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(soemtron.org cannot control and does not necessarily agree with the content of external sites)
Wolfgang Robel's website - http://www.wolfgangrobel.de/museum/etr220.htm
Here Wolfgang details the repair of his ETR220 which was helped along by details and circuits from the Soemtron.org website. He experienced the usual common faults in the Soemtron ETR series; oxidised keyboard switches, non-functional Germanium diodes and mains filters that wanted to explosively de-construct themselves !. The site is in German, Google Chrome will translate it for you if needed.
Historisch-technisches Museum - Sömmerda - www.dreysehaus.de
Opened in 2005, the Historical and Technical Museum in the city of Sömmerda is centrally located in the Grade II listed building formally occupied by Nicolaus von Dreyse, inventor of the needle gun. The museum pays tribute to the life and work of the inventor and a 200-year history of industrial development in his hometown of Sömmerda. This excellent museum has a small fledgling technical section with exhibits and an extensive collection of writing, calculating machines and electronics from 1923 to more recent times when the various Soemtron brands and the V.E.B. Büromaschinenwerk Sömmerda cooperative were in full production.
Supported by the local community and government this museum is well worth a visit if you are in the locality, it cannot be praised highly enough, and deserves all the support they can get. New developments and renovations near to the museum should see the collections of calculators, electronics, computers and other machines from their extensive archive move into a new purpose built facility.
Early Electronic Calculator Technology Reference - www.cs.ubc.ca
Brent Hilpert's website about the technology used in the construction of early electronic calculators.
Technical data for electronic calculators manufactured from the 1960s to the early 1970s is difficult to come by and most of these machines utilized proprietary and now obsolete integrated circuits. For SSI-based calculators, technical data is useful and often a necessity if one wishes to repair such units.
This is a fascinating website by Brent Hilpert with in depth reverse engineering and simulations of old electronic calculators. Lots of detailed data on obsolete integrated circuits that are used as the building blocks for calculators dating from the late 1960's and early 1970s.
12/9/2017 - Brent's website at http://www.cs.ubc.ca/~hilpert/eec/index.html is no longer responding, the main link above now points to the Wayback Machines last copy of 05/02/2017.
The Old Calculator Web Museum - www.oldcalculatormuseum.com
This museum is simply devoted to preserving, documenting, and sharing the technology of desktop automatic calculating machines from the electro-mechanical calculators of the 1950's and 60's, through the beginnings of the pocket wonders we have today which had their roots in the early 1970s, and on into the beginnings of the desktop computer of the mid-1970's through the early 1980s.
Serge Devidts - www.calcuseum.com
Based in Belgium this is Serge's website covering old technology and in particular vintage electronic calculators. The site includes desktop, pocket and special calculators, pocket computers, educational calculators, a catalogue of electronic calculators (11,000+ references), calculator statistics, time keepers, components used in calculators, advertisements, publications and brochures.
Vintage Calculators - www.vintagecalculators.com
A celebration of old calculators showing the evolution from the mechanical calculator to pocket electronic calculators. Featuring mechanical & early electronic desk calculators, strange hand-held calculators, articles photographs and databases from the archives of the International Association of Calculator Collectors, information and photographs for those interested in the history of technology, British and Sterling currency calculators.
Xnumber world of Calculators - www.xnumber.com
The forums for all things in vintage calculators, electronic and mechanical.
Soemtron ETR220 overview at DoPECC - www.dopecc.net
A very nicely presented website with a section about the Soemtron brand and V.E.B. Büromaschinenwerk Sömmerda in the Soviet East Germany. DoPECC is a private effort to share an interest in early electronic technology with fellow enthusiasts and those with a general interest in the subject. Very nicely presented, clear with excellent photos and sections covering several calculator brands and computers. It has a small technical library section; the whole site is currently still being expanded with new information.