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Monroe 770 Desktop Calculator

Updated 5/17/2002

The Monroe 770 is the 'big brother' machine to the Monroe 740 [See Advertisement]. The 770 adds additional capability in the form of a second accumulator-style memory register, and an automatic constant function for multiplication and division. Other than these differences, the machines are very much the same, sharing virtually identical cabinetry, chassis, and power supply components. Like it's stablemate, the 770 was made for Monroe by Olympia, A.G., in, Wilhelmshaven, West Germany.

The Olympia RAE 4/30-3
Image Courtesy Serge Devidts

Olympia marketed a similar machine of its own in Europe, introduced in 1966, called the RAE 4/30-3. The RAE 4/30-3 was functionally equivalent to the Monroe 770, but had some minor cosmetic differences, including different configuration and placement of the negative/overflow/memory status indicators, slight differences in keyboard color scheme, and placement of the power switch.

Profile view of the Monroe 770

The museum received two Monroe 770's through the courtesy of Arthur McAleenan, who hauled these old machines around with him for many years after the New York-based insurance company he worked for took them out of service sometime in mid to late 1970's. Fortunately, Art couldn't bear to see these works of electronic art simply thrown away, so he took them home, and provided a safe haven for them through all the years. The machines both have tags on the bottom of the cabinet indicating their delivery date to the original purchaser. The earlier machine was delivered on 3/18/68, and the later machine, on 11/17/69. The 700-series Monroe calculators seem to have debuted sometime in late 1966, so it is clear that even as the 1960's were drawing to a close, the machines were still being actively marketed by Monroe, even though electronic calculator technology was changing at a breakneck pace in the late 1960's.

Early and Late Monroe 770 Keyboard Designs

It is interesting to note that the two examples of the Monroe 770 in the museum have some differences between them. The earlier machine, built in the late part of 1967, has a European-style keyboard, with the divide key having the ":" nomenclature. The digit keys are also a brick-red color. The later 770, built in the late part of 1968, has the American-style divide symbol, and uses dark gray for the numeric keycaps.

There are also signs of some internal design changes from the older machine to the newer one, including an improved circuit board retaining method, slight differences in the power supply circuitry, and some subtle wiring harness changes.

The major functional difference between the Monroe 740 and the 770 is the addition of a third accumulator memory register to the two registers available on the 740. On the 770, one register is a store/recall register only, with the other two registers providing accumulator-style functionality, with individual keys to add and subtract the content of the display from the memory register. The keyboard nomenclature for the various memory functions on the 770 are quite a bit more obvious than the somewhat cryptic keycap legends used on the 740. Memory registers are denoted as |, ||, and |||. Memory registers | and || are general-purpose accumulators. Function keys are provided to add or subtract the number in the display from memory register | or ||, to temporarily display the content of memory register (as long as the function key is held down), to recall the content of the memory register to the display, and to recall the content of the memory register to the display, while simultaneously clearing the memory register. Memory register ||| is a store/recall only register, that also serves as a 'double precision' result register for multiplications which exceed the 15-digit capacity of the machine.

Memory Accumulation Function Keys

Adding and subtracting from memory registers | and || are controlled by keys on the right hand end of the keyboard. The "+ |" and "+ ||" keys add the current number in the display to the content of the memory register, leaving the number in the display intact. The "- |" and "- ||" keys subtract the number in the display from the memory register.

Memory Display Function Keys

An unusual feature of the Monroe 700-series calculator is a set of keys that allow the user to view the content of the memory registers without actually recalling the memory register to the display register. The set of keys pictured above temporarily changes the display to show the content of the memory register corresponding to the key being pressed, for as long as the key is held down. When the key is released, the display reverts back to its previous content. This allows the user to do a quick check on a memory register's content without disrupting any calculations currently in progress. This is a very handy feature, which I'm surprised didn't end up being implemented in more calculators.

Memory Recall/Clear Function Keys

The keys shown above recall the selected memory register to the display, then clear the memory register. The 770 provides no independent means to clear a memory register other than to use these keys. Most later calculators abandoned this 'recall/clear' methodology, and adopted separate memory recall and memory clear keys.

Accumulator Memory Recall Function Keys

To recall the content of the accumulator memory registers to the display without clearing them, the above pictured set of keys are used. These keys simply pull the content of memory register | or || into the display, leaving the content of the memory register intact and obliterating any previous content of the display register.

