Original prüfgeräte (test equipment) used by V.E.B. Büromaschinenwerk Sömmerda for the Soemtron 22x range of calculators no longer exists having been scrapped when the factories were closed many years ago. A photo (right) of the Soemtron 220 production line, shows several of the test stations with a Prüfgerät and oscilloscope, both of which can be seen in the photo to the left of the white coated technician with his back to the camera.
In the Russian technical manual for the Soemtron ETR 220, Chapter 5 “Maintenance”, from a short section reproduced here, details the use of the Prüfgerät in the case of faults that cannot be diagnosed with the logic diagrams, describing that - “the display panel of an electronic invoice machine « Soemtron-381 » of the type 8601 is connected”.
All of the machines in the Soemtron ETR 22x range had a set of internal "service" connectors which were used to connect a Prüfgerät to the various internal signals of a machine under test, which would then allow further diagnosis of any faults. With the description in the Russian manual that the Prüfgerät was for an - “Electronic invoice machine Soemtron-381 type 8601”, it would seem that these "service" connectors were a common design feature in both the ETR 22x series machines and other complex digital electronics product produced by V.E.B. Büromaschinenwerk Sömmerda at the time.
We have tabulated the differences between the service connectors in the Soemtron 22x machines by comparing the service connectors of our ETR 220 machines with the listings and drawings in the Russian Technical Manual, we then added the connections from our ETR 222 and 224 machines.
From this information it would appear that the original Prüfgerät had two (common ?) 32 pin leads for signal acquisition from the main card cage of the machine being tested, with a display and test point matrix of five rows of ten signals on its front panel, along with a few other test points and switches. Comparing the service connector pinning to the Prüfgerät front panel drawing on page 185 of the Russian Technical manual gives a very close match suggesting that the two cables are one-to-one pinned and that there were probably front panel overlays for the display matrix to change the signal names between the various models.
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.
With no original information available about the internal structure of a Prüfgerät, other than that in the Russian manual and the production line photo, we have decided to design our own. Using connections derived from the 220 logic and circuit diagrams, Bernard Green's recollections that the Prüfgerät contained “high impedance” inputs, the data gleaned from our own 220, 222 and 224 machines along with whatever other information we could deduce from the Russian manual, we have put a circuit together. Quite a lot of the circuit has been based on the very useful eight channel monitor board we designed a year or two back. Although we are now using relatively modern op-amps and comparator circuits for the signal detection and LED's for the display, we believe our "Prüfgerät" quite closely mirrors the functionality of the original production line unit. We have also gone a little further than the original Soemtron equipment by adding a pair of four digit panel meters to monitor the internal supply voltages which should prove very useful when setting the core memory supply voltages against temperature.
The more eagle eyed of your will have noticed that our Prüfgerät has six horizontal rows of signal lamps where the original unit has only five. During compilation of the service connector signal differences table (see below), we found out that there were several small discontinuities between the 220, 222 and 224. With that in mind we decided to add another row of signal detection for the extra signals. These main differences are on the V2 power connector for the 224. What these extra signals are for will be determined when we get our 224 electrically operational.
Documents for the Prüfgerät can be found on the Prüfgeräte downloads page.
One of the first bits of test equipment we built was an extender board. 31 pin DIN style connectors for the Soemtron boards are no longer available and attempts to remove them from scrap boards were not very successful. So we built or own utilising a custom PCB to replicate the pitch of the 31 pin connector and an aluminium panel. The extender is also capable of holding an 8 way display board with flying lead test clips so that various signals on a board an be monitored. Pins for the connectors were sourced from Harwin (P#H2170-01 pins, and H3194-05 sockets) and our grateful thanks go to them for their help with this project. This bit of kit has proved invaluable in decoding the inner workings of the Soemtron ETR series calculators.
To date the closest specification we can tie the 31 pin board connectors to is DIN41517. They are very close to 41617, pin pitch / diameter, overall size is the same, its just the row pitch thats wrong, 3.8mm in 41617 and 5.0mm on the Soemtron boards.
Documents for the Extender board can be found on the Prüfgeräte downloads page.