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FSS-7   SP-15/NL
Dutch stay-behind version of the SP-15 - wanted item

The SP-15 was a complete self-contained modular spy radio station, developed in Germany in the early 1960s by Wandel & Goltermann and H. Pfitzner, for use by the BND, Special Forces (SF) and the German Stay-Behind Organisation (SBO). In the late 1960s, the set was also introduced with the Dutch Stay Behind Organisation O&I, where it became known as the FSS-7 1 (also as FS-7).

The set was supplied in a green military water­tight container that was supplied by the Dutch Army, and contained an FS-7 transmitter, an FE-8 receiver, a mains AC power supply unit (PSU) and a DC battery PSU. The accessories, like the wire antennas, connectors, lamp fittings, morse key, headphones, isolators, tools, cables, etc., were supplied in two grey plastic lunch boxes.

Initially, the transmitter was crystal operated and a suitable set of crystals was supplied in a small metal container. The FE-8 receiver had a built-in VFO and was suitable for all SW frequencies.
FSS-7 with watertight container

Around 1975, by which time the Germans had already swapped their SP-15 sets for the newer SP-20, the FSS-7 was given a mid-live upgrade by the addition of a purpose built synthesizer. This required modifications to all FS-7 transmitters and the addition of an extra junction box.

This junction box acted as an interface that connected all parts of the transmission chain together. It was made by the Dutch themselves and contains additional circuitry for calibrating the receiver, switching the antenna between transmitter and receiver, powering the Racal synthesizer and performing FSK modulation.

FSK, or Frequency Shift Keying, was necessary to support the much higher speed of a new burst encoder that was also part of the set. The image on the right shows the upgraded FSS-7 set 2 including synthesizer, junction box and key.
Complete setup with standard morse key

The unit at the front left is the FE-8 (BN-58) receiver which is also part of the set. Although it works independently from the transmitter, its antenna is connected via the junction box and an extra cable provides it with the side tone from the keyer. The FSS-7/SP-15 was used in The Netherlands until the introduction of the DZO-81 (PRM-4150) and later the AZO-91 (FS-5000).

  1. In the Netherlands the SP-15 was named after the FS-7 transmitter (Funk-Sender), but it was also known as FSS-7 (Funk-Sender-Synthesizer). Some (internal) literature refers it as FFS-7 (Feld-Funk-Sender).
  2. The FSS-7 set shown here is from the collection of Museum Jan Corver in The Netherlands. Many thanks for allowing us to take detailed pictures of this special version of the SP-15 and its container.

Container Container FSS-7 with watertight container SP-15 with Synthesizer and Junction Box Dutch FFS-7 (SP-15) set with Speicher burst encoder Complete setup with standard morse key
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FSS-7 with watertight container
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SP-15 with Synthesizer and Junction Box
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Dutch FFS-7 (SP-15) set with Speicher burst encoder
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Complete setup with standard morse key

The Dutch Stay Behind Organisation

Like most other Europeans countries during the Cold War, The Netherlands had its own Stay-Behind Organisation (SBO), a secret army that would be activated in the event of an invasion by the Soviet Union. The Dutch SBO was established in 1946 or 1947, just after WWII had ended, and became known as O&I (later: A&B). After much debate, the organisation was dismantled in 1992.

During the initial years, O&I used its own ZO-47 radio sets, develop by Dutch electronics giant Philips, but once the North Atlantic Treaty Organization (NATO) was established in 1949, newly developed American radio sets were used instead, such as the RS-1 and later the RS-6.

Around 1968, it was decided to replace the American radios by the SP-15 radio set, that had been in use by German organisations since the early 1960s. As the Dutch operated in small unlinked cells, each cell was given its own radio set and radio operators were trained on its use.

Each radio operator was given two watertight metal containers like the one shown above. One was filled with money, juwelry, a pistol and ammunition. The other one contained the radio set. The containers were generally stored in the attic or burried in the garden on in a secret cache and were only retrieved for test, trainings and, of course, in the event of a war. After a series of incidents, both nationally and internationally, the secret network became known to the public which led to public debates and investigations. As by that time the Berlin Wall had come down and the Soviet Union had collapsed, the Dutch government dismantled the organisation in 1992.

