The Victor MEC/225 is an early-1974 vintage desktop/portable calculator. The calculator is a prime example of the transitional machines between the AC-only powered desktop calculators, and truly 'handheld' battery-powered calculators. The MEC/225 is powered by an external AC adapter, which doubles as a charger for internal Nickel-Cadmium batteries that can power the machine when away from mains power.
A view of the MEC/225 guts
The MEC/225 is quite a loaded machine for a 'portable' calculator of the day. The machine is a bit of a handful to be considered a handheld calculator, but it is small enough to be considered truly portable. It provides a full 12-digits of capacity, rare for a machine of this size. It is also fairly capable, providing the standard four functions, percentage calculations, and a memory register/accumulator. The 225 uses 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 brain of the machine is a single 15500 LSI IC made by Rockwell. A significant compliment of discrete components make up the display drive, power supply, and clock circuitry of the machine.
A closer view of the LSI and other circuitry of the Victor MEC/225
The Victor MEC/225 is 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 "-" 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, and the "MRC" key recalling the memory register to the display on the first press, and clearing the memory register on a subsequent press. 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, the 225 has an unusual key on the keyboard with an 'arch', under which the word "SET" is printed. This key, like on the 1800, is used to set the round-off function. 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 looked at to perform the round-off function. By default at power-up, the 'round off' function is set to 2 digits behind the decimal point.
The keyboard of the MEC/225 is very 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 keys. This design makes for a very reliable keyboard that is free from the problems created by other keyboard designs that use switches or contacts. 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 of the MEC/225 (note unusual negative indication)
The 225 indicates 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. The error/overflow condition can be cleared by pressing the "C" or "C ALL" keys. The sign of the number on the display is indicated in an unusual fashion, with negative numbers denoted by a '-' segment that lights up which is located below and slightly to the right of the right-most digit of the display. Speed-wise, the 225 is 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.