Sperry Remington 661D Handheld Electronic Calculator
This is an unusual, and somewhat rare machine. My guess as to the date of this machine is sometime around late 1973 or early 1974. This calculator was given to my brother as a birthday gift by my parents. Years later, my brother gave it to me, and the machine is still in very good condition.
The Sperry Remington 661D is unusual in that it has only a 6-digit display, with a special key () on the keyboard that displays the remaining digits of any result that is larger than 6 digits when pressed and held. This arrangement was common for a short time on some early handheld calculators (Casio and Sperry/Remington) because the most expensive part of the machines were the display elements, and the cost difference between a 6 digit and an 8-digit display was significant.
First part of 12345 X 23456 = 289564320
The display technology used is individual tube-type vacuum fluorescent seven segment displays. The 7-segement representation is fairly standard with the exception of the zero, which is half-height.
Last 3 digits of 12345 X 23456 (displayed when 'arrow' key held down)
The machine uses a single IC (Hitachi HD 32154P) for its (rather simple) brains, and another chip apparently for a display driver (NEC µPD129C).
The inside view of the 661-D
A small board underneath the main board is a small switching power supply to derive the various voltages needed by the calculator chip and display drive, as well as generating the master clock for the IC. The calculator is powered by 4 AA-cell batteries, and also has a jack for an external 6V DC power supply.
The 661D does fixed point math, with the decimal point being positioned by a slide switch at 0 or 2 places. The machine has no indicator for error conditions, opting to simply lock up, requring a press of the [C] key to restore the machine to normal operation