 More about O&I

Transmission chain
The diagram below shows the complete FSS-7 transmission chain. The FE-8 receiver is not shown here. The transmitter and the PSU, that are normally connected to each other, are separated and the new junction box is placed inbetween them. The new synthesizer is attached to the front of the junction box and a high-speed burst encoder is connected to the DIN socket at the top.

Note that both the RX and TX antenna connections are routed via the new junction box as well, so that from now on only one antenna is needed. Switching between RX and TX antenna is done automatically by the circuitry inside the junction box. It also contains a calibrator for the receiver.

(modified) FS-7 transmitter
FE-8 (BN-58) receiver
Mains AC power supply unit
DC battery power converter
Additional synthesizer for the transmitter Junction box with several interfaces
Tools and accessories High-speed morse burst encoders
Transmitter   FS-7
The FS-7 transmitter was developed by Pfitzner in Germany and is built around two valves (EL95 and EL81) and just one transistor (OC450).

Around 1975 it was modified for use in combination with the new Racal synthesizer. As a result, the crystal socket at the far left of the front panel was removed.

 More information
Modified FE-7 transmitter

Receiver   FE-8
The FE-8 receiver was developed by Wandel & Golterman (W&G) in Germany in 1958 and can be used stand-alone. It can be powered by an internal battery or by an external DC source.

The receiver has two ranges (2-5.1 MHz and 5.1-9 MHz) with permeability tuning, resulting in a linear scale for both ranges. This receiver was also used with the later SP-20 radio sets.

 More information
FE-8 (BN-58) receiver

AC Power supply
The FSS-7 was supplied with a highly compact mains power supply unit (PSU) that was suitable for all common AC mains voltages in the world, between 95 and 235 V. It produces the LT and HT voltages for the transmitter.

It is normally slotted into the left side of the transmitter or, in the case of the modified Dutch FSS-7, into the left side of the junction box.
Mains AC power supply unit

DC Power supply
When no mains AC network was available, it was also possible to power the FSS-7 from a 12V DC source, such as the battery of a car. This was done by swapping the AC PSU for a DC one.

It contains a power inverter that converts the 12V DC input into 6.3V for the filaments and a HT AC voltage for the transmitter's valves. Like the AC PSU, it is slotted into the left side of the transmitter or the junction box.
DC battery inverter

Around 1974, the German SBO swapped their SP-15 units after 15 years of service for the newer SP-20 with had a synthesizer-driven transmitter. Although The Netherlands desperately wanted to switch form crystal-based operation to synthesizers, their sets were just 6 years old, making a transition to the newer sets not economically justifiable, and another solution had to be found.

It was decided to give the FSS-7 a mid-live upgrade by adding an external synthesizer to the transmitter. A suitable one was subsequently developed by Racal (UK) and 160 units were ordered for a total price of NLG 2 million. 1

A budget for the development and manufacture of the units was arranged and the sythesizers were delivered by Racal during the following year. A typical unit is shown in the image on the right. Is has four rotary selectors for setting the frequency in a recessed bay at the top front, and a single 9-pin male sub-D connector at the rear.
Racal synthesizer for the SP-15/FFS-7

All voltages and HF signals are guided to and from the synthesizer via this connector. The device is higher than the other modules and has a typical Racal-look. In order to use it with the FS-7 transmitter, the latter had to be modified and a special junction box had to be fitted inbetween.

Taking away the bottom panel of the synthesizer reveals a rather complex structure of metal cans of varying sizes and a lot of wiring to connect it all together. Although this Racal unit definitely would not have won the prize for most beautiful design, it reflects the state of technology in 1974, when synthesizers were just beginning to find application in professional equipment.

All in all, the Racal synthesizer proved to be a very reliable addition to the radio set and greatly improved the overall flexibility of the set as it eliminated the limitations of the old crystals.
Synthesizer interior detail

Once connected to the set, the four rotary selectors on top of the device are used for setting the transmission frequency. The leftmost knob can be set from 2-8 MHz. The other three knobs have the full 0-9 range. This means that frequencies can be generated from 2.000 MHz to 8.999 MHz.

