The Großer Klappenschrank could be assembled with an Untersatz C to set up an SB switchboard exchange. All Feldfernsprecher 33 (FF33) field telephones connected to Untersatz C would have to be equipped with an SB Zusatz 33 handset cradle to allow the OB phones to operate on the SB exchange.
Unlike the Untersatz A that uses signal flaps actuated by magneto current, the Untersatz C uses Sternschauzeichen actuated by each SB Zusatz to indicate when the calls have ended.
The Sternschauzeichen is an electo-mechanical device with electromagnetic coils (spulen) that rotate a shaft when the coils are energized. On the end of the shaft is a star-shaped vane. The paddles of the vane are brightly colored and they show though a star-shaped window on the Sternschauzeichen. When the coils are de-energized, a spring rotates the shaft in the opposite direction and the window shows black.
Referring to the above electrical schematic… When a call is in progress, the FF33 handsets are off the cradles, and switches U on both SB Zusatz 33s are open. When the call is completed, and the caller on Line 1 (Teilnemer 1) puts the handset back in the cradle, switch U on the associated SB Zusatz closes. This causes 12 volts direct current from the Kontrol Batterie (KB) to flow from the Untersatz C, through the SB Zusatz, and back to the Untersatz C. When this happens, the Sternschauzeichen (SZ1) coils are activated and the star vane rotates, indicating to the operator that the call has ended.
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WW2 German Field Telephone Equipment 