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S 90/40
Abwehr spy radio transmitter

S-90/40 was a clandestine 40 Watt short-wave (SW) radio transmitter, developed around 1939 at OKW-Aussenstelle Berlin-Stahnsdorf 1 (Germany) for use during World War II (WWII) at the central control stations (Köpfe) 2 of the German Intelligence Service, the Abwehr, and its foreign counter­parts. From 1942 onwards, the transmitter was produced by OKW Aussenstelle Wurzen [1].

The S-90/40 was commonly used as part of a complete SE-90/40 spy radio set – housed in a black suitcase – along with the E-90 receiver, but it was also deployed as a standalone trans­mitter, for example in Abwehr base stations.

The image on the right shows a typical S-90/40 transmitter with serial number 2. It has Italian text on its front panel, which suggests that it was intended for use by the Italian equivalent of the Abwehr — the Servizio Informazioni Militare (SIM) 3 . The design of the S-90/40 transmitter was later copied by the Italians as the RN-6.
  
S-90/40 transmitter with Italian front panel

When used as a standalone transmitter, it was commonly used in conjunction with an existing HF receiver, such as the Siemens R-IV or the Radione R3. The device is housed in a dark grey metal enclosure that measures 27.5 x 19.5 x 10.5 cm and weights 3128 grams. It should be powered by an external power supply unit (PSU) that provides 12.3V AC (LT) for the filaments and +700V DC for the anode of the RL12P50 valve (tube) of the power amplifier (PA). The device is suitable for CW (morse) only and has a freely adjustable oscillator (VFO) built around a CC2 valve.

  1. OKW = Oberkommando der Wehrmacht (Supreme Command of the Armed Forces) in Nazi Germany during the Second World War. Aussenstelle = Outpost.
  2. In German known as Funkmelde-Köpfe (radio message head-end stations). These were fixed Abwehr stations (such as Wohldorf-Hamburg) or temporary (mobile) stations for a specific operation.
  3. The Servizio Informazioni Militare (SIM) was Italian's military intelligence organisation from 1900 to 1949. During WWII, it was fascist dictator Benito Mussolini's equivalent of the German Abwehr.

S-90/40 transmitter with Italian front panel S-90/40 placed horizontally Front panel Power socket Connected to a power source Band selector Antenna current meter Tuning dial
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A
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S-90/40 transmitter with Italian front panel
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S-90/40 placed horizontally
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Front panel
A
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Power socket
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Connected to a power source
A
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Band selector
A
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Antenna current meter
A
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Tuning dial

Controls
All controls and connections of the S-90/40 are located at the front panel, which is shown in the diagram below. At the top left is the socket for the power supply unit (PSU). At the top right is the antenna current meter, with the antenna sockets to its left. The antenna should be connected to the rightmost socket (T) and to one of the other sockets (1-4). At the bottom left is the frequency dial of the free-running variable frequency oscillator (VFO), which has a liniar scale from 0-180°.


A standard morse key can be connected to the banana sockets at the bottom left. Once the desired frequency is set, the PA tuning knob at the bottom right should be adjusted for maximum power output, using the antenna current meter at the top right as an indicator. On some versions an extra meter adjustment is present between the meter and the PA tuning knob. The white lever at the centre is used to select the required frequency band (1) or (2). The basic model runs from 3.5 to 8.5 MHz, but other frequency ranges have been found as well, such as 5.3 to 9.3 MHz [1].


S 90/40 in use
Although the S-90/40 was sometimes used as a standalone transmitter, it was developed specifically for the SE-90/40 spy radio set shown in the image below. The set is housed in a rectangular suitcase covered with black book linen. Inside the suitcase are three compartments: one towards the rear for the mains power supply unit (PSU) and the ancillaries, one at the front left that holds the E-90 receiver, and one at the right that holds the S-90/40 transmitter.

SE-90/40 spy radio set in black suitcase. Photograph via [1].

The image below shows an unknown Abwehr radio station in which two Siemens R-IV receivers are clearly visible. The one at the right has the power supply unit on top, whilst the one at the left has two plug-in units on top. In between the receivers are two Abwehr transmitters: an S-89/80, with the smaller S-90/40 placed on top. Thanks to Jørgen Fastner for supplying this picture [4].

Abwehr S-90-40 abnd S-89/80 together with Siemens R-IV receivers, in an unknown Abwher station. Jørgen Fastner Collection [4].

According to contemporary witness Rudolf Staritz [5], the setup shown in the picture above is rather unusual, as it was common practice with the Abwehr to keep transmission and reception stations apart, often at two different locations. Nevertheless, the S-90/40 and S-89/80 were used exclusively by the Abwehr. In this case they were issued alongside the Siemens R-IV receiver.

Ast Oslo (Funkstelle) during World War II. Source unknown.

The photograph above is from an unknown source, but is thought to show the radio room at Ast (Abwehrstelle) Oslo during WWII. At the left is a Siemens R-IV receiver with a plug-in module on top. To its right is an Abwehr E-90 receiver and an S-90/40 transmitter both placed on top of the accompanying power supply unit (PSU). The fact that the setup is very similar to the one in the photograph above it, gives rise to the thought that they might have been taken at the same place.


Italian clone   RN-6
The transmitter of the Italian RN-6 spy radio set was a clone of the S-90/40. The circuit is nearly identical and is built around the same valves (CC2 and RL12P50), albeit complemented by Italian components. The RN-6 was made by Nova Radio in Milan shortly after the introduction of the Abwehr's SE-90/40 spy radio suitcase set.

