The Soemtron ETR 224 Calculator

The museum's Soemtron 224

Above ©2009 Ruediger Kurth, below ©2009 www.soemtron.org Click any image for a larger version.

Inside the 224

Manufactured from 1968 to 1974 by V.E.B. Büromaschinenwerk Sömmerda, we believe only 526 Soemtron ETR 224's were built during the six year production run. The processing logic of the Soemtron ETR 224 calculator, believed to be derived from the ETR 222, is all transistorised, consisting of pairs of PNP Germanium transistors coupled as Flip Flops with a few resistors and capacitors, and sets of diodes for gating functions. This model in the Soemtron electronic calculators series has a printing capability which according to Ruediger Kurth was apparently a TSD16 thermal printer. However with information about the 224 in the Historisch-technisches Museum in Sömmerda, recently received from Ruediger (see below Feb 2009) after he visited the museum, it appears that the ETR 224 printer was an internally mounted rotary impact type and that the 224 did not retain the 15 digit nixie display. This Streifendrucker, was capable of printing in two colours using Red for negative values and Black for positive, using a typewriter ribbon and typical feed mechanism, technologies that V.E.B. Büromaschinenwerk Sömmerda were well versed in from many years making typewriters and mechanical calculators.

To date we have found that very little information, if any, exists about the Soemtron ETR 224 and certainly no technical information appears to have survived at all. We now know that probably only four Soemtron 224's remain in existence -

1. A partially working machine in the Historisch-technisches Museum in Sömmerda.
2. Our machine currently undergoing restoration in the www.soemtron.org / .com collection (see September 2009 below).
3. One in the hands of a private collector in Sömmerda.
4. And the fourth machine in the Museum für historische Bürotechnik in Naunhof.

If you know of, or have any more history or information about the Soemtron brand, the 220 222 224 or the very rare 221 machine, test equipment (Prüfgeräte) or you just know more about the Soemtron companies in general then please let us know, contact us here.

• Photos of our Soemtron 224 before restoration began.
• Photos and description of the restoration progress.

May 2010 - Cleaning the ETR 224 we picked up last September is progressing well, all the case parts have been cleaned and re-sprayed as they were too heavily corroded to recover. The printer mechanism is now clean and has been partially reassembled, but during this process we found that one of the print wheel arms was bent and would not contact it's idler gear or the print platen correctly, this bent arm has subsequently been straightened and further re-assembly continues. Having got most of the mechanism back together and run the motor it is evident that the transverse fan shaft bearings at the fan end are loosely toleranced or badly worn and have a large amount of "bearing slap" resulting in a lot of noise reverberating through the fan blades themselves. These worn bearings have been replaced and a new rubber disk fitted to the friction drive, with the result that the fan is now working well and is very much quieter.

Print wheel layshaft assembly

On disassembling the printer mechanism, it was found that a cam operated bar used to return the 18 selector racks was broken in two so the print wheels would potentially not return to their correct rest positions. The broken part was temporarily welded back together for testing purposes but the part has now been accurately measured and a mechanical drawing made, subsequently, two new bars have been manufactured so that we can effect a full repair in the future. During preliminary testing of the re-assembled unit we have also noticed a large amount of rotational miss-alignment of the print wheels at their rest position through the idler gears back to the selection racks. This appears to be related to another cam actuated bar that simultaneously releases all eighteen print wheels to make the print impression through the ribbon. As it appears there is no adjustment for this as the bar is pinned in position, we may need to look for other possible problems with the mechanism, re-pin the bar or have a new part made and re-pin that.

January 2010
Firstly - A happy new year to you all.
With the new year we have started the restoration of our 224, which we expect to take about a year, again with progress documented in these pages. Like the 220 pages we hope that the information gathered from this process will evolve into a full technical documentation set for the Soemtron 224.

November 2009
Our ETR 224 is serial numbered 32612, which seems to roughly agree with the 1969 date codes on most of the boards, however the Core memory and Driver boards are dated 1972, so we presume they were replaced at some point in the machines history. These two boards are 220 part coded and are the only boards that appear to be common over the whole Soemtron 22x range. The machine is in quite a poor condition cosmetically having been used, we think, in a meat processing factory, but it is not that far gone mechanically that it cannot be completely refurbished. The paint work in places is corroded and damaged so we will probably have the whole case stripped and re-sprayed.

Internally the condition is quite good and does not show signs of the patchy external corrosion. The machine closely resembles the Museum's ETR 224 (see the bottom picture), but has an extra mechanical drive through the printer mechanism to an additional fan housing on the card cage. The right angle friction drive from the motor coupling for the fan is not functioning properly, probably due to misalignment in the fan shaft assembly. What looks to be the drive electronics for the printer solenoids is behind the printer, whereas in the Museum's 224 those boards are mounted on the side of the card cage, that location in our machine has the fan housing.

