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Victor MEC/223 & MEC/225 Portable Calculators

Updated 4/24/2019

The Victor MEC/223 and MEC/225 are early-1974 vintage desktop/portable calculators. The machines are prime examples of the transitional calculators that bridged the gap between AC-powered desktop calculators and truly handheld battery-powered calculators. These machines are just large enough that they can't quite qualify as handheld calculators. Though they will fit in a large hand reasonably well, using them has a handheld calculator is a bit unweildly for long-term use. They are much more comfortably used when sitting on some kind of surface to operate. The only difference between the MEC/223 and MEC/225 is the color scheme of the machines, with the MEC/223 color scheme being the classic Victor Green, while the MEC/225 adopts the later Victor burnt-Orange color scheme. Other than the color scheme differences, the internals of the two machines are identical, and the calculators operate identically. Apparently Victor felt that it was necessary to offer two (as known at this writing) color schemes for this calculator design to allow the buyer a choice of color. The calculators are powered by an external AC adapter, which doubles as a charger for an internal Nickel-Cadmium(NiCd) battery pack that can power the machine when away from mains power for a period of about 90 minutes of continuous use.

A view of the insides of the MEC/223 & MEC/225

The MEC/223 and MEC/225 are quite feature-rich for a portable calculator of the day, with twelve digits of capacity (rare for a portable, as most portables had only eight digits of capacity), keyboard settable round-off position, comma indicators, leading zero suppression, and a memory register that operates as an accumulator that can be added to or subtracted from, as well as percentage calculation capability.

Perhaps the best word to describe the usage of these machines is "portable". The machines are well-built, and deisgned to work in a variety of environments, making them very useful as a mathematical workhorse at a job site, or just as easily used as a desktop calculator for an accountant, billing, or payroll personnel.

The Rockwell-made Large Scale Integration Calculator Chip

The brain of the machines is a single Rockwell-made 15500 Large Scale Integration device, in a plastic zig-zag configuration 42-pin package. A significant complement of discrete components make up the display drive, a rather complex power supply, and clock circuitry of the machines. The power supply is worthy of mention, as it creates the necessary voltages to operate the calculator IC, as well as generating the roughly 120 Volts DC required to operate the Burroughs Panaplex II display.

A Close-Up View of the Burroughs Panaplex II Display

The calculators use a Burroughs Panaplex II display, which is somewhat unusual for the portable/rechargeable battery-powered form-factor, due to the high voltages required to drive the display.

The Rockwell LSI and Other Circuitry of the Victor MEC/223 & MEC/225

The calculators are quite straightforward in operation, with a couple of minor exceptions. Addition and subtraction operate arithmetically, but the keys are marked as if they operate algebraicly, with [+] and [-] keys rather than [+=] and [-=] as was the common nomenclature for arithmetic-logic machines. Multiplication and division (along with the [%] function) operate algebraicly, with the [=] key generating the result. The memory functions operate as expected, with the [M+] and [M-] key adding/subtracting the display to/from the memory register. The [=+] key performs as the [=] key, but after performing the pending multiplication or division, the resut is automatically added to the memory register, a function useful for invoicing and sum-of-products calculations. The [MRC] key recalls the memory register to the display on the first press, and clears the memory register without disturbing the value in the display if pressed again. The [C] key clears the display, for correcting erroneous input, and the [C ALL] key clears the entire machine except for the memory register and the roundoff digit selection.

Close-Up View of Victor MEC/225 Keyboard

Like another Victor machine in the museum, the Victor 1800, these calculators have an unusual key on the keyboard with a ∩ symbol, under which the word "SET" is printed. This key, like on the 1800, is used to set the round-off position. When pressed and held, the display blanks with the exception of the right-most digit of the display, which shows a single digit between 0 and 9. While holding this key, any digit key pressed will change the displayed digit to the key pressed. This digit represents which digit behind the decimal point will be observed to perform the round-off function. If the selected digit is 5 or more, the next higher significant digit will have one added to it, and the rest of the digits behind it will be set to zero. If the selected digit is 4 or less, then that digit and those behind it will be set to zero, effectively truncating the number at the selected digit. By default at power-up, the round off position is set to 2 digits behind the decimal point, making the machine convenient for financial functions when first powered on.

The Keyboard Keystalks with Pads to Trigger Pads on Keyboard Circuit Board

The keyboard of the calculators is interesting, and rather unusual. The machine uses a capacitive keyboard design. In the first internal view above, you can see a series of black discs on the keyboard circuit board. These discs are sensitive to capacitance, and can be activated by a simple finger touch. The keys themselves, contained in the upper half of the case, have a similar material on the end of the key stalk that provides the small capacitive kick that activates the circuit for the key. This design makes for a very reliable keyboard that is free from the problems created by other keyboard designs that use physical contacts or switches. The keyboard circuit board connects to the main board of the calculator with two ribbon-style cables.

A Close-up View of the Burroughs Panaplex II Display in Operation (Note Unusual Negative Indication)

The 223 and 225 indicate overflow or error conditions by clearing the display and lighting commas on all but the last three digits of the display. The reason the whole display isn't filled with commas in overflow/error condition is that the last three digits of this particular Panaplex display panel do not have comma segments, since there is no need for commas at those display positions. When the machine is in overflow mode, all keys except the [C] or [C ALL] keys are ignored, with either of these keys resetting the machine and clearing the display to zero. The sign of the number on the display is indicated in an unusual fashion, with negative numbers denoted by a "_" segment that lights up, located below and slightly to the right of the right-most digit of the display.

The Tag on the Nickel Cadmium Battery Pack

The Nickel Cadmium battery pack, which consists of six AA-cell 1.25 volt Nickel Cadmium batteries manufactured by General Electric, is situated in the lower part of the bottom cabinet. The battery cells are connected in series, so they generate approximately 7.25 volts. The battery pack connects via short red and black wires from the positive and negative terminals of the battery pack to gold-plated spring terminals that screw into holes in the base of the calculator. The terminals press against similar contacts on the main circuit board of the calculator by spring pressure, providing a solid connection between the battery pack and the calculator electronics. This arrangement made it possible to fairly easily replace the battery pack should the NiCd cells wear out, an operation that could potentially be performed by a careful end-user of the calculator, but Victor strongly recommended that such a battery replacement be done at an authorized service center.

The Power Switch and Power Adapter Socket on the Victor MEC/223

The power switch, located on the back panel of the calculator, has three positions, "OFF", "BATT", and "AC". The AC position is for operating the calculator when the external adapter is plugged into an AC power outlet and the adapter is plugged into the calculator. During use an AC power, the batteries in the calculator are also being charged. When the power switch is in the "BATT" position, the calculator runs on its internal battery power, even if it is plugged into the AC adapter. Obviously, when the power switch is in the "OFF" position, the calculator is turned off, however, the NiCd batteries will be charged if the AC adapter is connected to the calculator and plugged into an AC outlet.

The Molded in Model/Serial Number Identification on the Victor MEC/223

Speed-wise, the calculators are fairly slow, generating the result of the 'all-nines' divided by 1 calculation in approximately 1/2 second. During calculations, the most of the display is blanked, with the exception of the decimal points, which light up while the machine is busy calculating. The MEC/223 and MEC/225 can support division using the full twelve digits of capacity, utilizing a separate internal register to keep track of the divison counter rather than using a digit of the working register as many older calculators did.


Sincere thanks to Mr. Greg Tayman for donation of the Victor MEC/223 Calculator

Text and images Copyright ©1997-2021, Rick Bensene, The Old Calculator Museum.