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SCM Marchant I Electronic Calculator

Updated 6/7/2024

This is one of the museum curator's favorite vintage electronic calculators. It had been on the museum's WANTED page for a very long time. It seemed that the chances of finding a good example of the SCM Marchant I were slim to none after so many years of searching. Those that occasionally came up on auction web sites sold for amounts that were too high for the museum's self-funded budget, and the calculators are just uncommon enough, as well as having the "cool factor" of Nixie tubes, that it seemed unlikely that one would be found at a swap meet, garage sale, or antique shop, and searches in those locations also proved fruitless.

The long-held dream came true when the museum received an EMail just after Christmas, 2017, from a very kind gentleman that said he had one of these calculators that he'd be willing to donate to the museum, and better yet, he had a trip planned to the area in early February of 2018. He offered to deliver the calculator to the museum in-person. It's always very special to be able to meet the donor of a great old calculator in-person, as well as being an honor to give the donor a tour of the museum. Sadly, just before the trip was to occur, there was an illness in his family, and the trip had to be canceled. There were thoughts that perhaps a trip could be made at a later date, but the situation being as it was, it didn't transpire. In August of 2018, the gentleman Emailed that he had decided that the calculator should be shipped to the museum since a visit was unlikely any time soon. On September 6th, 2018, UPS delivered a package to the museum that contained the very carefully packaged SCM Marchant I calculator, two original owner's manuals for the calculator, and its original power adapter.

The donor indicated that the Marchant I was originally purchased by his father's uncle, who took great pride in owning this piece of bleeding-edge technology. In time, the donor's father acquired the calculator from his uncle, and treated it to the save level of care as did his uncle. Some years ago, the donor's father gave the calculator to him as a gift, which he kept through the years in a small collection of vintage technology that he maintained, until he decided that it might better serve in a museum where it could be shared with the world. Some web searching led him to the Old Calculator Museum's Wanted page, where the listing for the Marchant I had been waiting for someone like him for over fifteen years.

The Marchant I in its Closed State

The calculator made the trip to the museum with no ill effect thanks to very careful packing. Visually, the Marchant I looked to be in astoundingly nice aesthetic condition after all these years. This was a clear indication of the care that was afforded the calculator through all of its owners during the over 45 years of ownership. All of the keyboard keys moved smoothly, and each made the distinctive tiny "tink" noise that only magnetic reed-switch keyboards make when a key is depressed and released. The display lens was clear without scratches or marring, and it appeared that all eight Nixie tubes were intact. There were no cracks anywhere in the cabinet. The unique keyboard cover when slid forward toward the display moved very smoothly. When the cover is moved so that the keyboard is fully exposed, the cover over the display panel flips up with a snap to reveal the display. At the same time, the small rocker-type power switch moves itself to the "ON" position, thus powering up the calculator. When the keyboard cover is slid back over the keyboard, the display panel cover snaps back down to provide protection for the display, and the power switch moves itself to the "OFF" position. This mechanism is a bit complicated, and the fact that it worked so smoothly after all these years was yet another testimony to the care that the machine had received. The flip-out feet on the bottom of the calculator were intact (many times these are broken or missing), and worked perfectly. The calculator was very clean, with only a light accumulaton of dust from years of storage. There was only one area of concern; the power jack for plugging in the external power adapter, located at the top right corner of the bottom panel of the calculator looked somewhat greenish in color. This was a hint of corrosion, more than likely caused by the rechargeable Nickel-Cadmium (NiCd, aka NiCad) batteries inside the calculator having leaked after many years of storage. Leakage from NiCad batteries is highly corrosive to just about any metal, and there was genuine concern that there could be corrosion damage lurking inside the calculator, potentially damaging circuit boards as well as electronic components from leakage of the battery cells.

The Unusual Screws Securing the Cabinet on the SCM Marchant I

As with any calculator received by the museum, the first thing that happens is that it is carefully taken apart both for purpose of inspection as well as taking photographs of the internals of the machine for documenting the calculator, as well as for use in the online exhibit. This was a particularly important step for the Marchant I, because of the concerns about the corrosion visible around the power jack. The disassembly of the calculator was complicated by SCM's use of two proprietary screws to secure the cabinet. These screws were quite unlike any other screw top that the curator has encountered. They resisted just about every attempt loosen them. They were clearly designed to be removed using a specialized tool, likely to limit service of the calculator to those authorized service organizations that have the special tool. As well as having a very unusual head, the screws were also torqued to an unusually high level, which made getting them loose all the more difficult. Only two of the screws that secure the cabinet are of this type (the rest were conventional Phillips-head screws), but without removal of these two screws, it would be impossible to get inside the machine. Finally, after some careful manipulation with a set of snap-ring pliers, they were able to be loosened and removed without damaging them.

