The APF Mark 42 is an LED calculator from the late 1970s. With its compact size and bright red display, it exemplified the LED calculator era before LCD displays took over in the 1980s.
The Mark 42 measures just 67 x 126.5 x 27.1 mm and weighs only 70g without batteries. It is powered by a single 9V battery or an optional 7.5V AC adapter. The two-piece black plastic case has an aluminum front panel for the display and branding. The display itself is an 8-digit red LED module with bubble lenses for a crisp and vivid readout.
Despite its small form factor, the Mark 42 has the full set of basic four-function math operations, plus a percent key. The plastic keys have a soft click when pressed. There is a “CE” (clear entry) key to delete the current number entered, and a “C” (clear) key to reset the entire calculator.
The logic of the Mark 42 is fairly simple. It does not show negatives with a leading minus sign, instead indicating a negative number by showing the minus on the digit just before the units digit. This means the largest negative number it can display is 7 digits. Overflow is indicated by flashing digits, recoverable with the CE key. Divide by zero also flashes a zero, again recoverable. One quirk is that it treats decimals in a fixed manner instead of floating – if you enter 1 + 100 =, it will show 1.000 and keep 3 decimal places until you need more precision.
Inside the Mark 42 is just a single chip – the Texas Instruments TMS0972 LED driver/processor, and an 8-digit National Semiconductor LED display module. This minimal component count helped APF produce it very cheaply. The main circuit board sits loose on top of the keyboard.
The Mark 42 came in a compact box suitable for hanging in stores. It included a detailed 18-page manual. As one of the last LED calculators before the transition to LCDs, the Mark 42 makes an interesting collector’s item from the 1970s.