Soemtron ETR220 Restoration Project

Recombobulating a 1970s vintage digital electronic calculator from Eastern Germany

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This beast of a calculator is magnificent in every respect. It uses diode-transistor logic with discrete germanium components, a ferrite core memory and a multiplexed display with 15 Nixie tubes. My objective is to bring it back to action! A challenging job. The documentation is available on but the schematics use non-standard symbols for logic blocks and thus are pretty hard to follow.

1. Acquisition

I found this calculator in one of the back rooms of the Book Art Museum (Łódź, PL), where I worked for five years. As a lover of old technology (and a new kid on the block in retro computing), I was more than happy to bring it to my lab and start restoring it to a working order.

2. Cleaning

I did it in my old lab at the museum. Prior to firing it up, I cleaned the Soemtron with compressed air already at the museum; it collected mighty amounts of dust, so I did the blowing outside. This also involved pulling the twelve PCBs from the arithmetic/logic unit cage, blowing air on each one of them, with extra care not to damage the delicate components like signal transformers or the ferrite core memory.

3. Initial Power Up Attempt 

Connected the calculator to a variac and powered it up, bringing the voltage gradually up to nominal value (240V). Nixie tubes lit up showing zeros. Attempts to enter numbers were failed - nothing changed on the display when doing it as the documentation specified. At some point all tubes went dark and had intermittent contact, suggesting the power rail going down somewhere. Time to call it a day...

4. Transport

I took the calculator to my lab in Gdańsk on my return trip from the museum.

5. Debugging the Nixie display issue

First I powered the thing up, then measured the voltages, made sure they're within spec. While the Nixie positive and negative voltages were present on the PSU, the anode driver didn't get the positive. Reflowing solder on the connectors seems to have helped - almost certainly a cold joint. I reflowed other connections on the motherboard too, as they're under some mechanical stress from inserting and removing the ALU boards from their sockets.

6. Debugging the keyboard

Some keys were sticky, so I decided to take the keyboard apart and make sure that the switches work properly. The keyboard uses mechanical micro-switches in SPDT configuration, where all the NC contacts are wired in series to the ground (detecting the inactive state of the keyboard), and NO contacts are routed to a number decoder and function decoder logic. These decoders translate each key to BCD-coded numbers.

7. Still not alive...

Despite the keyboard now working properly, the calculator still doesn't even want to show the number I just entered. The function and number decoders proved OK after some testing. Documentation study and more work will be necessary - I'm suspecting a faulty ferrite core memory driver, or the memory itself. Got to take a deeeeeeeeeeeeep dive...

  • Repaired at last!

    Keri Szafir10/01/2023 at 18:13 0 comments

    The Soemtron journey has come to a glorious end with a machine working and performing all operations.
    In order to get it to this state, I had tons of figuring out how it works based on info gathered by including Soviet technical manuals. I was new to discrete transistor logic (especially running on negative 12V level) and ferrite core memory - this has been an excellent learning opportunity!

    It has been a few months on standby and a month of trying to determine what was wrong. I got back to the project after my vintage computing nerd friend named Konrad gave me a whole spare arithmetic logic unit (ALU) soon after I saw he had one with no use for it. I hoped it was in a working order, that would make things easy... Unfortunately, things are never that easy in the world of vintage electronics, right? His ALU has parts missing on several modules, and borked transistors on some more. The second board i.e. the memory addressing controller board was damaged in a different way than mine, and by some trickery I managed to pin down the failing circuit.

    How did I pull it off?

    My memory addressing controller board didn't let me enter any number - it was all zeros no matter what I entered, despite having recombobulated and tested the keyboard. After I got the spare ALU, I swapped the boards and could enter digits 0...7. Digit 8 was converted to 0 and 9 was converted to 1, and since the device uses BCD encoding for numbers, I instantly knew that the 4th bit (8s) was dead somewhere. I suspected the flip-flops, but studying the machine's inner workings (mainly the ferrite core memory) plus some poking around with a scope led me to either a failing inhibit line for 8, or failing read amplifier. The failing inhibit would prevent the bit from being stored, or to be more precise, not erased when other bits were stored. First I swapped the HV4 and HV8 read amplifier inputs to see if I'd get 8 in place of 4... I got - so, the read amplifiers and their outputs were working right. By elimination, inhibit was suspicious. I checked the transistors in the BL8 driver circuit and one of them was shot, CE shorted. This possibly led to the ferrite core memory releasing some magic smoke as well; fortunately I had a spare in the donor unit and swapped them.

