Our Own Test Equipment

Soemtron 8601a Prufgerate, click image for a larger version
Soemtron 8601a
Click image for a larger version

Soemtron ETR220 production, ©1995 DESOTRON,, click image for a larger version
ETR220 production testing

January 2022 – A rare 8601a Prüfgeräte for an Electronic Invoice Machine « Soemtron–381 » (EFA381) was found on Ebay, this machine is now in the Soemtron collection, click the Tester 8601A link in the menu section for a description.  These units were used on the ETR22x production line as the photo (right) shows.

Soemtron designed test equipment.
(Our Prüfgerät - Extender)

Original prüfgeräte (test equipment) used by V.E.B.[1] Büromaschinenwerk[2] Sömmerda for the Soemtron ETR22x range of calculators no longer exists, having been scrapped when the factories were closed many years ago.  A photo (right) of the Soemtron ETR220 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 ETR220, Chapter 5 “Maintenance”, from a short section reproduced here, it details the use of a "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 ETR22x 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 a - “Soemtron–381 type 8601”, it would seem that these "service" connectors were a common design feature in both the ETR22x series machines and other complex digital electronic products produced by V.E.B. Büromaschinenwerk Sömmerda at the time.

We have tabulated the differences between the service connectors in the Soemtron ETR22x machines by comparing the service connectors of our ETR220 machines with the listings and drawings in the Russian Technical Manual, we then added the connections derived from our ETR222 and ETR224 machines.

ETR220 Minitester overlay panel, click image for a larger version
Minitester panel
Click image for a larger version

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 selection 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.

  1. Overlay drawing of the Prüfgerät front panel with connections.
  2. Pages 184–186 from the Russian Technical Manual detailing the Prüfgerät and its usage.
  3. Service connector signal differences for the Soemtron ETR220, 222 and 224 machines.

Soemtron logo

If you have any further historical information, circuits, drawings, photographs, data or manuals about the Soemtron ETR220, 221, 222 and 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.

May 2017 – A recent posting on the forum has thrown up a photo of another Prüfgeräte called the "Minitester" (photo above right).  It has a very similar layout to the production line Prüfgeräte, but with a smaller and slightly different panel arrangement it would appear to have been possibly used as a piece of field test equipment.  There was one panel for an EFA382 and several overlay panels for an ETR220, one of them has now been donated to the Soemtron collection, many thanks to Markus Zluwa. - new prüfgerät, click for larger image
Our own Prüfgerät
Click either image for a larger version
Eight channel monitor board, click image for a larger version
Eight channel monitor designed test equipment.
(Soemtron Prüfgerät - Extender)

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 ETR220 logic and circuit diagrams; Bernard Green's recollections that the Prüfgerät contained “high impedance” inputs; and the data gleaned from our own ETR220, 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 with switching to monitor the internal supply voltages which should prove very useful when setting the core memory supply voltages against temperature.

  1. New Prüfgerät circuit diagram,
  2. New Prüfgerät front view,
  3. New Prüfgerät rear view.

The more observant 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 ETR220, 222 and 224.  With that in mind we decided to add another row of signal detection for the extra signals.  The main differences are on the V2 power connector for the ETR224, along with a couple of new signals on both the V7 and V8 service connectors.  What these extra signals are for will be determined when we get our ETR224 electrically operational.

Documents for the Prüfgerät can be found on the Prüfgeräte downloads page.

Extender board, click image for a larger version
Click image for a larger version extender board.
(Soemtron Prüfgerät - Our Prüfgerät)

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 but data for this is hard to come by.  They are very close to DIN41617, with the pin pitch, diameter and overall size being the same, its just the row pitch that's 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.

  1. Wikipedia entry for V.E.B. = Volks Eigener Betrieb, or Peoples Owned Company V.E.B.
  2. Büromaschinenwerk = Office Machine Works. [back] [top]

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