Once the screws were out, a further complication was found in the rather ingenious mechanism that links the sliding keyboard cover to the flip-up display cover and power switch. Two somewhat complex metal castings, along with a strategically-placed spring, create the mechanism. It took some time to figure out how the mechanism fit together so that it could properly be re-assembled later. A number of photos were taken with the cabinet partly apart to capture the location of these parts, as well as a video taken to show how the parts moved as the keyboard cover was moved toward the dispay to cause the display cover to flip up and the power switch to be moved to the ON position. This documentation proved to be invaluable in putting it all back together after repairs to the calculator were made.

Original Nickel-Cadmium Battery Pack from the SCM Marchant I

Once the calculator had been disassembled, concerns over the NiCd battery pack leaking were proven legitimate. The battery pack is located in the top center of the lower part of the cabinet. It was immediately obvious that the batteries were most certainly the originals that came with the calculator when it was new. It was no wonder that after all this time that they were leaking, and probably had been for a very long time. The ends of the battery pack were covered in a grayish-white fuzz, and the body of the batteries had a greenish tint to their metal surfaces, both being clear signs of leakage. Cadmium, one of the components of this type of battery, is a poisonous metal. Leaking NiCd cells must be handled very carefully, and also disposed of properly, as they are definitely a hazardous material. Chemical resistant gloves were used to carefully extract the battery assembly from the calculator cabinet. The copper wires leading to the battery assembly were clipped and were found to be completely corroded inside the insulation. The battery pack consists of five AA-sized NiCd cells connected in series to create a 6 Volt battery supply. Normal AA-sized alkaline batteries deliver 1.5 Volts, but NiCd batteries provide 1.25V each, which is why it takes five of them wired in series to make 6 Volts. The five cells were arranged around a dense cardboard cylinder to provide a base for the cells to form a pentagonal arrangement. The cells were adhered to the cardboard cylinder with something like rubber cement, which had mostly disintegrated due to the corrosive action of the leaking battery cells. The cardboard cylinder was able to be withdrawn from the package without damage, and was able to be cleaned up for re-use in constructing a new battery pack. The old cells were wrapped in aluminum foil. The foil package was then sealed up with duct tape, then placed into a thick plastic bag. The cells were taken to the county's hazardous materials disposal station for proper disposition. All batteries, but most especially Nickel Cadmium and newer Lithium Ion batteries must always be taken to a hazardous material disposal site, as the metals used in their creation, even common single-use batteries, are very hazardous to the environment. Batteries of any sort should never be thrown out for normal trash collection.

The leaking battery pack was fortunately located in a position that kept the leakage from getting to the main logic board and power supply of the calculator. However, the jack that the external charger plugs into, as well as the wiring associated with that jack were definitely victims of the corrosive nature of the gas and fluid leakage from the old battery pack. The power jack was quite green from corrosion, and the wires soldered to the terminals on the jack literally fell off with the Copper conductors turned into greenish-white powder as the jack was carefully being removed from the calculator chassis.

The power jack was placed in a bath of De-Ox-It®, an effective and reasonably safe chemical for removing oxidation from electronics. De-Ox-It is commonly used for removing oxidation from edge connector fingers and sockets. It is an indespensible tool for the restoration efforts of the Old Calculator Museum. A soft brush was used to gently clean the jack using the De-Ox-It liquid, and the results were quite satisfying, as is generally the case with this product. The greenish oxidation dissolved away, revealing clean metal (which appeared to be a Silver layer plated over Copper) contacts for the jack as well as the solder terminals for connecting wires to the jack.

It was also noted that there was a Micro-Switch® brand switch located such that the metal actuator lever of the switch would be pressed down, activating the switch when the power plug was inserted in the jack. The metal lever of the microswitch was very corroded, and, like the power jack, the wiring to the switch was also corroded beyond re-use. The switch was removed, tested, and was found to no longer work properly. The switch likely had internal corrosion due to leakage from the battery pack.

Fortunately, an exact replacement for the failed microswitch was found in the museum's supply of spare parts. Since the wiring related to the power jack, the microswitch, the power switch, and the battery pack was corroded from the battery pack leakage, it was all replaced with wire of the correct gauge, type, and insulation color.

Restored Power Jack and Microswitch

A replacement battery pack was fabricated using new, higher-capacity AA-cell Nickel Cadmium batteries. NiCd batteries (versus newer technology rechargeable cells such as Nickel-Metal Hydride or Lithium Ion) were used because the charging circuitry in the calculator is designed for NiCd cells, and the use of newer-tech rechargeable batteries could result either in damage to the charging circuitry, or incorrect charging of the batteries. The higher capacity of the new batteries will allow for a longer run-time on battery power than the original batteries at the expense of a longer recharge time.