Memory Register Recall and Store Function Keys

Memory register ||| has a dual purpose. Like register | and ||, it's contents can be temporarily shown using the "uparrow |||" key, and its content recalled (clearing the register) using the "||| *" key, but because it functions only as a store/recall (it has no accumulating function) register, the designers opted to use different nomenclature on the keys that store and recall into this register, using an "M" designation rather than "|||". The "diamond M" key recalls the content of memory register ||| to the display, and the "M" key stores the current content of the display into memory register |||. Along with the store/ recall functionality of this register, it performs an additional purpose, serving as a double-precision register that holds the low-order fifteen digits of products that exceed the fifteen digit capacity of the machine.

Double-Precision Multiplication Result of 111111111 X 11111111

This double-precision feature is rather interesting, and bears a little further description. When a multiplication is performed that results in a product that exceeds the capacity of the calculator, the overflow indicator lights, and the display contains the most-significant fifteen digits of the product. The least-significant fifteen digits of the product are stored in memory register |||. By pressing and holding the "uparrow |||" key, the display switches to show the least significant digits of the answer. For example, as shown above, if 1111111111 X 111111111 is performed, pressing the "=" key results in the overflow indicator lighting up, and "123" to show on the display, indicating the most significant digits of the result. Then, pressing and holding the "uparrow |||" key will show "456789987654321", the remaining fifteen digits of the result. The overflow indicator stays lit until the calculator is cleared, and any further calculations will result in answers that are indeterminate.

The "Shift/Round" Key

Like the Monroe 740, the 770 has a "shift right and roundoff" key. This key causes the number on the display to be shifted to the right one position, and if the number that shifts off the right end is 5 or more, one is added to the last digit on the display. For example, if the display contains 122.96458, successive presses of the shift/round key will result in 122.9646, 122.965, 122.97, 123.0, and 123.

The Monroe 770 uses algebraic entry, with an "=" key terminating problems and causing the result to be calculated. Chain calculations are entered as written, followed by the "=" key to display the result. The 770 adds an automatic constant to multiplication and division, a feature not found on the 740. An example of the constant function would be to calculate successive powers of two. To do so, the user would enter "2", followed by "X", followed by successive presses of the "=" key for each power of two desired. This also means that the machine can do two key squaring, by simply entering the number to be squared, pressing the "X" key, then the "=" key to generate the square.

The Display Subsystem

The front panel of the machine contains the clear plastic display window that the fifteen Telekunken ZM1080 Nixie tubes peek through.

A close-up view of the Telefunken logo and Part Number on the Nixie tubes

Along with the display window, there are four jeweled indicator windows arranged left to right, with the left-most being a yellow jewel indicating that the number on the display is negative. The red jewel lights to indicate an overflow condition. The right-most two jewels are green, and light up when memory register | and memory register || contain non-zero numbers. Behind the jewels are small incandescent lamps that are lit when the specified condition exists. The display subsystem connects to the main chassis via two pin-type connectors.

A close-up of the Nixie Tube Display in Operation

Amazingly enough, the display system on the 770 is multiplexed, which is quite unusual for a machine of this early of a design. Other machines designed in the 1965/1966 timeframe (such as the Sharp Compet 20 and Wang LOCI-2) use separate driver circuitry for each digit of the display. The 770 provides fully-floating decimal, indicated by small discrete neon tubes situated between the Nixie tubes.

Monroe 770 with Top Cover Removed

The electronics that make up the 770 are of the same general nature as those in the 740. A total of fourteen (same count as in the Monroe 740) 11 1/2" x 3 1/2" circuit boards make up the logic of the machine. The circuit boards are made of phenolic, and have circuit traces on the wire side, and components and jumper wires on the component side.

A Typical Circuit Board from the Monroe 770

Each board has a pin-type connector at each end, one which plugs into a fixed connector in the backplane on the right-hand side of the machine, and another which connects to a removable connector wired into a wiring harness on the left side of the machine. The backplane consists of a maze of hand-wired connections that must have been done by people with great eyesight, and lots of patience. Each pin connector has 24 pins, and the pins themselves appear to be silver-plated for good conductivity. The circuit boards occupy a card cage across the bottom of the machine. The circuit boards are relatively dense, with components packed quite tightly for the rather primitive circuit board technology used.

A Close-Up View of one of the Flip Flop Modules

An unusual modular approach is used for some of the components on the circuit boards. Some of the components are grouped together onto small daughter cards that are soldered (via short wire leads) onto the main circuit boards. It appears that these daughter cards make up modules that contain a flip flop circuit, consisting of two transistors, and a combination of resistors and diodes.

Valvo and Telefunken Transistors

A total of 355 transistors are used in the logic, with a mix of manufacturers including Valvo and Telefunken, both European electronic component manufacturers. Given that the 700-series calculators were made for Monroe by Olympia in West Germany, it's no surprise that the components used to make the machines were all of European origin. The machines were made in Germany, then shipped to the US, where they were distributed by Monroe dealers under the Litton/Monroe brand-name.