  1. This means that each synthesizer had a price tag of NLG 12,500 (EUR 5680) in 1975.

SP-15 with Synthesizer and Junction Box Racal synthesizer for the SP-15/FFS-7 9-pin male sub-D connector Connecting the synthesizer to the junction box Interior Synthesizer interior Synthesizer interior detail Interior detail
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SP-15 with Synthesizer and Junction Box
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Racal synthesizer for the SP-15/FFS-7
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9-pin male sub-D connector
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Connecting the synthesizer to the junction box
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Synthesizer interior
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Synthesizer interior detail
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Interior detail

Junction Box   IF-7
When the Racal synthesizer was added to the FSS-7, the transmitter had to be modified as it would no longer be used directly with crystals. Furthermore, the signal from the synthesizer had to be fed to the oscillator stage of the transmitter and the synthesizer itself had to be powered.

In order to fit all units together in the upgraded set, a special junction box or common interface was built by the technical department of O&I. It consists of a small rectangular metal box with connectors on all sides. It is fitted in between the transmitter and the PSU. The synthesizer is connected to the sub-D connector at the front.

The FS-7 transmitter is modified and can no longer be used stand-alone. The crystal socket at the front left is removed and its lines are now routed via the large connector at its left side, where the new junction box is now connected.
Junction box seen from the front right

The signal from the synthesizer is fed to the transmitter via the junction box. For emergency purpuses, e.g. when the synthesizer is broken, it is still possible to use crystals, by inserting them into the socket at the front of the junction box. For this the synthesizer has to be removed.

On top of the junction box is a 6-pin 270° female DIN socket, for connection of the burst encoder or, in case the burst encoder is broken or can't be used, the small standard morse key.

Towards the front right is a small red push button that can be pressed to activate the built-in reference oscillator that is used for calibrating the receiver, much like the separate calibrator does with the standard version of the SP-15. Just behind the CAL button is a 2-position slide switch marked FS/OO. It is currently fixed to OO. The function of this switch is currently unknown.
SP-15 with Synthesizer and Junction Box

The junction box also handles automatic switching of the antenna between transmitter and receiver, a feature that is missing from the standard SP-15 set which requires separate send and receive antennas. The antenna connections are at the rear, close to the internal antenna relay.

In addition to all of this, the junction box also contains a small PSU for the synthesizer which needs its own stabilized power source. It works by taking the HT voltage from the main PSU to the FS-7, and feeding it to a small transformer.

The image on the right shows the interior of the junction box as seen from the rear right. At the bottom right is the long conector that is fitted to the socket at the left side of the transmitter. At the left is the transformer of the synthesizer's PSU. It also powers the electronic circuits inside the junction box. The stabilizer is at the right.

The junction box contains a modulator for Frequency Shift Keying, or FSK, which is needed to support the much higher data rate of the Speicher burst encoder. At the rear of the junction box, between the antenna sockets, is a 3.5 mm jack socket that provides the side tone from the keyer. It should be connected to the side-tone input of the receiver and is used to monitor the sound. Although the junction box may seem like a simple device at first, it implements the following:

  • Antenna switcher
  • Interface between synthesizer and transmitter
  • Frequency Shift Keying (FSK) modulator
  • Crystal oscillator
  • Side-tone generator
  • Calibrator
  • PSU for synthesizer and circuitry
Junction box seen from the rear right Junction box seen from the front right Junction box seen from the front left Junction box seen from the rear left Front panel Rear side Modified transmitter: the crystal socket is no longer accessible. SP-15 with Synthesizer and Junction Box
Interior Interior Interior Interior seen from the right side Interior seen from the left side Interior seen from the top PSU detail and antenna relais Switch detail
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Junction box seen from the rear right
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Junction box seen from the front right
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Junction box seen from the front left
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Junction box seen from the rear left
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Front panel
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Rear side
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Modified transmitter: the crystal socket is no longer accessible.
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SP-15 with Synthesizer and Junction Box
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Interior seen from the right side
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Interior seen from the left side
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Interior seen from the top
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PSU detail and antenna relais
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Switch detail

Like most spy radio sets of the era, the FSS-7 (SP-15) was supplied with a wide range of accessories, many of which were used to rig up the set under improvised conditions. There are various types of mains plugs and even adapters for 'stealing' power from a lamp fitting.