The Top Section of the Marchant I with Replacement Battery Pack and Power Supply Board in Place
Note Magnets Embedded in Keystalks of Keyboard to Activate Magnetic Reed Switches

The five new cells were fully charged before the battery pack was put together to facilitate testing without having to charge them using the charging system...eliminating a potential failure vector. The cells were adhered to the original carboard center tube using rubber cement. Jumper wires were carefully soldered to the cells to connect the batteries in series, and the appropriate red (positive) and black (negative) wires were soldered to the battery pack to connect it into the circuitry. The battery pack was wrapped in a layer of high-quality electrical tape to cover the jumper wires to prevent any possibility of a short circuit.

Side Profile View of the Marchant I

Once the battery pack, power jack, and microswitch were put back in place, and the wiring all reconnected as it should be, testing was done to make sure that everything was connected up properly, with the connector that goes to the power supply board unplugged just in case there was any error made in the wiring. Everything tested out OK, so the next thing to do was to re-assemble the machine, and give it a try on battery power.

Re-assembly of the calculator was a bit of a challenge, owing mainly to refitting the mechanism that pops the display lid up and turns the power switch on when the keyboard cover is slid up toward the display. While the the mechanism had been well-documented, fitting it all together was a bit fiddly. Once it was all back together, the only thing left to do was to give it a try.

The keyboard cover was gently moved up toward the display, and just before the keyboard was fully exposed, the cover over the display quickly flipped up, and the power switch flipped to the ON position. The battery meter, indicating the state of charge of the battery pack, jumped to the black zone indicating the battery charge was good, and low and behold, the gorgeous geniune-Burroughs Nixie tube display immediately lit up with "00000000". (Note the highlighted least-significant digit, more on that below.)

The Battery Charge Meter to the Left of the Power Switch

To the left of the power switch is a tiny meter that indicates the state of charge of the battery. When the calculator is powered by the battery pack, a needle (not visible in the photo above because the calculator is turned off) moves to show the charge level. If the needle moves into the black area of the meter, the battery charge is good. If the meter registers in the red area, the charge is low, though the calculator will still operate until the needle is deep into the red zone. The user manual states that the calculator should either be powered off or connected to the AC power adapter when the meter reaches the middle of the red zone.

The SCM Marchant I, introduced in the late fall of 1970 for $495, is one of the earliest so-called "hand-held" portable, rechargeable battery-powered calculators produced. It is somewhat difficult to classify the Marchant I and its contemporaries as truly hand-held, as the Marchant I is 10.2 inches long, 4.8 inches wide, and 2 inches tall, weighing in at a hefty 2.4 pounds. There weren't all that many portable rechargeable battery-powered electronic calculators on the market at the time, but by late 1970, it became clear that the era of the "mini calculator" was upon us. By the mid-part of 1971, just about every major calculator manufacturer had a rechargeable battery-powered electronic calculator on the market.

The Canon Pocketronic and Monroe 10
Image of Monroe 10 Courtesy of Serge Devidts, Calcuseum

Among the other early players in the rechargeable battery-powered "mini" calculator marketplace was Canon's Pocketronic and Monroe's OEM copy, the Monroe 10. The Canon Pocketronic became a consumer product as a result of an internal skunk-works project at Texas Instruments initiated in the mid-1960's called "Cal-Tech", a calculator way ahead of its time using Large Scale bipolar discretionarily-wired gate-array technology. Cal-Tech never became a consumer product and remained an engineering exercise, though a few of the machines were manufactured. The Pocketronic came out literally years after the Cal-Tech project as a result of a partnership between Canon and Texas Instruments. The Pocketronic definitely benefitted from Texas Instruments' Large-Scale IC technology as it only required a three-chip chip set for its calculating electronics.

The Sanyo ICC-0081

Another an early player in portable rechargeable battery-powered electronic calculator market was the Japanese electronics giant Sanyo, whose ICC-0081 and ICC-82D calculators were also a handful. Unlike the Marchant I and Canon Pocketronic, these calculators offered an easily replaceable Nickel-Cadmium battery-pack. Along with this nice feature, the Sanyo machines had the AC power supply and battery charging circuitry built-into the calculators, eliminating the need for an external power pack that was required by most mini calculators on the market at the time. The ICC-0081 had a compartment on the bottom of the calculator that the AC power cord could be stashed in. When AC power was needed, the compartment lid could be removed, and the power cord pulled out and plugged into a standard AC power outlet. This feature made the ICC-0081 calculator more portable than all the competitor's machines because it required no external devices for providing AC power.

Sanyo enlisted the help of a US chip-maker, General Instruments, to design and (at least initially) fabricate the four-chip LSI chip set that made up the calculating logic of both of the calculators.

