The Soemtron ETR 224 Calculator
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 -
If you have any further historical information, circuits, drawings, photographs, data or manuals about the Soemtron ETR 220, 221, 222, 224 or their manufacture, test equipment (Prüfgeräte) or you just know more about the Soemtron companies in general then please let us know, contact us here.
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.
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.
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 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 photos of our newest acquisition to follow soon.
February 2009 -
19/2/2009 - With an update on the 224 printing mechanism, an email from the Historisch-technisches Museum at Sömmerda, Frau Speiser relates (translated) -
August 2008 -
"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 -
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.