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S 87/20
Abwehr spy radio transmitter

S-87/20 was a clandestine 20 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, where it was introduced in 1939. From 1942 onwards, the transmitter was produced by OKW Aussenstelle Wurzen [1].

The S-87/20 was a standalone transmitter, that was commonly used in combination with an existing receiver, such as the Abwehr E-75, the Radione R3 or the Siemens R-IV. It was primarily used for communication with agents in foreign countries, and with front reconnaissance groups.

Contrary to other clandestine transmitters of the era, it is not crystal-operated but features a VFO (variable frequency oscillator), built around an EDD11 double-triode valve (tube). The 20 Watt output power is delivered by two EL11 or EL12 valves in a so-called push-pull configuration.
  
S-87/20 transmitter

The device is housed in a metal case, measures 25 x 19.5 x 10.5 cm and weights 2500 grams. It should be powered by an external power supply unit (PSU) that provides 6.3V AC (LT) and +420V DC (HT). It has two front panel selectable frequency bands: (1) 4.5-7 MHz and (2) 8-12 MHz.

The device is suitable for CW (morse) only, and the key is connected in series with the cathode of the EDD11 oscillator valve. Note the use of civil EL11/EL12 valves, as the Abwehr was not (yet) allowed to use military parts at the time.

The balanced antenna output is intended for use in combination with a symmetrical centre-fed open dipole wire antenna, which makes the device less suitable for field (agent) use, but all the more interesting for fixed stations, not least because of its superior radiation pattern and much reduced antenna matching problems.
  
Fixed Abwehr station consisting of S-87/20 transmitter and Radione R3 receiver

The S-87/20 was produced in small numbers. It is likely that no more than 25 units were made [5]. The device featured here has serial number 6, which is screen-printed at the top left corner of the front panel. The S-87/20 was succeeded in 1939 by the more powerful SE-90/40.

  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.

S-87/20 with morse key S-87/20 transmitter S-87/20 in (normal) vertical position Tuning dial Band selector Antenna tuning indicator Antenna sockets Wehrmacht morse key
A
×
A
1 / 8
S-87/20 with morse key
A
2 / 8
S-87/20 transmitter
A
3 / 8
S-87/20 in (normal) vertical position
A
4 / 8
Tuning dial
A
5 / 8
Band selector
A
6 / 8
Antenna tuning indicator
A
7 / 8
Antenna sockets
A
8 / 8
Wehrmacht morse key

Controls
All controls and connections are located at the front panel of the device. At the top left is a 4-pin socket for connection of the external power supply unit (PSU). A 6.3V AC or DC voltage is needed for the filaments of the valves (LT), whilst +420V DC is needed as HT voltage. At the bottom right are two banana sockets for connection of the antenna (marked H). The device is best used with a centre-fed open dipole. At the top are two banana sockets for connection of the morse key (T).


The device is turned ON with the power switch at the left centre. It controls the supply of the LT voltage. At the bottom left is the frequency dial, which has a 180° scale. A table – for conversion to the actual frequency – was usually supplied with every unit. A lever just below the meter (marked U), is used to select the required frequency band: 1 (4.5-7 MHz) or 2 (8-12 MHz).

Once the frequency is chosen, the two knobs at the bottom right should be used to tune the transmitter for maximum power output, using the meter at the top right as an indicator. The leftmost one (0-180°) is part of the tuned circuit in the output of the power amplifier stage (PA). The rightmost one controls the coupling between the tuned circuit of the PA and the antenna.

Front panel Power socket ON/OFF switch (LT voltage) Tuning dial Band selector Antenna tuning indicator Antenna sockets Morse key socket (Taste)
B
×
B
1 / 8
Front panel
B
2 / 8
Power socket
B
3 / 8
ON/OFF switch (LT voltage)
B
4 / 8
Tuning dial
B
5 / 8
Band selector
B
6 / 8
Antenna tuning indicator
B
7 / 8
Antenna sockets
B
8 / 8
Morse key socket (Taste)





Interior
The interior of the S-87/20 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 contains the freely adjustable oscillator, built around an EDD11 with a balanced tuned circuit. It also holds an array of wire-wound power resistors that are part of the power supply. A large adjustable resistor is used for adjusting the current through the EDD11.

The larger section holds the power amplifier (PA), which is built around two EL11 valves in a push-pull arrangement. The current through the PA valves can be adjusted with a wire-wound adjustable resistor in the oscillator/power supply section, mounted aside the EDD11 adjustment.
  
S-87/20 interior

The tuned circuit of the PA section is fully balanced, just like the antenna loading coil, which is loosely coupled with the tank coil. The latter is mounted on an isolated adjustable axle, so that it can be moved in and out of the tank coil by means of the rightmost knob at the front panel.

The image on the right shows a close-up of the main tank coil which is fully symmetric. At the center – in the space between the two halfs of the tank coil – is the antenna loading coil, which consists of a cylindrical pertinax body with two symmetric windings. The two brown wires at the left are routed to a current transformer which is mounted to the back of the meter. The output of the current transformer is rectified by a Sirutor, converting the meter into a tuning indicator.