    After getting the BL8 to work, I could enter full range of digits and the machine started performing all operations.

    The question was, why my original addressing controller was failed...

    First I noticed that the -12Vsp (memory driver power bus) was broken; finding this filled me with joy, which lasted shortly. After repairing the broken trace and testing the board, I put it back into the Soemtron hoping for issue resolving itself... It didn't happen - still the same, all zeros.

    Then, since it was all zeros, I incorrectly suspected a circuit on the controller board that was common to all the drivers. It was a monostable trigger cutting a clock pulse to 4µs; it was working though. Well, at least I calibrated it with my scope. Again, still all zeros - something else was wrong.

    Using the spare controller, I entered and copied a large number to the persistent accumulator (AC1...AC3) that wouldn't be erased after pressing the CLEAR key. After swapping the boards back to mine, I tried retrieving that number from the accumulator - and got only half of the digits, the rest being zeros. I tracked this issue to read line not working on half of the memory, regardless of digits being displayed (all memory planes affected) and regardless of register (AC1...AC3 same as MD and MR). So, column addressing was suspected, with half of the memory going down, which led me to Z8L or /Z8L drivers. After checking them and replacing a failed transistor, the machine started working like a charm. Not bad...

    I spent a month working on the machine. Definitely exhausting, but also satisfying.

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Mike Hatch wrote 12/22/2022 at 09:11 point

Logic levels for the Soemtron ETR22x units are 0V and -12V. +12V is mainly a bias for the logic gates, the core drivers use a -13.5V (adjustable) non constant current supply, the core current is set by 3x120r (paralleled) in the respective drivers. There is no loop wire in the cores to monitor these currents. 

Logic symbols on the original German drawings is something rather obscure (AND is a circle, NAND filled circle).  Full redrawn circuitry and logic for the ETR220 to MIL-STD-806 / IEEE Std 91/91a is available at - - circuit and logic drawings for each board, psu, display, backplane, wiring, block diagram. Some pages have basic timing diagrams.

The 220 downloads page has the German logic keys and drawings, manuals etc. That page also has a full English translation of a very comprehensive Russian technical manual describing the 220 in much detail. 

The keyboard is the Achilles heel of any 22x series machine (220, 222, 224), all parts need to work well, a common line through every key combined with decoded keys serve to de-bounce the signals by delay timing the falling and rising edges. The switches are silver plated so corrode with non use, and heavy handed use breaks keyboard tracks and/or solder joints.

If you have 15 0’s after resetting (Lo key) the main 5 bit Z counter controlling the display sequence and core control is basically good. The master reset Lo does not clear the 3 temporary stores I, II, III, so there maybe data still in them, the down pointing triangle key is “recall”, might show data if the core is cycling correctly.

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Keri Szafir wrote 10/01/2023 at 17:30 point

Hey Mike, you're the legend! Thanks for all the info you gathered on
That's one more Soemtron back in the world of the living - with a Z8L amplifier's transistor failed open on my module, and BL8 transistor failed C-E short on a spare ALU that helped me diagnose mine.

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Tom Nardi wrote 12/12/2022 at 19:26 point

What a glorious machine, glad to see its being well taken care of.

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Keri Szafir wrote 12/13/2022 at 03:51 point

I'm pretty pressed at a time, but next year I'm going to tinker with it some more. Hopefully I'll manage to bring it back alive.

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Dan Julio wrote 12/09/2022 at 02:50 point

Those schematics!  Good luck with this!

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Keri Szafir wrote 12/09/2022 at 10:14 point

Thanks, I'll need some! Gotta crank up the gain on my brain...

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Dan Maloney wrote 12/08/2022 at 22:48 point

+100 just for "recombobulation"

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Keri Szafir wrote 12/08/2022 at 23:41 point

Such a Fran-tastic word, I love it :)

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Ken Yap wrote 12/08/2022 at 03:27 point

Wow, fascinating project. Will follow with interest. All the best.

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