In addition there are the standard accessories, such as the antenna wires, counterpoise, wind-up antennas, morse key, earpieces, isolators, tools and even a small pocket lamp. Examples of some of the accessories in the images below.
Two lunch boxes with accessories

Two lunch boxes with accessories Luchbox with accessories Luchbox with accessories Small manual morse key Mains cable Antenna and counterpoise Screwdrivers Pocket lamp
Wind-up antennas Side-tone cable Spare earpieces and connectors Morse indicator lamp Antenna cable Receiver power cable
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Two lunch boxes with accessories
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Luchbox with accessories
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Luchbox with accessories
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Small manual morse key
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Mains cable
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Antenna and counterpoise
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Pocket lamp
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Wind-up antennas
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Side-tone cable
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Spare earpieces and connectors
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Morse indicator lamp
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Antenna cable
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Receiver power cable

Burst encoders
In the early days of the SBOs, radio messages were sent by means of morse code, for which operators had to be trained especially. The downside of using manual morse code is the fact that the transmitter is on the air for long periods of time, especially when sending long messages, giving the enemy a chance to locate the station by means of Radio Direction Finding, or RDF.

With the arrival of the first American radio sets, like the RS-1, the use of high-speed burst encoders was introduced. These devices allowed a message to be pre-recorded on some kind of medium, and then sent via a transmitter at very high speed. This reduced the on air time and, hence, the chance of detection and interception.

In the early days of the FSS-7 the old American GRA-71 burst encoder was used, as supplied with the RS-1 and RS-6 sets, but they were soon replaced by the newly developed fully electronic German Speicher (Memory) that is shown here.
Dutch FFS-7 (SP-15) set with Speicher burst encoder

The Speicher was initially connected directly to the morse key DIN socket on the transmitter, but later (after the introduction of the synthesizer) to the black junction box at the centre. It required a text-based message to be converted into numbers first, using some kind of manual encoding scheme. The numbers were then entered into the device in groups of five, separated by spaces.

Once the message was complete, the transmitter was powered up and Speicher was set to PLAY, after which the entire message was sent in just a few seconds, with far less risk of being captured.

Around 1973/1974, some (but not all) Speicher units were replaced by the more advanced MMP that is shown in the image on the right. It is suitable for alphanumerical messages. The MMP was not supplied with the FSS-7 set shown here.

 More about the Speicher
 More about the MMP
 More about GRA-71

MMP connected to the FS-7 transmitter (SP-15)

Below is the pinout of the 6-pin 270° DIN socket on the IF-7 interface. Although this socket is compatible with the 5-pin DIN socket on the FS-7 transmitter, it carries voltages that are not available on the transmitter socket. The morse key or burst transmitter should be connected to this socket, rather than to the transmitter. When using the GRA-71 burst encoder, the keyer is powered by the -12V DC voltage on the socket.

  1. Not connected
  2. KEY
  3. Ground
  4. (-)12V DC
  5. 6V AC
  6. Unknown (brown wire from synthesizer DB9 socket)
  1. Museum Jan Corver, Exhibition Secret Messages
    The Dutch version of the SP-15 was on display during this exhibition.
    Many thanks for allowing us to take detailed photographs of this set.

  2. Louis Meulstee, Wireless for the Warrior, volume 4
    ISBN: 0952063-36-0, September 2004.

  3. Museum Verbindingsdienst, Burst Encoders for Stay-Behind use
    Dutch Signals Museum. Photographed by Crypto Museum. 25 February 2009.

  4. Geschiedenis van de Sectie Algemene Zaken, Hoofdstuk VI, Consolidatie
    History of the Section General Affairs, Chapter 6, Consolidation. pp. 79 - 80 (Dutch)
    Describing the period May 1970 - December 1981. Dutch National Archives. Top Secret. Partly declassified and released in 2007 under the FOI Act.
Further information
Complete FSS/7/FS-15 set from the collection of Museum Jan Corver

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Crypto Museum. Created: Saturday 19 September 2015. Last changed: Friday, 03 November 2017 - 16:30 CET.
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