Rick Bensene.

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