The Sharp discretionary-8B. Note "Docking Station" AC Power/Recharger
Image Courtesy of the Smithsonian National Museum of American History, Kenneth E. Behring Center

Sharp's entry in the fray was their QT-8B calculator, which was a modification of the history-making Sharp QT-8D, the second commercially-available AC-powered electronic calculator to use only Metal Oxide Semiconductor (MOS) Large-Scale Integration (LSI) integrated circuits for its calculating logic (the first was the Victor 3900, introduced in 1965). Like all Japanese calculator companies at the time, Sharp had to go to a US chip-maker, the Autonetics division of North American Rockwell, to develop and fabricate the LSI chips that made the QT-8D possible. At the time, Japanese integrated circuit manufacturers (primarily NEC, Toshiba, and Hitachi) did not have the capability to produce Large-Scale MOS integrated circuit devices in production quantities, although they were learning and developing the technology at an amazing pace. The QT-8B was identical in function and size, as well as using the same chips as the QT-8D, but was slightly heavier due to the weight of the internal Nickel Cadmium batteries. The QT-8D had a docking station that it slid into that provided the charger for the machine, as well as providing a means to operate the calculator on AC power.

The Sharp EL-8

Not all that long after Sharp introduced the QT-8B, Sharp's engineers were put to task to cut the size of the QT-8B roughly in half, to make a truly hand-held (but certainly not pocketable) calculator, which was introduced in Japan as the Sharp EL-8 in January of 1971. The EL-8 used essentially the same chips as the QT-8D/QT-8B, but the design of the chips was modified to reduce their power consumption to allow longer battery-powered runtime in a much smaller package than the QT-8B. The electronics of the Sharp EL-8 were sold as an OEM product to Swedish calculator manufacturer Facit, who packaged the electronics in their own somewhat boxy cabinet, and introduced the Facit 1111, and the Addo-X (A Facit subsidiary) 9364.

Model/Serial Number Tag on the Back Panel of the Marchant I

While the Marchant I was apart, the various components of the Marchant I were photographed. Once sufficient photos had been taken, the calculator was carefully re-assembled, being mindful to properly refit the battery pack, and the tricky mechanism for the keyboard and display cover. The cabinet was secured with its original standard Philips-head screws, as well as two more Phillips-head screws of the proper size, thread pitch and length rather than the two fussy proprietary anti-tamper screws, though the original screws were kept.

The next step was to test the external power supply. The supply is specified to provide 9 Volts DC nominally under load. It was plugged into a Variac, and the mains voltage ramped up slowly while monitoring the power supply output with a digital voltmeter and an oscilloscope; the voltmeter to watch the voltage, and the oscilloscope to monitor the ripple. As the mains voltage reached 115 Volts AC, the output of the power supply was running stable at around 12.2 Volts, and there was approximately 12 millivolts of AC riding on top of the DC voltage. The voltage seemed a bit high to me, it was decided to make a dummy load to simulate the load the calculator places on the power supply. The label on the calculator indicates that it draws 3 Watts at 9 Volts, which would be 1/3 Amp. Using Ohm's Law, the resistance of the calculator would be approximately 27 Ohms. Some power resistors were connected together to provide a dummy load of approximately 27 Ohms with a dissipation capacity of 5 Watts in order to serve as an approximation of the load the calculator would put on the power supply. This dummy load was connected to the power supply, and the voltage and ripple monitored. The voltage was stable at around 8.9 Volts, and the ripple actually decreased to about 6 millivolts. It appeared that the power supply was in good condition.

The power supply was connected to the calculator, the machine powered up, and it operated properly with no drama. While the external power supply is connected, the calculator runs from the external supply and the batteries are also trickle charged. If the calculator is powered off with the external power supply connected, the batteries are charged to full at a higher charging current, then a light trickle charge is kept on the cells to keep them topped off. The circuitry that manages the charging is located on the power supply board within the calculator.

The Bottom Side of the Marchant I
Note Fold-out Legs

In late January of 2022, the museum received an Email from Mr. Warren Parks indicating that he had the main electronics circuit boards from both the AMI and TI versions of the SCM Marchant I calculator, and that he and his wife Amy would be interested in donating the boards to the Old Calculator Museum to add to the museum's exhibit for the Marchant I. On February 7, 2022, the skillfully packaged boards arrived at the museum. The AMI logic board had one of the Nixie tubes broken (as it was before shipped), and one of the glass-encapsulated reed switches for the keyboard was missing. These faults will be repaired as the museum has spares that can be used to replace the damaged/missing parts. The TI logic board was complete and in very nice condition, including the three chip Texas Instruments-fabricated calculator chip set. The Old Calculator Museum curator is very grateful for this fantastic donation, and wishes to sincerely thank Warren and Amy Parks for their kindness. This donation made a great addition both to the physical museum presentation for the Marchant I, as well as in this exhibit, providing documentation of the Texas Instruments logic board and chip set.

