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Monroe 740 Desktop Calculator

Here's a pretty amazing machine that brought forth (with me, at least) the realization that Monroe not only resold calculators designed and manufactured by Canon and Computer Design Corporation, they also resold early electronic calculators made by the West German company Olympia. The Monroe 740 is a re-badged (with subtle cosmetic changes) version of an early Olympia calculator. The machine is built to the typically high quality levels of German engineering -- a precision instrument built with extreme craftsmanship.

Profile view of the Monroe 740

The 740 appears to be the first generation of electronic calculators that Monroe resold in the US. It is an all-transistor machine, built in the mid-1968 timeframe. The machine uses magnetic core memory for storage of working registers, and classic Nixie tube displays. Though this particular machine appears to have been manufactured in 1968, the design is more reminiscent of machines designed in the mid-1960's. My guess (and it is just a guess at this point) is that Olympia introduced this calculator technology in Europe in the 1966 timeframe.

The next generation of calculators from Monroe, such as the Monroe 990 switched to the emerging small-scale integrated circuit technology, and abandoned rather expensive magnetic core in favor of an acoustic delay-line for working register storage.

This machine is in wonderful cosmetic and functional condition, looking nearly new with the exception of a few small scratches here and there, with very little signs of wear. Its condition and functionality are testimony to the high quality of the design, as well as the care that owners of the machine lavished upon it over the years. I feel quite fortunate to have been able to add this wonderful example of German engineering to the museum.

Model/Serial Number Tag

At the time this machine was sold, the electronic calculator business was really starting to heat up. Just about all the major forces in the electronics business were involved with calculator technology by 1968. Monroe, a famous US maker of wonderfully-designed mechanical calculators, was faced with a real problem in the mid-1960's. Electronics were taking over, and the days of mechanical calculators were seriously numbered. As a result of the pressure, Monroe management decided to seek out electronic calculator makers outside the US. These manufacturers in Europe (and later Japan) weren't selling in the US marketplace, and Monroe could use them to design and build machines which they could market in the United States to fend off the electronic competition from other US makers like Friden and Wang, as well as the aggressive Japanese makers Casio and Sharp. Early on, Monroe partnered with the West German electronics manufacturer, Olympia A.G., to resell their first-generation transistorized calculators into the US market under the Monroe label. The Monroe 740 shown here is an example of this relationship. Sadly, I haven't been able to find out the model numbers of the corresponding Olympia machines. Along with the Model 740 exhibited here, it appears that Monroe also offered a model 730 (perhaps with no memory functionality), and the Model 770 (which provided an additional memory storage register) that were 'clones' of machines produced by Olympia. These three machines appear to be the only desktop calculators sold by Monroe that were designed and manufactured by Olympia. It seems that Monroe abandoned Olympia after these machines were no longer viable in the marketplace, in favor of Japanese manufacturer Canon, and later, calculators designed by California-based Computer Design Corporation.

Display with Negative(Yellow) and Overflow(Red) Indicators.

The Monroe 740 is a basic four function machine, providing addition, subtraction, multiplication and division. Unusual for the time is the fact that the machine operates algebraicly, with an "=" key used to calculate the result of a given operation. Many calculators of the time use arithmetic logic. The calculator has two memory registers, one that can serve as a general purpose accumulator (with add and subtract functions), and a second memory register that is store/recall only. The 740 operates with fully floating decimal point, which is quite unusual for the time, since floating decimal point logic is more complex than that required for fixed decimal operation. The display of the 740 uses Nixie tubes, with fifteen tubes making up the display panel. The Nixie tubes contain the digits zero through nine only. Decimal points are made up of small discrete neon tubes that are positioned between the Nixie tubes. The machine does not provide leading nor trailing zero suppression, leaving it to the user to ignore insignificant zeroes. Three incandescent status indicators are positioned behind yellow, red, and green jewels on the front panel of the calculator. The yellow indicator lights when the number on the display is negative. The red indicator indicates an overflow condition, and the green indicator lights to show that the memory register has non-zero content.

