In 2011, a unique Feld-Hell machine was re-discovered. It was professionally modified to transmit a start-pulse at the beginning of each character. It is assumed that it was used around 1952, as a prototype in the development of a start-stop Feld-Hell machine for the East-German military and police.
The machine is "factory new", with an electronics box from 1943. There are no acceptance stamps anywhere on the machine. Possibly, the machine was still at the factory (Siemens-Halske in Berlin, or Mende Radio in Dresden) at the end of the WW2. It appears to have remained in the East-German/Soviet-occupied zone until it was sold on a flea-market in Berlin in the early 1990s, after the German reunification.
The characters of the Feld-Hell font consist of 7 columns of 14 pixels. The first and the last column are always empty, and normally used for spacing between characters. The start-pulse is conveniently placed in the upper half of the first column, by embedding a metal (brass?) strip across the entire cylinder, flush with the surface. See the photo below. Based on the photo, the start-pulse seems to be 4 pixels long, i.e., 4 x (1000/2.5) / (7 x 14) = 16.32 msec.
Fig. 1: standard Feld-Hell character drum (above) and Feld-Hell character-drum with start-pulse (below).
Start-stop teleprinters require a blank "space" character, as paper transport only takes place when a character is printed. To implement this, both the keyboard and the character-drum were modified. The keyboard modification was simple: the "?" key was replaced with a "Zw" key for "Zwischenraum", i.e., inter-character "space". "Zw" was not simply painted onto a blank key, but properly engraved. See the photo below. The pixels of the "?" character were removed from the character-drum, by carefully machining them out of the drum and covering them with a hard black lacquer - the surface of the drum had to remain smooth.
Note that this Feld-Hell machine was only modified to transmit characters with a start-pulse. The printer mechanism was not modified. Modifying a Feld-Hell machine to detect start-pulses would have implied a major redesign of the electronics box. Likewise, a major mechanical re-design would have been needed to make a start-stop printer.
Fig. 2: Feld-Hell keyboard with "Zw" key instead of the "?" key (lower right-hand corner) and the character-drum with start-pulse
(original unedited photo: courtesy G. Koenig)
©2004-2016 F. Dörenberg, unless stated otherwise. All rights reserved worldwide. No part of this publication may be used without permission from the author.