The SCM Marchant I is a curious mix of advanced integrated circuit technology, with the beautiful display technology of Nixie tubes, which, by the time the Marchant I was introduced, were beginning to fall from favor as a display technology due to their cost and high-voltage power requirement. The Large Scale integrated circuits that were used in the Marchant I were the epitome of technology, while the Nixie tubes used in the display were comparatively ancient technology. SCM went to chip-maker American Micro-systems, Inc. (AMI) to have the large scale ICs for the Marchant I developed. At the time, AMI had perhaps the most advanced MOS/LSI design and fabrication capabilities in the world. The operator's manual for the Marchant I claims that the AMI chips each contain the equivalent of over 1,000 discrete transistors, making them among the most advanced Large-Scale Integration devices in a commercial consumer product of the time. AMI had developed LSI chip sets for SCM's earlier desktop electronic calculators, which included the SCM Marchant Cogito 412, SCM Marchant Cogito 414, and the SCM Marchant 410.

When SCM selected to AMI to create the chips needed to make their mini calculator, they had asked AMI to try to create a single chip that would contain all of the calculating logic needed for the calculator. However, once AMI looked into the realities of doing so, they said they believed that they could indeed fit all of the logic onto a single chip, but convinced SCM that a more conservative two-chip design would be more time- and cost-effective. SCM had great confidence in AMI, but out of business prudence they later contracted with Texas Instruments (TI) to design and develop a chip set that was functionally equivalent to those that had been successfully created by AMI. Having a second source for any advanced technology used in a commercial product is insurance against unforeseen problems such as labor issues, process faults, and the proverbial "Murphy's Law" that can strike at any time, creating the possibility that production of the product could halt due to a lack of availability of the critical technology used.

A photo of worker at SCM factory installing the edge-mount chip socket on a Marchant I calculator logic board

AMI suggested to SCM that they may want to consider the use of its edge-mount LSI package for the Marchant I. This relatively new LSI IC package technology had the IC mounted vertically in a special connector that the package plugged into that had all of its connections along one edge of the package. The package was similar in concept to today's DIMM modules used in computers. SCM had been the first large-scale user of this IC packaging technology in its earlier desktop calculators, the Cogito 412 and Cogito 414, but was a bit concerned about using this type of package in a portable calculator, because it tended to be subject to more physical forces than the typical desktop calculator. The concern was that the connections between the edge-lead chip and the socket could suffer failures from the shocks that a portable calculator can receive when carried around in a briefcase or handbag. SCM had not suffered any problems with the packages in the desktop calculators, but a desktop calculator is typically not subjected to the same forces as a portable calculator would be. SCM decided to go ahead with the use of the edge-mount chips in the Marchant I because of the overall lower-cost of the packaging, and ease of installation during manufacturing, despite the concerns. As it turned out, there were very few problems that were associated with using this technology, and as a result of doing so, SCM was able to decrease the size of the main logic circuit board versus the use of Dual-Inline Packages (DIP), as well as saving money on manufacturing and service cost, all very important factors in a very competitive marketplace. The edge-mount package resulted in an average of about a 40% decrease in circuit board real estate as opposed to the standard Dual-Inline Package (DIP), at the expense of some vertical height requirement. The vertical height issue wasn't a problem with the Marchant I, as more vertical height for the cabinet was required for the Nixie tubes, leaving plenty of room for the vertically-mounted LSI packages. With these thoughts in mind, SCM opted to go ahead with the edge-mount packages in the Marchant I.

AMI began to produce the two-chip edge-mount chip set in volume, and SCM began cranking out the calculators and shipping them to distributors. Since TI didn't yet have their chips ready when the Marchant I was introduced, the early production Marchant I calculators were all produced using the AMI chip set. Once Texas Instruments completed their chip layout, it was clear that it was going to require three chips to contain the logic that AMI managed to pack onto two chips. SCM decided this would be acceptable, and that they would simply make two different versions of the main logic board of the calculator, one that would use the AMI chips, and another that would use the Texas Instruments chips. The logic boards were virtually identical other than an additional edge-mount socket for TI's third IC, and differing interconnect wiring between the chips and to the chips. All other aspects of the board, including the magnetic reed switch array for the keyboard, the connector to the power supply board, as well as the circuitry for driving the Nixie tubes. Even test points (used during manufacturing test) located around the periphery of the lower part of the logic board are identically located.

