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SE 108/10   SE 100/11
Abwehr spy radio set · 1942

SE 108/10 1 is a clandestine transmitter and receiver, also known as a spy radio set, developed during WWII, in 1942, by OKW-Aussenstelle Wurzen 2 for use by the German Intelligence Service, the Abwehr [1]. The set consists of three small units in tin enclosures, and is often referred to as the Keksdosen (Biscuit tins). It was released in 1942 and was manufactured by OKW-Wurzen at Nischwitz Castle (Germany), alongside the similar, but less powerfull, single-unit SE-109/3.

SE-108/10 in storage cassette, seen from the top. Copyright Günter Hütter [2].

The image above shows the complete set, in the bottom section of a storage cassette. From left to right are receiver (RX), power supply unit (PSU), and transmitter (TX). The three units are inter­connected by means of 4-pin Brechkupplungen (break connectors), so that the set can be used without removing it from the cassette. At the front of the transmitter is a miniature morse key, that can be stowed inside the transmitter's enclosure (otherwise the cassette can not be closed).

Strangely, the mains power cable is hidden inside the PSU. To access it, the top lid of the PSU has to be removed first. Furthermore, the top lid has to be left off as long as the PSU is connected to the mains, as there is no hole in the enclosure to guide the cable through.  User manual

  1. In Louis Meulstee's book Wireless for the Warrior Volume 4, the SE-108/10 is referred to as SE-100/11 [1]. Both names are believed to be correct.
  2. OKW = Oberkommando der Wehrmacht (Supreme Command of the Armed Forces) in Nazi Germany during the Second World War. Aussenstelle = Outpost.
  3. Note that different frequency ranges were used for specific missions [1].

SE-108/10 without top lids, seen from the top. Copyright Günter Hütter [2].
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SE-108/10 without top lids, seen from the top. Copyright Günter Hütter [2].

Versions
  • SE 108/10
    Initial version of 1942 of which the transmitter was built around an EL2 valve (tube), which produced an output power of 10W. This comprises all units up to serial number 100. Due to wartime shortages, the EL2 was replaced in 1942/43 by the UBL21, which was more common and was readily available at the time [4]. For this, the PSU was changed as well.

  • SE 100/11
    This is a later designator for the modified SE-108/10 (with the UBL21 valve in the transmitter). It is not exactly known when and why this designator was introduced, but it seems likely that both designators refer to the same set. There are many manufacturing variations, which may have been caused by supply shortages. There are also variants with a different frequency range, as this was usually tailored to a specific Abwehr radio net.
Encryption
In the mid-1990s, an SE-100/11 with serial number 268 was found in Finland, together with radio instructions and cipher material for a so-called Sonderkommando Nord (special command north) radio station named 'Land'. The cipher material consisted of two plexiglass Caesar Discs.

Original plexiglass cipher discs as found with SE-100/11 serial number 268

The image above shows the original plexiglass cipher discs, as found with the SE-100/11 in Norway. In 1996, Finnish radio amateur Esko Jokinen (OH3QS) made a detailed description of their construction and use, based on the documentation that had been found with the set [3].

 Description of cipher discs


Interior
The SE-108/10 consists of three same-size tin boxes that are interconnected by means of so-called Brechkupplungen (break-connectors). The image below shows the three units after the top lids are removed. From left to right: receiver (RX), power supply unit (PSU) and transmitter (TX).

SE-108/10 without top lids, seen from the top. Copyright Günter Hütter [2].



Circuit diagram
Transmitter
Below is the circuit diagram of the transmitter, as it appears in Louis Meulstee's book Wireless for the Warrior Volume 4 [1]. There are two versions of the circuit. Transmitters up to serial number 100 were built around an EL2 valve. It is likely that this version was known as SE-108/10.

Due to supply problems during the second half of the war, this valve was replaced by the more common UBL21, which needs a 55V filament voltage. It produced a slightly higher output power (11W instead of 10W) and it is likely that this version was designated SE-100/11. Note that this version needs a different power supply unit (PSU) than the SE-108/10, as the voltages for the transmitter are different. The diagram below shows the circuit of the S100/11 transmitter.



Connections
Transmitter power socket
Power is distributed between the three units, by means of 4-pin circular connectors that are known as break connections (German: Brechkupplungen). These connectors were also used by the German Air Force during WWII. Below is the pinout when looking into the male socket.

  1. HT 237V DC
  2. Meter
  3. GND 0V
  4. LT 55V AC
Receiver power socket
  1. HT 170V DC
  2. LT GND (connected to 3)
  3. HT GND (connected to 2)
  4. LT 6.3V AC
IMPORTANT — Note that the notch in the socket can be at the bottom (as shown here) oas well as at the top. This is not related to the layout of the wiring though. It is unclear why the notch on some devices was at the opposite side. If transmitter and receiver are used with the original PSU , this should not be a problem.
EL2 transmitter valve   S/N up to 100
Initially, the transmitter was built around an EL2 valve that produced an output power of approx. 10W. It's anode (HT) is powered at 237V DC, whilst the filaments (LT) are powered by 6.3V.

 EL2 datasheet



UBL21 transmitter valve   S/N 100 onwards
As the EL2 was in short supply during the war, the set was redesigned and the EL2 was replaced by an UBL21, which has similar specifications, but was much easier to obtain at the time. The only problem was that the UBL21 needs 55V for its filaments, for which the PSU was changed as well.

 UBL21 datasheet



Specification
Transmitter
  • Bands
    2
  • Frequency
    (1) 300 kHz - 6.2 MHz, (2) 6.2 MHz - 12 MHz 1
  1. Different ranges are known to exist and depended on area, user and Abwehr radio network.

Documentation
  1. SE-100/11 Operating Instructions (German)
    S/N 736385. Date unknown. 1

  2. Description of the caesar cipher discs and their construction
    Esko Jokinen (OH3QS), 20 March 1996 [5].

  3. UBL21 valve, datasheet
    Unknown publisher. Obtained via [3].

  4. EL2 valve, datasheet
    Unknown publisher. Obtained via [3].
  1. Between 1942 and 1945.

References
  1. Louis Meulstee, SE 108/10
    Wireless for the Warrior - Volume 4. ISBN 0952063-36-0. September 2004.

  2. Günter Hütter, Images of SE-108/10
    Received May 2020.

  3. The National Valve Museum, UBL21 datasheet
    Obrtained via Frank Philipse. Retrieved May 2020.

  4. Rudolf Staritz Archive, Information and Circuit Diagrams of SE-108/10
    Retrieved June 2012.

  5. Esko Jokinen (OH3QS), Description of the SE-100 cipher discs
    20 March 1996. Obtained via [4].
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© Crypto Museum. Created: Thursday 14 May 2020. Last changed: Sunday, 25 October 2020 - 08:36 CET.
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