The band selector is operated by a small lever at the front panel, just below the meter (marked U).
  
Antenna loading coil fully out

The lever is mounted to a metal axle that runs from left to right through the transmitter's interior. At either end of the axle is a two-position rotary switch that shorts-out part of the windings of each coil (the hexagonal ceramic bodies), thereby effectively selecting the high band (8-12 MHz).

Transmitter removed from the case S-87/20 interior Bottom view Top view Rear view Left side Right side Wire-wound power resistors
Antenna coupling knob Main tank coil with antenna loading coil at the centre Antenna loading coil halfway out Antenna loading coil fully out Oscillator Power Amplifier (PA) with 2 x EL11 valve Current transformer at the back of the meter One side of the band selector (PA)
C
×
C
1 / 16
Transmitter removed from the case
C
2 / 16
S-87/20 interior
C
3 / 16
Bottom view
C
4 / 16
Top view
C
5 / 16
Rear view
C
6 / 16
Left side
C
7 / 16
Right side
C
8 / 16
Wire-wound power resistors
C
9 / 16
Antenna coupling knob
C
10 / 16
Main tank coil with antenna loading coil at the centre
C
11 / 16
Antenna loading coil halfway out
C
12 / 16
Antenna loading coil fully out
C
13 / 16
Oscillator
C
14 / 16
Power Amplifier (PA) with 2 x EL11 valve
C
15 / 16
Current transformer at the back of the meter
C
16 / 16
One side of the band selector (PA)

Circuit diagram
Below is the complete circuit diagram of the S-87/20 transmitter, with the exception of the power supply unit (PSU), as it was originally published by Rudolf Staritz [2]. This diagram was taken from the original wartime drawings but does not fully match with the actual device as we found it.

Original circuit diagram as published by Rudolf Staritz [2]

As the actual circuit is different from the circuit diagram above, and because we had to restore our transmitter, we decided to recreate the circuit diagram from scratch. The result is available below. For clarity we have altered the orientation somewhat, so that the balanced nature of the circuit is more obvious. At the bottom right is the power socket with a network of wire-wound resistors to obtain the desired voltages. The device is turned on with switch S1 (6.3V AC LT).

Actual circuit diagram (Crypto Museum)

At the bottom left is a free-running oscillator, built around an EDD11 touble triode. It is activated by the morse key, which is connected in series with the cathode. The oscillator signal is fed to the grids of the power amplifier (PA), that is built around two EL11 valves in push-pull configuration.

The output of the PA is fully balanced, with its loading coil mounted on an axle, so that it can be moved in and out of the PA tank coil with a knob at the front panel. The center of the loading coil is looped through a current transformer that drives the meter, using a Siemens Sirutor 1 to rectify the alternating antenna current. This way, the meter can be used as an antenna tuning indicator.

  1. A Sirutor is an early type of semiconductor diode, which consists of a series of stacked copper-oxide tables. At the time they were mainly used in high-voltage low-current and high-frequency (HF) applications [3].

Restoration
When we received our S-87/20 transmitter, it was not in working condition. Some of the large wire-wound resistors appeared to be broken, and in a earlier restoration attempt – to replace a couple of physically damaged resistors – some wires to the PA valves had been misconnected.

At one corner of the oscillator compartment is an array of five wire-wound power resistors, that are used to create the internal voltages for the valves, all derived from the main +420V supply.

Three of these resistors appeared to be broken (i.e. ∞ resistance). As these are the high-ohm resistors (30k and 2 x 20k) this is probably due to oxidation of the thin resistance wire. Luckily, we were able to repair the 30k one at the corner, by removing the end caps, cleaning the contacts and the wire and re-assembling it. It measures as 27k, which is OK for the current application.
  
New 10k power resistor installed (green)

The two 20k resistors appeared to be beyond repair, but finding a proper replacement turned out to be dificult. As they are connected in parallel (to form a 10k resistor), we decided to replace them with a slightly larger single 10k resistor from new-old-stock, that is correct for the era.

The image above shows the green replacement 10k resistor that supplies the EL11 g2 voltage.

The next challenge were the two large adjustable capacitors (in the oscillator and the PA), both of which were binding and had to be removed for repair. The one in the PA stage was easily fixed, but the oscillator one had to be disassembled completely. Eventually both were fully restored.

Whilst the PA tuning capacitor was removed, we were able to take a closer look at the parts that are mounted below the sockets of the PA valves.
  
Re-assembed PA section

In a previous restoration attempt, the two 20k resistors (connected between the g1 of each EL11 and ground) had been replaced by modern types, as the original ones were physically damaged. In the processing of doing this, the two 10nF capacitors (connected between the cathodes of the EL11 valves and ground) had accidentally been connected to the g1 of the rearmost EL11 valve.