The image on the right was kindly supplied by Antonio Fucci and shows the interior of the RN-6 transmitter in his collection [6].
  

B
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B
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B
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Interior
The interior of the S-90/40 can be accessed by loosening the four large bolts at the corners of the front panel, after which the front panel can be lifted from the metal case shell. All internal parts are mounted to the front panel, wich is actually a metal frame divided in two compartments.

The smallest compartment – here visible on the right – contains the freely adjustable variable frequency oscillator (VFO), built around a CC2 valve, a large coil and an adjustable capacitor.

The larger compartment contains the power amplifier (PA), which is built around an RL12P50 valve that is powered at +700V and delivers an HF power output of 40 Watts. It also has a large coil and a large adjustable capacitor, but the latter is mounted on a Pertinax panel, so that its axle is isolated from the chassis. The antenna is connected to a gavanically separated tank coil.
  
Bottom view

Five banana sockets are present at the front panel for connection of the antenna, of which socket (T) is the common rail. The other sockets (1-4) are each connected to different taps on the tank coil. The wire to socket (1) passes through a current transformer, so that the meter on the front panel can be used to measure the antenna current when tuning the transmitter for maximum power output. The image above shows the interior of the S-90/40 as seen from the bottom.


Circuit diagram
Below is the circuit diagram of the S-90/40 with serial number 2. Note that a number of different version of this transmitter were produced, in particular for different frequency ranges and with a different antenna output circuit. The diagram shown here is of the so-called 'tropicalized' variant of the S-90/40, and is based on the wartime circuit diagram published by Rudolf Staritz [2].


At the left is a free running oscillator (VFO) built around a CC2. Note the two orange 100 pF temperature-compensated capacitors in the tuned circuit. The output of the oscillator is fed to the g1 of the PA valve (RL12P50) via a 20 pF capacitor and a 10 ohm resistor with 5 windings. Also note that the centre contact of the variable capacitor in the tuned circuit of the PA is not connected to ground as is usually the case. Instead the axle is isolated with the Pertinax plate.

S-90/40 with case shell removed Interior seen from the rear right Bottom view Interior seen from the rear left Rear view Oscillator Antenna coil and PA valve Antenna coil
C
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C
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S-90/40 with case shell removed
C
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Interior seen from the rear right
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Bottom view
C
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Interior seen from the rear left
C
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Rear view
C
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Oscillator
C
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Antenna coil and PA valve
C
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Antenna coil

Connections

Power
The transmitter is powered by an external power supply unit that must be connected to the 4-pin socket at the top left of the front panel. Only two voltages are needed: 12.3V for the filaments (LT) and +700V for the anodes of the valves (HT). The pinout of this socket is a s follows:


Finding a suitable connector for this socket can be very difficult, as they were probably purpose-built at the time. As a gap-fill solution it is possible to use female banana plugs and place them over the pins in the socket. The best solution however, would be to make a replica from era-correct materials. Below are the dimensions of the plug (in mm). The body is made of pertinax.

 Download drawing as PDF

Abwehr 4-pin power plug. Click to download as PDF.

WARNING — When inserting or removing the power plug, be careful not to touch the screws at the front side of the plug as they carry a high voltage that will cause a serious – and potentially harmful – shock. This may also be the case after the external power supply unit has been switched off or disconnected, as the capacitors inside the S-90/40 transmitter may still be charged. Always hold the plug by the short sides.
Valves
Below are the connections of the valves as seen from the bottom (the solder side) of the sockets. At the far right is the RL12P50 power amplifier valve, which has two contacts at the top: one for the g3 (connected in parallel to the g3 contact at the socket) and one for the anode (a).



 CC2 datasheet
 RL12P50 datasheet


Specifications
  • HT voltage
    700V DC
  • LT voltage
    12.3V AC
  • Output power
    40 W
  • Valves
    CC2, RL12P50
  • Stages
    Oscillator, Power Amplifier (PA)
  • Frequency
    3.5 - 8.5 MHz
  • Tuning
    Variable Frequency Oscillator (VFO)
  • Antenna
    Symmetrical dipole (wire) or long wire with counterpoise
Documentation
  1. Original S-90/40 circuit diagram
    Copied from original wartime circuit diagrams by Rudolf Staritz [2].

  2. CC2 valve datasheet 1
    Unknown source, 1935/1936. Retrieved May 2018.
     Tungsram CC2 datasheet

  3. RL12P50 valve datasheet 1
    Telefunken, December 1941.
  1. Obtained from Frank's Electron Tube Pages [7].

References
  1. Louis Meulstee, S 90/40
    Wireless for the Warrior, Volume 4, Supplement, Chapter 117.
    Retrieved May 2018.

  2. Rudolf F. Staritz, Original S-90/40 circuit diagram
    Obtained via [1].

  3. Wikipedia, Servizio Informazioni Militare
    Retrieved May 2018.

  4. Photograph of Siemens R-IV and Abwehr transmitter in unknown Abwehr station
    Jørgen Fastner Collection. Obtained May 2017. Reproduced here by kind permission.

  5. Rudolf Staritz, Personal correspondence
    June 2017, via Arthur Bauer.

  6. Antonio Fucci, Image of Italian RN-6 transmitter
    Retrieved May 2018. Reproduced here by kind permission.

  7. Frank Philipse, Valve datasheets
    Website: Frank's Electron tube Pages.
Further information
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© Crypto Museum. Created: Sunday 06 May 2018. Last changed: Saturday, 12 May 2018 - 21:13 CET.
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