A Close-Up of Core Memory

Back in the mid-1960's, when this machine was designed, there were a limited number of choices as to how to implement the working registers of a calculator. At the time, integrated circuits were just beginning to appear, and were way too touchy and expensive for use in an electronic calculator. The basic technologies for storage of data involved the brute-force approach, using individual transistorized flip flops or ring counters for each digit of working register (Sharp Compet 20); use of an acoustic delay line (Friden 130), in which the bits representing the working registers were represented as acoustic pulses of energy in a special type of wire which propogate through the wire; and lastly, core memory, the technology developed to replace electrostatic storage tubes and mercury delay lines in early computers. The Monroe 770 opts for core memory technology as its data storage medium. Magnetic cores use small doughnuts of magnetic material to store a magnetic field. The magnetic field in the core can exist in one of two states. Using these states, and other properties of the magnetic material the cores are made of, it is possible to store and recall a binary digit (1 or 0) in each core. The cores are arranged on a grid of wires through which electric currents can be placed to control the direction of magnetic field in a given core, and read the state of the magnetic field of any given core core. The nice thing about magnetic core memory is that once a bit is stored in a core, it remains there forever, requiring no additional electricity to maintain the magnetic field. This means that the bits stored in the core memory are retained, even when the power to the device is turned off.

The Core Memory Board of the Monroe 770

The core memory board of the 770 contains six planes of 64 bits each (arranged in an 8 x 8 grid), for a total of 384 bits of storage. The magnetic cores used in the 770's memory are some of the largest cores I've seen, with a diameter of about 1/5 of an inch. The cores are threaded onto an array of fairly heavy gold wires. A horizontal and vertical grid of wires allows each core to be individually selected by sending current through the horizontal and vertical wires that intersect at a given core. Two other wires thread their way through the core array, known as the 'sense' and 'inhibit' wires, that are involved in the reading and writing of bits from/to the cores.

Display Subsystem Connector and Partial Backplane

Like the 740, the chassis of the 770 is very stout. Heavy-gauge metal makes up the chassis, with a large number of machine screws used to hold the chassis together. The chassis is made up of the card cage across the bottom of the chassis, the power supply and fan section above the card cage, the display subsystem in front of the power supply, and the keyboard module at the front of the machine.

Even though transistors were a great advance over the vacuum tubes they replaced, they still generate a bit of heat. With over 300 transistors, along with countless other components, each dissipating a small amount of energy, the heat builds up quickly. A large squirrel-cage fan pulls outside air through vents in the bottom part of the cabinet, across the circuit boards and power supply components, and exhausts the heated air through vents in the back panel of the plastic cabinet. Even with the forced-air cooling, there are a couple of asbestos sheets glued to areas on the inside of the upper half of the cabinet to protect the plastic from melting due to the heat.

Decal Advertising the New York Sales office for Monroe Calculators, Located on Front Panel of both Monroe 770's

The power supply of the 770 is virtually identical to that of the Monroe 740. The supply is a basic linear design, using computer-grade components throughout. Voltage regulation is done both actively, with a big pass-transistor mounted on a massive heatsink regulating the main logic supply, and passively, with a number of other voltages controlled simply by resistors which are selected based on the load imposed by the circuitry to assure the correct voltage. It appears that a small switching power supply, located down near the keyboard, generates the high-voltage needed to run the Nixie tubes. This switching supply is powered by the logic voltage, and uses switching technology to generate the high voltage for the Nixies. This was likely done because the design had to run on a number of different line voltages depending on what market it was sold into, so rather than rely on using the line voltage to derive the high-voltage for the Nixies, it is generated internally.

Fancy and Expensive Cloisonne' Badging

The keyboard assembly, though having more keys than the 740, uses the same complex arrangement of levers and bars that create a mechanical interlock that prevents more than one key from being depressed at a time. Each key acts on a linkage that activates a microswitch. All of the key microswitches are wired together in a harness that connects to the calculator's logic through a couple of pin-type connectors. The keycaps are made of a high quality plastic, with nomenclature moulded into them to prevent the legends from wearing off with use.

Dust Cover for Monroe 700-Series Calculators

Calculation speeds are virtually identical to those of the Monroe 740. Addition and subtractions take around 80 to 100 milliseconds. Multiplication and division take significantly longer, with simple multiplies and divides taking perhaps 100 to 200 milliseconds, and more complex operations taking as long as one second to complete. Like the 740, the 770 is fairly unexciting to watch while calculating, with the display showing all zeroes, and a very slight flickering of the uppermost few digits of the display.

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