The two resistors were replaced by era-correct (red) types, and the wiring of the capacitors was corrected. We also added the missing decoupling capacitor to the anode voltage rail. The result is shown above (with removed tuning capacitor).

To repair the tuning capacitor of the oscillator's tuned circuit, it was necessary to remove the tuning dial, allowing the latter to be cleaned more thoroughly. For this, it was necessary to remove the oscillator coil first, as the mounting nut of the index hairline (shown in the image on the right) is located under the hexagonal coil.
  
Close-up of the tuning dial

The tuning dial was disassembled, cleaned and reassembled. The bakelite knob at the center of the dial was polished in order to remove a couple of severe scratches. Once the individual parts were ready, the entire unit was reassembled again. The outer case shell – which had several dents – was straighted, before the front panel was screwed-in again.

So far, the following has been restored:

  • 30k power resistor repaired
  • 2 x 20k broken power resistors replaced by a single 10k type
  • PA tuning capacitor repaired (was binding)
  • Oscillator tuning capacitor repaired (was binding)
  • 2 x 20k resistors at PA g1 replaced by era-correct types
  • 2 x 10nF capacitor at PA cathode connection corrected
  • 1 x 20nF capacitor at 420V rail added (era correct type)
  • One EL11 glass glued
  • Tuning dial cleaned, bakelite knob polished
  • Front panel cleaned and paint partly restored
  • Case shell straightened.
  • Case shell clained and paint partly restored
PA stage (as found) Oscillator/power section (as found) Misconnected capacitors Green replacement resistors Wire-wound power resistors New 10k power resistor installed (green) Replaced (red) resistors Re-assembed PA section
Repaired tuning capacitor (refitted) Cleaned and restored front panel Restored unit Restored unit Close-up of the tuning dial Tuning dial Dial lock
D
×
D
1 / 15
PA stage (as found)
D
2 / 15
Oscillator/power section (as found)
D
3 / 15
Misconnected capacitors
D
4 / 15
Green replacement resistors
D
5 / 15
Wire-wound power resistors
D
6 / 15
New 10k power resistor installed (green)
D
7 / 15
Replaced (red) resistors
D
8 / 15
Re-assembed PA section
D
9 / 15
Repaired tuning capacitor (refitted)
D
10 / 15
Cleaned and restored front panel
D
11 / 15
Restored unit
D
12 / 15
Restored unit
D
13 / 15
Close-up of the tuning dial
D
14 / 15
Tuning dial
D
15 / 15
Dial lock

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: 6.3V for the filaments (LT) and +420V 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-87 transmitter may still be charged. Always hold the plug by the short sides.
EL11 or EL12 power amplifier valve
EL11 and EL12 are penthodes from the so-called Harmonische Reihe, developed in 1938 and produced by many manfucturers, including Philips, Telefunken, Siemens, TeKaDe and Valvo. The EL11 has a maximum anode voltage of 250V, whilst this is 375V for the EL11/375.



 EL11 datasheet
 EL11/375
 EL12 datasheet
 EL12/235, /350 and /375


EDD11 oscillator valve
EDD11 is a double triode, introduced in 1938 by Telefunken, mainly for use as an audio amplifier in early car radios. In the S-87/20 it is used as a balanced oscillator, which forms the device's variable frequency oscillator (VFO).



 EDD11 datasheet


Specifications
  • HT voltage
    420V DC
  • LT voltage
    6.3V AC or DC
  • Output power
    20W
  • Valves
    EDD11, 2 x EL11 (or EL12)
  • Stages
    Oscillator, Power Amplifier (PA)
  • Frequency
    4.5 - 7 MHz and 8 - 12 MHz
  • Tuning
    Variable Frequency Oscillator (VFO)
  • Antenna
    Symmetrical dipole (wire)
Documentation
  1. EDD11 valve datasheet 1
    Telefunken. September 1938, April 1939.

  2. EL11 valve datasheet 1
    Telefunken. June 1938, June 1940.

     EL11/375 specifications

  3. EL12 valve datasheet 1
    Telefunken. August 1939 — July 1943.

     EL12/235, /350 and /375 specifications
  1. Obtained from Frank's Electron Tube Pages [4].

References
  1. Louis Meulstee, S 87/20
    Wireless for the Warrior, Volume 4, Supplement, Chapter 117.
    Retrieved March 2018.

  2. Rudolf F. Staritz, Original S-87/20 circuit diagram
    Obtained via [1].

  3. Radio Museum and contributors, Siemens Sirutor
    Retrieved September 2016.

  4. Frank Philipse, Valve datasheets
    Website: Frank's Electron tube Pages.

  5. Various collectors, Personal correspondence
    The following collectors have contributed to this page: Gerd Reinel (DH2FAI), Günter König (DJ8CY), Manfred Bauriedel (DK4NQ), Rudolf Staritz (DL3CS). March 2018.
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© Crypto Museum. Created: Thursday 15 March 2018. Last changed: Saturday, 31 March 2018 - 07:52 CET.
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