Monroe 740 with Covers Removed

Bottom View Minus Cabinet (Note Tank-Like Mechanical Structure)

The electronic brains of the Monroe 740 consist of a total of fourteen circuit boards (approximately 11" wide and 3 1/2" high) crammed with discrete transistors, diodes, resistors, and capacitors. The boards are positioned low in the chassis, with card guides holding the circuit boards in place. Each printed circuit board has traces on the back side, and components and jumper wires on the front side.

The Left-Side of the 740 Chassis, with Connectors and Wire Harness

The Right-Side of the 740 Chassis, with Display Subsystem Connectors

Each circuit board connects into the others via pin-type connectors at each end of the board. One end of each board plugs into a fixed set of connectors on the right side of the machine, and a removable connector plugs into the left end of each board. The connectors are wired together with a maze of wire arranged in an extremely nicely bundled harness.

The chassis of the machine is built like a tank, composed of heavy-gauge stamped sheet metal, with lots of threaded holes and machine screws holding everything together. The quality of the hardware used to put the chassis together is impeccable, reminding me of the insides of a Curta mechanical calculator than that of an electronic instrument.

An Example of "Modular" Flip Flops

An interesting twist of the electronic design is that it appears that standardized flip-flop modules are used. A small (approx. 1" x 1 1/2") circuit board, with two transistors, and an assortment of resistors, diodes, and capacitors makes up a module that serves as a flip flop. These modules connect to the main circuit board by solid wire leads. It appears that most of the flip flops used in the sequential logic of the machine are built this way, with the remainder of the logic (traditional diode-resistor gating, and transistor buffers and inverters) populated directly on the circuit boards.

A View of the Monroe 740's Core Memory Plane

Core memory is used in the 740 for storage of the machine's working registers. The core stack is quite primitive, with physically large cores. The core plane appears to have been hand-wired - a tedious operation that had to be done by people with extreme levels of patience. Each core has four wires that pass through it, the "X", "Y", sense, and inhibit wires. Given the age of the machine, and the fact that it operates perfectly, I've been reluctant to disassemble the machine enough to take boards out to get detailed photos and document the boards. For this reason, I'm not entirely sure of the size of the core array.

Power Supply and Cooling Fan Detail

The electronics of the machine are quite densely packed, and given the sheer number of components, even though solid-state, quite a bit of heat is generated. The solution to potential problems with overheating was to add a rather large "squirrel-cage" type fan that pulls through vents in the bottom of the cabinet, up through the circuit card cage, and out vent holes on the back panel of the cabinet. The fan is surprisingly quiet, given that it operates at a relatively low RPM, making up for lack of speed with sheer size. Along with the fan and display assembly, the top part of the chassis is filled with power supply electronics. The power supply is a traditional transformer-based linear supply with transistor regulation. As with the rest of the machine, the power supply appears to have been over-designed, with large, heavy gauge heatsinks and components with lots of specification headroom.

Monroe 740 Keyboard Detail

The 740 uses a very intricate mechanical keyboard design. Each key has a complicated arrangement of levers and bars that create a mechanical interlock that prevents more than one key from being depressed at a time. The lever arrangements actuate a microswitch for each key on the keyboard, with the individual switches wired back to the logic through a couple of pin-type connectors. The keyboard layout is fairly conventional, with the left group of switches controlling memory functions, decimal point positioning/rounding, and clearing the machine. The center cluster of switches is a standard 10-key numeric entry pad, with digits zero through nine and a decimal point. The rightmost group of keys control the math and memory accumulator functions. The keycaps themselves are made of a high quality plastic, and appear to have the nomenclature moulded into them. The keys themselves show virtually no wear, and the keyboard mechanism operates smoothly and quietly (as mechanical keyboards go).

