hitachi kk 221b

here is a fascinating old model from hitachi, possibly from 1972 or so.

the red and black case is rather stylish resembling mr spock's tricorder and the model would have been the height of suavity for sherlock holmes.

at first, it seems like another four function model with a memory but in fact there are many hidden features. the display shows 8 digits but the machine itself calculates to 12 and indeed you can enter and work with 12 digit numbers. the special "<>" display exchange key facilitates the banking of the lower 12 digits and 4 upper. for example, pressing 1234567890 shows 34567890, whence the <> key toggles the display of 12. the display banking feature was also used by the casio memory personal mini to reduce the cost of the then expensive vacuum fluorescent displays. which could also be the reason here, although this model looks like it was not a budget design so perhaps 8 was most you could get at that time. a spare leftmost digit isnt wasted for a minus sign. instead there is a custom digits on the rightmost which can display a dot (overflow) M for memory and a bar for minus. unlike the personal mini, a 12 digit result could be chained on.

general operation is like a desk calculator with the += and -= postfix operation style. for example 2-3 is entered 2 += 3 -= and 7/-3 is entered 7 / 3 -=. 

the three switches control the decimal rounding and tally enablement, and constant operation and the rightmost on/off switch. decimal rounding is interesting, on power up the default is no digits after the point. ie round to integer. to select a decimal place, switch to ds, press exactly one number key and switch back from ds either to the middle position or to the summation position (also enables the tally). the maximum is 7 and the 8 and 9 keys are interpreted as 7 for this purpose.

another secret feature is that the unit can extract square roots! this is accomplished by the sequence / +=. for example, to find the square root of 3, first ensure that 7 digits of decimal are requested eg switch ds, 7, switch back. enter 3 press / then += to get 1.7320508. only 7 digits of decimal may be extracted (not 12) and the result is not rounded in the last place. roots of numbers from 0 to 100 maybe found, excluding both limits (0 gives underflow indicated by the dot and 100 gives 1) anything greater than 100 gives 1. i think it performs x/x in this range.

the unit is powered by a special rechargeable battery pack which forms part of the back of the case itself. it can also be powered from a mains adapter. nominal rating 6v, but i gave it 5 (the batteries are dead) and it is happy with this. note that the middle pin is positive which is confusing since inside this is a black wire!