Texas Instruments claimed that SCM had specified in their specifications that three chips were expected, SCM claimed that the design specifications given to TI requested that the logic fit onto a single chip, just as had been specified to AMI. Something doesn't quite ring true to TI's claim that SCM requested a three-chip design given that the original specification to AMI stated a single chip. While the design and manufacture of a separate circuit board for the TI three-chip set did involve some extra expense, the benefits of having a second source for the ICs outweighed the cost issues. Once TI's three chip set was available in production quantities, SCM began manufacturing Marchant I calculators with either chip set depending on availability of chips at the time of production.

The edge-mount AMI Chip set installed in special sockets made by Litton Industries under license from Texas Instruments

  

The AMI Chip set edge-mount LSI packages (Left-479A, Right-480B)
These devices are later than those in the exhibited calculator with date codes in February, 1972, and May, 1971
Sincere thanks to Warren & Amy Parks for their donation of the Marchant I logic board that contains these devices

The edge-mount chip packaging consists of a rectangular ceramic base (manufactured by Coors Porcelain, Inc., a division of the famous Coors Beer company) that contains a circular depression in the center, with a gold base metal in the bottom of the depression. The IC chip is bonded to this gold base. Arrayed around the circular inset where the chip is mounted, there are 40 gold contact points, with screen-printed metal traces leading away from them to one edge of the ceramic base. Tiny gold wires are bonded from contact points on the chip to gold contact points arrayed around the inset of the package, providing connections from the chip to the 40 fingers at the edge of the chip. A gold-plated metal lid is then sealed over the chip using a low melting point glass called "frit" to hermetically seal the chip and its delicate connections underneath. The completed package measures approximately 3 1/2" x 1 1/4", and plugs into a special socket mounted on the main logic board. The socket provides a secure connection between the traces on the edge of the packaged to the rest of the circuitry using springy gold-plated contacts, as well a pair of pressure springs that help secure the package in the socket. The socket is made by Litton Industries (under license from Texas Instruments), a company that specialized in aerospace, military and defense electronics that also ended up in the calculator business when it purchased the Monroe Calculating Machine Company in 1958). Oddly, no Litton/Monroe calculator found thus far seems to have used this technology.

The AMI-made chip set has part numbers of 0479A and 0480B. The chips in the exhibited Marchant I are date-coded in the last week of 1970 (7052), meaning that the chips were manufactured sometime between 28th and 31st of December, 1970. It's interesting to note that the 0479A chip appears to have had some original markings on the chip scuffed off and new labeling printed. It is not known why this occurred, but it is clear that the ICs are the originals in the exhibited calculator due to the coincidence of the date codes with other components in the calculator. It can only be guessed that perhaps during manufacturing, incorrect labeling was put on the chip, and the incorrect markings were scuffed off and new labeling applied rather than rejecting the entire package.

  

The Main Logic Board of SCM Marchant I utilizing (Left) two-chip American Micro-systems, Inc. LSI chip set, and (Right) logic board with Texas Instruments three chip LSI chip set
Sincere thanks to Warren & Amy Parks for donation of the SCM Marchant I logic board with the TI Chipset pictured above

The TI chip set has part numbers of TMC 1771, TMC 1772 and TMC 1773. The "TMC" part number prefix is indicative of a custom chip designed and fabricated specifically for a Texas Instruments customer.

  

The Texas Instruments-made Three-Chip Edge-Mount LSI Chip set
TMC1771, TMC1772, and TMC1773, all with date codes from October, 1970
Many thanks to Warren & Amy Parks for their donation of two SCM Marchant I main logic boards that contain these devices

The logic board of the calculator has the two (or three in in the case of the TI-chip version) LSI ICs that contain the calculating logic, as well as 32 discrete transistors used for driving the Nixie tubes. Also included are fifteen hybrid circuit modules (brown and orange single-inline packages) that contain resistor networks. Along with the resistor hybrids there are a number of discrete resistors and diodes used on the board. The Nixie tubes are mounted to their own circuit board that is connected to the main board by stiff wire jumpers. A black plastic fixture that attaches to the logic board by a single screw supports the Nixie tubes at the correct angle for proper viewing, as well as preventing the tubes from touching each other. The logic board has the array of eighteen glass-encapsulated magnetic reed switches that are actuated by magnets on the end of the key-stalks of the keyboard keys. The logic board also has a fourteen-pin dual-inline plug that is wired to the board with multi-colored Spectra cable. This plug provides power and master clock signals to the logic board. The plug is inserted into a standard 14-pin dual-inline IC socket on the power supply circuit board. One pin on this plug is clipped off, and one pin in the socket is plugged to insure that the power cable cannot be plugged in incorrectly. This connector provides power and clock signals to the logic board, along with negative and overflow signals from the logic board to the power supply board.