Store to Memory Register 1 Recall Memory Register 1
Recall and Clear Memory Register 2 Recall Memory Register 2
Display Memory Register 2 Shift Decimal Right and Round
Add to Memory Register 2 Subtract from Memory Register 2
Display Dividend Divide

Keycap Decoding

Keycap nomenclature on the machine is rather unconventional. Once its known what the functions of each key are, the keycap legends make sense, but it took a little while to figure out just what the keycaps meant. At first glance, it appears that the machine is missing a divide key. However, one must remember that this machine was built in Europe, where division is typically rendered as a ratio, for example, 1:2 indicating 1 divided by 2. So, the divide key is rendered as a ":". The rest of the math function keys are labeled as expected. The other cryptic keys on the keyboard are related to the memory functions of the machine. The calculator has two memory registers, one which is usable only for storing and recalling a single number, and the other as an accumulator that can have numbers added to, or subtracted from. The store/recall register has two keys that control it, one to store the number in the display into the register, and another to recall it to the display. The other memory register has four control keys. All of the control keys for this register have a circle around the function legend. Two keys allow the number in the display to be added to or subtracted from the register. Two more keys recall the register to the display, one destructively, and the other non-destructively. A fifth function key for the memory register allows the memory register to be viewed without calling it into the display. This key has an upward facing arrow with a circle around it. When this key is depressed and held, the display temporarily switches to show the content of the memory register. Once released, the display reverts back to the previously displayed number. A key with a similar legend, without the surrounding circle, works similarly, but temporarily displays the dividend in division operations. The utility of this function isn't really clear, but that's what it seems to do. The last mysterious key is a key with a rightward-facing arrow, with a semi circle underneath. This key is used to reposition the decimal point one position to the right, while rounding off the number using a '5-up, 4-down' rounding algorithm. This function is useful for forcing results to a given number of decimal points. For example, a retail merchant wants to decide the retail price of an item that he sells. He purchased a lot of 200 of the items for a total of $199 at wholesale, and he marks up the items by 2/3rds for his profit. This means that the total sales price for all 200 units should be $331.666666. Dividing this by 200 results in a sales price per unit of $1.6583333. The merchant would press the "Shift/Round" key five times to result in a final sales price of $1.66. Here's how the Monroe 740 would display the results after each depression of the "Shift/Round" key: 1.658333; 1.65833; 1.6583; 1.658; 1.66. Further depressions would yield: 1.7, and finally 2. Continuing to depress the "Shift/Round" key would result in the decimal point wrapping around the left end of the display -- it appears no check is done to prevent the user from shifting the decimal point 'too far'.

The 740 is relatively robust in its detection of overflow conditions. When the machine is commanded to perform an operation which exceeds its capacity, it lights up the overflow indicator as soon as the overflow condition occurs, but continues the calculation until complete, displaying an answer that may not make much sense. Overflow conditions do not 'lock up' the machine like many other machines. The overflow indicator stays on until the user clears the machine with the "C" key. The machine does not properly error out when commanded to divide by zero, however. Rather it goes into 'churn' mode, with nothing but a bunch of zeros on the display. Pressing keys while the machine is in this state will cause apparently random and unpredictable results, ranging from flickering decimal points to random numbers on the display and sometimes an overflow condition. Pressing "C" when the machine is in 'divide by zero madness' state clears up the condition most (but not all) of the time. Sometimes the machine gets so confused that even pressing Clear won't do the trick, requiring that the machine be powered off and powered back on again before it will function properly. As with most machines with core memory, the content of memory registers is retained while the machine is powered off.

With regard to calculating speed, the 740 is no speed demon. Addition and subtractions return results after a slight but discernable flicker of the display, perhaps 80 to 100 milliseconds. Multiplication and division return answers in a period of time determined by the complexity of the operation, with 14 9's divided by 1 taking nearly one second to complete. Trying the same operation with 15 9's results in an overflow and incorrect answer -- a symptom that is fairly common on early electronic calculators. Multiplication of 9999999 by itself results in an answer in about 3/4 second. During the time calculations are taking place, the display shows all zeroes, with a very slight flickering of the uppermost few digits of the display.


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