Soemtron ETR 220 Calculator
S#54555 - Technical Description

This Soemtron 220 which we received in January 2008, has a serial number of 54555 so we think it should have been made in about week 41 of 1969.  The base panels of the Soemtron 220 and 222 all seem to be the same, and as we think the serial numbers were assigned to the bases before the machines were actually assembled, this theory seems to fit with the serial numbers and date codes of the machines we have, although admittedly this is a rather tenuous assumption !.  Once we get to repairing and refurbishing this machine we'll have a look at the board date codes and let you know !.

It unfortunately suffered quite a bit of damage when it was shipped in from Germany, it appears to have been dropped, the power supply is bent with a broken PCB, and the main electronics card cage is bent on it's mountings.  The power supply has been repaired and the machine actually does power up and seems to work correctly.

Power supply - circuit
The power supply is a fairly conventional series regulated unit, with outputs of -12V for the logic, +12V bias, and +13.5V for the core memory circuits and drivers.  A fourth supply provides approximately +180Vdc for the display nixies.  This display potential is actually split into +/- 90V DC rails.  Using the standard continental mains input connector for the time, the power supply has an internal input voltage selector for 110, 127, 220 and 240V AC.

Keyboard - circuit
The ETR 220 keyboard consists of 31, V23 microswitches capped with Red keys for control and functions, and White keys for numeric entry (see photo above), arranged in 4 logical groups thus -

• Clearing and decimal point keys - Master Clear  Lö , Clear entry  C  and decimal point  , 
• Numeric keys -  0-9 , arranged in the normal order as seen on modern computers and calculators.
• Arithmetic functions - Negate number  -# , Raise to power  Xn , Multiply  X , Divide  : , Add  + , Subtract  -  and Result  = 
• Register controls - for registers  I ,  II ,  III  - Recall with clear  ✶ , Recall  ∇ , Subtract  -  and Add  + 

All signals from the keyboard go through a 32 plug/socket arrangement (PL17/SK17 on the drawings), to the backplane PCB where they are routed to Boards 11, display Anode drivers and 12, display Cathode drivers, for encoding in diode matrices.  The Numeric keys  0-9 , clear entry  C  and decimal point  ,  are routed to Board 12 for encoding, with a separate signal for the Master Clear key  Lö  going to the core store.  Note that the diode encoding matrix does not include diodes for all of the keys pressed, presumably to save on components, and that the codes are encoded by some of the keys acting directly on the diode matrix outputs.  All of the other remaining keys are routed to Board 11 for encoding into five control signal lines for the three memory registers and functions.  As per the numeric encoding scheme, some of the keys act directly on the encoding matrix outputs.

Display - circuit
The display consists of 15, Z750M Nixie tubes, a power "ON" neon, "negative" result lamp and 10 fixed position decimal point lamps.  Each Nixie tube displays the numbers 0-9 in 13mm (0.51") high digits using a supply of 180Vdc at approximately 2mA.  Each digit is a shaped metallic cathode of the required numeric shape and is commoned onto a multiplexed signal buss derived from Board 12, the display Cathode driver board.  Each numerical digit has it's own individual inverter driver on board 12 with a slight difference for the "0" digit due to the Display decimal point logic on Board 4 used during a numerical entry.

Each Nixie tube anode is separately driven from Board 11, the display Anode driver board via fifteen individual drivers, whose control signals are again derived from a multiplexed data buss.  The ten Decimal Point lamps are directly switched from the last wafer of the front panel decimal point selection switch via a short cable loom and interconnecting 13pin plug and socket.  The other three wafers of the decimal point selection switch encode eight signal lines for the main calculator control logic.

Further technical description of this particular machine will follow in the future.

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