1971's Smallest Battery-Powered Calculator: Dictaphone 1680

[robve]

Luckily I won an auction on a working Dictaphone 1680 with carrying case, original manual and AC supply. I paid far less than what other units were sold for. But it needs a thorough cleaning and a new NiCD battery.

For your viewing pleasure I've scanned the Dictaphone 1680 manual to PDF.

After spending hours carefully prying the hardened rubber feet off with the aid of some alcohol and WD40 to dissolve the rubber (it hardly did), the four screws that hold the unit together finally came loose. Luckily, there was no corrosion on the PCB caused by the original battery pack.

Next I went to remove the deteriorated brownish felt padding from the battery pack and screen edges, which took some time to remove with wet Q-tips to stick small fluffs of loose felt onto. This stuff gets everywhere if you're not careful!

Some things that I found interesting about this early electronic calculator.

It does not show negative results. Rather, it shows its 10s complement. For example -3 is 99999999 99999997. Pressing the minus key again flips the value to positive.

Another quirk of this machine is that multiplication and division cannot be chained directly, but requires pressing = before the next multiply or divide.

It has a "Sales Price Calculation" feature. Pressing 0.25x12500= gives the profit 3125 for a 25% profit rate on a $12500 cost. When pressing = again twice more, you get the sales price 15625. I assume this feature is very handy for accountants and CFOs. I can only speculate that this was a major selling point.

Surprisingly, this calculator has constant calculations (e.g. pressing 3x2= gives 6, then pressing 4= gives 8), which is one of the first (I believe) to have this feature. Please correct me if I'm wrong.

The display continuously shows the current value of the X register accumulator, which means that it shows the division process on the display as the result in register X is updated. This is similar to mechanical calculators of the past.

These early PMOS ICs had no non-volatile memory (i.e. no flip-flops, SRAM) and had to shift and rotate the digits of a register continuously around to maintain the register state. See Reverse-engineering an early calculator chip with four-phase logic (the reverse-engineered PMOS-based Sharp EL-8 is from the same period 1970/1971 with Rockwell LSI whereas the Dictaphone 1680 / SANYO ICC-82D and SANYO IC-0081 Mini Calculator have General Instrument LSI.)

- Rob

EDIT: corrected a minor typo.