The Nixie Display Board

The eight Nixie tubes, genuine Burroughs type B-5750 tubes, are soldered to a small daughter board that has the wiring for the tubes. Each tube contains cathodes in the shape of the digits zero through nine, stacked atop one-another in the tube, isolated from each other by ceramic separators. The tube also contains a right-hand decimal point. A fine anode screen is positioned in front of the stack of digits and decimal point. The digit and decimal point pins from all of the tubes are wired together via the circuit board they are connected to, e.g., all of the digit 1 cathodes are wired together, all of the digit 2 cathodes are wired together, etc. The common digit and decimal point cathode lines are connected to the main board via the wire jumpers to the logic board. Each tube's wire-mesh anode is brought out via 8 wire jumpers to the main board. Because the display is multiplexed, each digit in each tube is lit for a short period of time, with only the digit tube to be lighted having a potential applied to its anode to cause that tube to light up. The digits are displayed sequentially one tube at a time at a rate fast enough that the human eye perceives all of the digits as being lit at once. Multiplexing the display saves considerably on power consumption (especially important for a battery-powered calculator), as well as being a natural fit for the bit-by-bit serial processing performed by the logic of the calculator.

The Burroughs B-5750 Nixie Tube used in the Marchant I

Along with the main logic board, there is a second smaller circuit board, packed with discrete components, the power supply board. The power supply board is responsible for battery management (charging and monitoring of the battery voltage), generating the master clock signals for the calculator logic, generating the proper supply voltages for the integrated circuits, driving the negative and error indicators, and generating the high voltage for the Nixie tube displays, which operate at approximately 180 Volts. All of these functions make the power supply circuit board rather densely populated with components, although most all of the components are common, low-cost, commodity transistors, diodes, resistors, inductors, and capacitors. In order to efficiently generate the voltages required for the ICs and the Nixie Tubes, a switched-mode power supply (SMPS) design is used. Today, switch-mode power supplies are used in just about every piece of electronic equipment one can imagine, as they are vastly more efficient, lightweight, cost less, and are significantly smaller than traditional linear power supplies even though they are more difficult to design, more complex, and require more components than a traditional linear power supply. The added complexity is offset by the fact that the large, heavy, and rather expensive transformer used in linear power supplies is no longer needed since the transformer in a SMPS can be much smaller since it is running at a much higher frequency than the AC power line.

The SMPS in the Marchant I has a transistorized oscillator that runs at a frequency somewhere between 10KHz to 20KHz (10 to 20-thousand cycles per second). The oscillator is powered by the 6 Volt DC power from the battery pack/AC adapter. The oscillator generates a rapidly alternating current (AC) that is fed to the primary windings of a small transformer (the square metal block with "TDK 11411" label in the photo below). This transformer has numerous secondary windings that emit AC voltages that are rectified by diodes and smoothed by capacitors into direct current (DC) voltages that are close to the required voltages for the ICs and Nixie display. These DC voltages are then regulated by transistorized circuitry to yield the appropriate supply voltages needed by the ICs and the Nixie tube driver circuitry.

The use of a SMPS is imperative in a portable, battery-powered calculator. A traditional linear power supply would be far too bulky to fit inside a manageable portable package, would weigh too much, be moderately more expensive, and its inefficiency would not lend well to useful run-time on battery power.

Along with all of the power supply and clocking circuitry, the power supply board contains two neon lamps (visible at the top right edge of the board pictured below) used for the error and negative indicators along with their discrete transistor drivers. The neon lamps shine through translucent white plastic inserts in the keyboard panel with "-" and "E" legends printed on them. The 14-pin DIP socket on the power supply board receives a plug-in connector from the main calculator board, and provides the master clock signals, battery level, error and negative indicator signals, and a ground reference. A six-pin connector at the left edge of the power supply board provides the logic supply voltages for the ICs and the high-voltage Nixie display supply to the logic board, as well as a ground return.

The Power Supply Board

The Marchant I is a four-function, eight digit calculator with automatic decimal point placement. It is a very basic calculator, but at the time, its ability to operate from battery power, allowing calculation to occur anywhere the calculator could be carried, was a huge advancement. Using the Marchant I is a little obscure, and could it appear to someone without an understanding of how it operates that it is malfunctioning. Once the nuances of operating the calculator are understood, it is easy to use, but does take a bit of getting used to. The operator's manual very clearly explains the use of the calculator with a great many examples to help with understanding how the machine is to be operated.

The "E" Error Indicator (blinks when active)

When powered-up, the calculator is automatically cleared and the display comes up with "00000000". Note the bold right-hand digit. In order to conserve power, and also to make the display easier to read, insignificant leading zeros are displayed as dimmed. It is interesting to note that when the calculator is powered or on cleared, there is no decimal point shown on the display. Digit entry occurs with digits entering from the right, shifting any existing digits to the left. No decimal point appears during entry unless the decimal point [.] key is pressed. Entering too many digits in front of the decimal point will result in the calculator going into Overflow mode, which causes the "E" neon indicator on the front panel (above the [C] key) to blink, and all keyboard keys except the [C] key, to be ignored. Pressing the [C] key will clear the error condition, and clear all of the calculator's registers.