September 2009 - important update
We recently drove out to Germany to pay a visit to the Historisch-technisches Museum in Sömmerda - website. For further information and photos of the visit see this page.

We met with a group of past employees from the Soemtron factory and museum staff (photos here), discussed the Soemtron machines and told them how we became involved, showing them this website. A short while later during the tour of the archive, and quite out of the blue, we received a bit of a shock, an offer of a Soemtron 224 !. Having thought that with the very small production run of 224's (only 526), the museum's machine was probably the only one left, how wrong we were!, there were two more in a private collection!. The unexpected and gracious offer of this 224 completes our collection of Soemtron machines - 220 - 222 - 224, and as far as we know is the only complete collection of 22x series machines outside that of the museum in Sömmerda. Many, many thanks must go to the gentleman who offered us the 224 from his own private collection. This just confirms our thoughts on the visit of the friendly, eager, courteous and helpful reception we received from the people of Sömmerda. More news, information and pictures of our newest acquisition to follow soon.

February 2009 -
23/2/2009 - One of our regular correspondents, Ruediger Kurth has found a Soemtron 224 in the archive of the Historisch-technisches Museum in Sömmerda. At this time this is probably the only known example of a Soemtron 224 in existence !. Rüdiger has sent a couple of photos of the machine showing it to be quite different from a picture in the Büromaschinen Lexikon (click on "Soemtron" in their menu), of a model 221 we mentioned last September (2008). Ruediger has kindly allowed permission to use these photos and hopefully will also check if there is any documentation to go with the machine and if the museum will allow any internal photos - further news to come. Unbounded excitement - almost - we really did not expect to ever find an example of a Soemtron 224 in the wild. The keyboard colours are the same as a 222 machine, leading us to believe the 224 has the same operation logic as a 222, indeed our own 222 has logic boards with "224" part numbers, so some parts of the 224 were useable in the 222 at least.

19/2/2009 - With an update on the 224 printing mechanism, an email from the Historisch-technisches Museum at Sömmerda, Frau Speiser relates (translated) -
"The 224 followed the same method as the ETR 220. A mechanical printer was fitted instead of the numerical display. The output was achieved by a 12 column printer. Negative values were shown by in red print, that means the negative values were printed in red colour. The ETR 224 had an additional electronic module that was responsible for controlling the mechanical printer mechanism. The electronic basket (module) of the ETR 220 was not identical to the electronic basket of the ETR 224 and were not inter-changeable.

August 2008 -
Bernard Green who was Senior Engineer at Office and Electronic Machines in London UK (importers of the Soemtron 220 series calculators) has sent us his recollections of the Soemtron 224 printer mechanism -
"On the 224 we had, the printer was mounted on the top, semi-recessed into the case, and I recall it also had the Nixie tube display. I am beginning to think that we never actually sold a single 224 in the U.K, because either there was no market and / or the printer was too unreliable. There were photo diodes to detect the position of the print mechanism elements and that they were very unreliable and prone to ambient light".

"The print mechanism had 16 print wheels with characters and these all rotated on a common shaft. When the desired character was in the printing position, a hammer, one of sixteen, was energised to press the paper against the print wheel. A slotted disc, one slot per character rotated with the print wheels and a photo diode counted the slots into a counter, when the counter matched the digit for a column(s), the hammer(s) were driven. Another slot referenced the home position of the print wheels".

"I can also recall another print mechanism with the characters on vertical print bars. The bars were all lifted by another horizontal bar and then the bar lowered. The print bars came down with the horizontal bar until the required character was in the print position when an electro-magnet for that print bar operated to pull in a pawl hold the bar in position and prevent it falling further. Then, when the bar was fully lowered and in theory all print rods were held by their pawls, a single hammer struck to make the print. For the next row of print the bar was raised again and the individual pawls released as the bars were lifted".

So at this time we have three possible contenders for the printing mechanism on a Soemtron 224 -

1. A rotating "drum" type mechanism with individual pin solenoids, similar to 1970's and 80's style line printers.
2. A semi-mechanical lever and impact hammer system, likely to have been derived from a previous Soemtron mechanical calculator, with possibly two variants of the mechanism.
3. A thermal printer.

As we now have our own Soemtron 224 (see the September 2009 update above), it would seem that none of the above fit the bill exactly although #2 is probably the closest. So now that we have a 224 in restoration, we can over time provide a thorough breakdown of the printer mechanism.

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