The Soemtron ETR 222 Calculator

Soemtron ETR 222

Manufactured from 1970 to 1972 by V.E.B. Büromaschinenwerk Sömmerda, 3,203 Soemtron ETR 222's were built during the two year production run. With seven functions, the processing logic of the Soemtron ETR 222 calculator, derived from the ETR 220, is all transistorised, consisting mostly of pairs of PNP Germanium transistors coupled as Flip Flops with a few resistors and capacitors, and sets of diodes for gating functions. The keyboard is large and cumbersome, non multiplexed, utilising V23 microswitches with external supplementary springs for the keys themselves. The unit has three memories for intermediate calculations, each with their own set of Add, Subtract, Recall, and Recall/Clear keys, there is also a "C" key which clears the current entry and an "Lö" key which clears everything excepting the three operand stores. The multiplexed display of 15 digits is based on Z750M "Nixie" tubes and uses a supply of around 180V !.

Soemtron ETR 222 core store

Decimal point selection is by a 12 position thumbwheel switch to the left of the main numeric keypad and is indicated on the display by small incandescent lamps between each Nixie tube, whilst a range of decimal point selection signals are fed back to the logic circuits for use during calculations. This makes the Soemtron ETR 222 a "fixed point" machine rather than the usual "floating point" form found on modern calculators. Memory on the Soemtron ETR 222 has a "Ferritkernspeichern" or magnetic core memory of four planes (BCD) of 6 x 16 digit registers, a total of 384 bits or 48 bytes, showing the age of the machine from a time when magnetic core memory was THE "modern" technology before the advent of multi-megabyte semiconductor RAM. Over the two year production run for the 222, 144Kb of magnetic core storage or nearly 1,152,000 miniature ferrite cores were hand threaded onto probably hundreds of kilometers of copper wire.

This our third Soemtron calculator, an ETR 222, is complete but with a damaged keyboard, it has a serial number of 74005, which we believe places it roughly half way through the production run of the Soemtron 22x series, sometime in 1971, possibly about week eight or nine. The base panels of the Soemtron 220, 222 (and presumably 224's) all seem to be the same, so we think the serial numbers were assigned to the bases before the machines were actually built, this theory seems to fit with the serial numbers and date codes of the three machines we have, although admittedly it is a rather tenuous assumption.

Keyboard microswitches

October 2009 - After a bit of searching Bernard Green has found an old supply of keyboard switches at Febana (website), in Sömmerda, sadly one of the few paces we thought to visit in our recent visit, maybe next time. They turned up in the post just the other day, so we now have the bits to repair the keyboard on our ETR 222 and leave us with a few spares. Many thanks to Febana for sending them over.

June 2009 - We are looking for a supplier of the keyboard microswitches used in the Soemtron 22x series of calculators, said to be a V23 microswitch ?. This Soemtron 222, has several broken keyboard switches, which because of the connection arrangement, renders the machine unusable. They may be of German origin, they have the triangle 1 quality mark, are rated at "220V~2,5A", and a "C2"mark. The manufacturers logo is an inverted V in a circle.

We have designed an alternative drop in replacement which would retain the old switch body and use a V3 style microswitch in place of the contacts, but as this means some delicate machining and wiring we would like to find the correct or compatible parts first. A drawing of the switch can be found here

If you have any information about these switches please contact us here, or email us

Keyboard - with the advent of the Soemtron 222 the keyboard layout of the 220 was changed slightly for the new model, with the main differences being -

1. The sign or negate entry key (220 -#) is moved from the central functional key group to the space below the Clear key  C  previously occupied on the 220 by the decimal point key ( , ) - which then
2. moves to a new location under the numeric  1  key left of the now slightly smaller zero  0  key. The current decimal point position (overlaid 0 and 1 digits to show as a "barred" 0), is now not visible in the display during numeric input.
3. The 220 "Raise to Power" function key ( Xn ) has been removed. For powers on a 222 the base number X is first keyed in, then for the exponent n, the multiplication key is pressed n-1 times, then the equals key once. This key position has become the "return constant"  ∇  key.
4. Lastly the key position on a 220 held by the "sign or negate entry key" (-#) becomes the "Exchange operands"  #  key which swaps the contents of both operand registers.

The ETR 222 keyboard consists of 33, V23 microswitches with Blue keys for control and functions, and White keys for numeric entry (see photo above), arranged in 4 logical groups broadly similar to the 220 thus -

• Clear and sign keys - Clear  Lö , Clear entry  C  and negate entry  -# 
• Numeric keys -  0-9 , arranged in the normal order as seen on modern computers and calculators, and decimal point  , 
• Arithmetic functions - exchange operands  # , return constant  ∇ , Multiply  X , Divide  : , Add  + , Subtract  -  and Result  = 
• Register controls - for registers  I ,  II ,  III  - Recall with clear  ✶ , Recall  ∇ , Subtract  -  and Add  + 

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