A Close View of the Marchant I Nixie Tube Display

The Marchant I has three working registers. There is the "Visual Display" register, from which the Nixie display is generated. Numeric entry into the calculator, as well as results of multiplication and division are placed in the Visual Display register. Along with the Visual Display register are the non-displayed "Add" register, an accumulator-type register that is used for addition and subtraction operations; and the "Calculation" register, where multiplication and division are performed.

Addition and subtraction are the functions on the Marchant I that can be a little obscure to understand. Familiarity with SCM/Marchant's earlier LSI-based desktop calculators (Cogito 412/414) is a great help here, as the Marchant I operates in a similar fashion. The key concept to understand operation of addition and subtraction on the Marchant I is that add and subtract operations are performed in the non-displayed Add Register. When the calculator is powered-up, or cleared by the [C] key, the "Add Register" is set to zero. Numbers entered into the "Visual Display Register" are added to the "Add Register" by pressing the [+] key, and subtracted from it by the [-] key. The trick, though, is that when the [+] and [-] keys are pressed, whatever number that is in the "Visual Display Register" remains as it was before the operation key was pressed. This behavior is what could be confusing, because most calculators post the result of the calculation when the [+] or [-] key is pressed. As an example, on a similar calculator from the time, the Sharp QT-8D, the calculation of 12 + 14 would be entered as [C] (to clear the calculator), 12 [+=] 14 [+=], and the answer (26) would appear in the display. Performing the same key presses on the Marchant I would result in "00000014.". To an unknowing operator, it would seem that the calculator ignored the second [+] key press, which could lead one to believe that the calculator is malfunctioning. In fact, this is exactly how the Marchant I is supposed to work. The reason is that the content of the "Add Register" is not displayed until the [T] key is pressed. Performing the above calculation, then pressing the [T] key will result in the correct sum of "00000026." appearing in the display. The [T] key copies the content of the "Add Register" into the "Visual Display Register", and then clears the "Add Register". To continue to use the answer for further addition/subtraction, it is necessary to press the [+] key to re-add the number into the (now zero) "Add Register".

Multiplication and division operate conventionally, with these operations occurring in the "Calculation Register". Pressing the [=] key completes a multiplication or division operation, and copies the content of the "Calculation Register" into the "Visual Display Register".

The Negative Indicator

The Marchant I supports negative numbers, with a negative result lighting the "-" indicator located above the [T] key. Both the "-" and "E" indicators are lit up orange by neon bulbs located on the power supply board, situated beneath the white plastic diffusers on the keyboard panel.

The SCM F-80 Version of the Marchant I
Photo Courtesy of Serge Devidts, Calcuseum

Later in production of the Marchant I, SCM created another version of the calculator known as the F-80. As far as is known at this point, the only difference between the Marchant I and the F-80 is the color scheme. Internally, the calculators are identical. The F-80 upper cabinet is a light tan color, with the same black lower cabinet as the Marchant I. The keyboard function keys ([+], [T], [C], [÷], [X] and [=]) are brown versus the black function keys on the Marchant I. Other than these cosmetic changes, the F-80 functions exactly the same as the Marchant I. The likelihood is that SCM re-introduced the Marchant I as the F-80 at a reduced price after the machine had been on the market for a while, possibly as a marketing ploy to wring additional market lifetime out of the machine. At the time the F-80 was announced calculator technology was advancing at a break-neck pace. Manufacturers had to do everything they could to maximize the market lifetime of their calculators in order to recuperate development costs and realize a profit.

The Marchant I is about average in speed for calculators of the time, performing addition and subtraction in under 10 milliseconds. Multiplication can take as long as 3/10ths of a second, and division up to 1/3rd second. The worst-case division, 99999999 ÷ 1, indeed takes what is observed to be around 1/3 second. During the time that calculation is occurring, the display is left active, but since math operations are carried out in separate registers from the display register, there is little activity to be observed.


Sincere thanks to Mr. Steve Stroh for donation of this wonderful calculator, two copies of the instruction manual, and the external power pack, to the Old Calculator Museum. This exhibit is made possible through this very generous donation.

Many thanks to Warren and Amy Parks for donation of two original logic circuit boards from the SCM Marchant I calculator, one with two chip LSI chip set fabricated by American Micro-systems, Inc., and the other board utilizing the three-chip LSI chip set produced by Texas Instruments. Photos of the LSI devices, as well as the TI logic board were made possible through this generous donation.
Text and images Copyright ©1997-2024, Rick Bensene.

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