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Sharp Compet 15 Electronic Desktop Calculator

The machine exhibited here, a Sharp Compet 15 calculator, is a close relative of another machine exhibited in the museum, the Sharp Compet 20. The Compet 15 appears to be an attempt by Sharp to reduce the cost-of-entry for potential buyers of electronic calculator technology. The cost reduction was implemented by trimming down the features and capabilities available on the Compet 20. The Compet 15, with its reduced capacity and fixed decimal point operation, made the machine less complex, and therefore less expensive.

Profile View of Sharp Compet 15

Both the Compet 15 and earlier Compet 20 share virtually identical design in terms of the structural and mechanical aspects of the machines. They also use the same general architecture. The major difference between the two machines simply lies in the number of digits of capacity (12 in the Compet 15 versus 14 for the Compet 20); fixed-decimal point operation for the Compet 15, versus the floating decimal capability of the Compet 20; and lastly, the Compet 15 doesn't have circuitry in it to deal with negative numbers, while the Compet 20 does. Mechanically, the plastic upper cabinet half and the metal cabinet base are interchangeable between the two machines, as are many of the other mechanical components of the design.

The "Sharp COMPET" Dust Cover

This particular calculator has an interesting history associated with it. Apparently this calculator was purchased in Europe by a German engineer, sometime in 1967. The machine, because of its European heritage, is designed to run on 200, 220, or 240 volt AC power. The line voltage is adjustable via a socket and plug arrangement accessable by removing the bottom cover of the chassis. European line voltage standards vary, so the machine was designed to be able to accomodate a number of different European line voltages. Fortunately, the machine is not picky about the line frequency, which in European countries is 50Hz, versus the 60Hz line frequency in the US. All US homes have 220 volt service supplied to them, and most have available 220V circuits in the home for powering electric dryers and cooking ranges, so it is possible to power up this machine in the US, which I have done. The machine operates beautifully. Anyway, the engineer who originally owned the calculator ended up coming to the US (Massachusetts) sometime later, and brought his prized calculator with him. Sometime in the late 1990's, the engineer passed away, and his family put his cherished calculator up for sale at an estate sale. The calculator was purchased by a person who had an appreciation for the meticulous condition that the machine was kept in, and later ended up being acquired by the Old Calculator Web Museum as a prized addition to the museum.

The mysterious "ADDO" Logo

There's an interesting twist to this particular machine that could have some relation to its European origins. All of the nomenclature on the machine makes it clear that the machine was made by Sharp. The serial/model number tag has the Sharp logo, along with a label on the back of the plastic case that says "SHARP", along with an emblem on the keyboard panel that says "Sharp" and "Compet 15". However, there's an additional emblem on the upper right of the keyboard panel that says "ADDO". Addo sold calculators in the European market, starting out selling mechanical and electro-mechanical calculators, and later working themselves into the electronic calculator market by forming relationships with established electronic calculator manufacturers. I don't have much information on the history of Addo, nor on the machines it sold, but it is clear from this particular example that Addo had a remarketing agreement with Sharp. Since this machine was originally purchased in Europe, it was probably bought at an Addo distributor. Addo simply purchased the Compet 15 from Sharp, and added on the "ADDO" badge, and resold the machine through their own sales channels.

The Sharp Compet 15's Keyboard Layout

The Compet 15 is a very basic four-function electronic calculator. It provides a 12-digit Nixie-tube display. Each NEC LD-784 Nixie tube indicates the digits 0 through 9 and includes a right-hand decimal point. The calculator operates with fixed decimal point, with settings for 0, 2, 4 or 6 digits behind the decimal point. The decimal point setting is selected by a rotary switch on the keyboard panel. Two groups of settings are available, with one selection designated in red. At this point it isn't clear what the difference in behavior of the machine is between the use of the red decimal point settings versus the other (which are labelled on the selector switch in white). Further experimentation will be required to determine the difference between the red and white decimal point settings. Addition and subtraction work as expected, in 'adding-machine' style, with the keys performing a "+=" or "-=" operation, even though the keycap nomenclature simply shows "=" on the keycap, with a red key used to indicate subtraction. Multiplication and division keycaps have small red jewels in them that light up to indicate the pending operation. Multiplication on the Compet 15 operates conventionally, as opposed to the unusual method of displaying multiplication problems on the Compet 20.

The Keyboard Assembly of the Sharp Compet 15

The keyboard of the Compet 15 uses an interesting combination of mechanical and electrical, the same basic design as used on the Compet 20. The mechanical aspect of the keyboard assembly assures that only one key can be pressed at a time. The electrical aspect of the keyboard uses magnetically activated microswitches to close the electrical circuit when a key is depressed. The keyboard connects to the rest of the calculator electronics via a couple of ribbon cables that connect via a fairly large edge connector to a circuit board that provides keyboard signal conditioning and encoding functions.

Operating the Compet 15 is pretty straightforward, with the usual zero through nine and decimal point keys. The white [=] key is used for addition and terminating multiplication/division problems, and the red [=] key for subtraction (or complimenting of the number in the display). The [X] and [÷] keys' functions are obvious. The [RC] key swaps the operands of multiplication and division functions. The [CLE] key is used for clearing entry errors, clearing the display/entry register, and the [CL] key clears the entire machine.

Unusual Power-On Display

The calculator does not have a power on clear circuit, resulting in an interesting display when the machine is first powered up, with all digits showing simultaneous '7' and '9' Nixie digits lit at the same time. Pressing the [CL] key after power-up clears the machine and readies it for normal operation.

Inside the Sharp Compet 15

Like the Compet 20, the Compet 15 is an entirely transistorized calculator. There are no integrated circuits used in the calculator. 468 transistors (with most being the same pancake-packaged devices used in the Compet 20), along with countless resistors, diodes, and capacitors make up the circuitry of the machine. The electronics are contained on a total of eleven circuit boards that plug into a hand-wired backplane that interconnects the boards. Like the Compet 20, a number of the circuit boards in the machine have specific function.

One of Six "Digit" Boards in the Compet 15

Six of the eleven boards in the machine are digit boards, with each board having circuitry on it to drive two of the Nixie tubes, as opposed to the digit boards in the Compet 20, that contain the circuitry for only one digit. The two Nixie tubes are mounted to the board, supported by a plastic fixture that is secured to the circuit board by a couple of small screws. Individual wire leads from the Nixie tubes have "spaghetti" insulation placed on them for electrical isolation, and are soldered to pads on the circuit board. The Nixies are individually driven by two sets of ten discrete transistor drivers, one transistor for each numeral-shaped electrode in the Nixie tube. A separate transistor provides drive for the decimal point.

Detail of Nixie Tube Mounting

The circuit boards are made of phenolic, with traces on both sides of the boards. Feed throughs are accomplished with uninsulated pieces of wire placed through the feedthrough hole and soldered into place, providing a solid connection between each side of the board. Edge connector fingers are tin-plated. Ten of the eleven boards plug into the backplane in such a way that those boards with Nixie tubes on them can shine through the display panel of the calculator.

The Sequencing Logic Board

One larger board spans the rear of the machine, containing the main sequencing state machine for the calculator. The right-most board in the chassis provides keyboard signal conditioning and encoding funtions, and the other three boards seem to contain miscellaneous control logic functions.

The Compet 15's Power Supply

The Compet 15 uses a very simple linear power supply, contained in the base of the machine, underneath the keyboard assembly. A small transformer takes the line voltage and steps it down to one 170V secondary which is rectified and smoothed and used for Nixie tube drive. Two other 14V secondary windings are used to generate the logic supplies for the machine. The voltages from the transformer are rectified and filtered by a bank of capacitors. It appears that the logic power supplies are not transistor regulated. The logic voltages are tweeked using adjustments (variable resistors) at the factory to provide the proper voltages at the current load imposed by the circuitry of the calculator.

The Compet 15's Model/Serial Tag

The Compet 15, like many early transistorized calculators, has a couple of quirks. It generates incorrect answers for multiplication and division problems where the multiplicand/dividend occupy the most-significant digit in the display. For example, with the decimal point setting set at 6, dividing 123456 by 10 results in a completely non-sensical answer, while dividing 12345 by 10 results in the expected answer. As part of the cost-saving measures, the Compet 15 does not deal with negative numbers. For example, performing 10 - 11 results in 999999999999. (with decimal point set to 0). Pressing the red [=] key at this point will perform a ten's compliment of the number in the display, so in the above case doing so would result in 000000000001. Arithmetic overflow is not detected by the Compet 15, with the machine simply discarding any digits beyond its 12-digit capacity. The logic of the calculator can only handle decimal points in the lowest six digits of the display. Entering 1.2345678, with decimal point position set at 6 results in 000012345678 on the display -- the decimal point gets lost. The Compet 15 also does not detect input overflow -- any digits entered in excess of the capacity of the machine are discarded with no warning. Division by zero results in the machine churning furiously to try to find an answer that doesn't exist. Pressing the [CL] key halts the futility and returns the machine to normal operation.

The Compet 15 seems a bit slower than the Compet 20 -- perhaps the 15 was purposefully slowed down to provide another perceived advantage to the Compet 20, but more likely, it allowed wider design tolerances in the circuitry that made the machine simpler to build, and potentially allowing the use of lower-cost transistors. Addition and subtraction complete virtually instantly, however, multiplication and division can take a little while to perform. The "all-nines" benchmark can't be performed to the full capacity of the machine due to the 11-digit limitation. However, performing eleven 9's divided by 1 (with decimal point set to 0) takes about 1/2 second to perform. During calculation, the Nixie tubes are not blanked, resulting in that wonderful 'spinning' effect that is one of my favorite aspects of Nixie tube-based calculators.


Text and images Copyright ©1997-2011, Rick Bensene.