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FS-7   FSS-7
Valve-based spy transmitter

The FS-7 was a minature valve based transmitter that was developed by Pfitzner in Germany in the early 1960s. It was designed for the SP-15 spy radio set and had the same size as the FE-8 receiver. It was intended for espionage activities and Stay-Behind Organisations (SBO) and was suitable for the transmission of morse code signals on the HF radio bands. The transmitter is the successor to the KSG-Sender and is also known as BN-22 and (when modified) as the FSS-7.

The transmitter was built around two valves (EL95 and EL81) and produced a power output of 10W or 20W, selectable from the power supply unit (PSU). A transistor (BC458) was used for the manual morse keyer and the burst encoder.

The FS-7 was crystal operated and was suitable for all frequencies between 2.5 and 24 MHz, divided over 6 colour-coded ranges. The crystal was inserted into a socket at the far left of the front panel. In The Netherlands, some FS-7 units were modified for use in combination with an external frequency synthesizer (see below).

The transmitter is only suitable for morse code (CW) and can be keyed in two different ways [1]: either by switching the cathode of the oscillator valve (EL95) or by keying the grid of the PA valve (EL81). The former allows transmission speeds up to 100 baud and is typically meant for manual keying. The second method allowed speeds up to 800 baud and was typically used with external burst encoders, such as the GRA-71 and the RT-3, and also with the later Speicher burst encoder.

It is likely that Frequency Shift Keying (FSK) was later added to the FS-7 by means of an external modification, in order to support the faster and more versatile MMP burst encoder in the early 1980s. As the transmitter does not produce an audio tone when sending morse, the telegraph key can be connected to the transmitter and the receiver simultaneously (sidetone).

The FS-7 was introduced by the German Intelligence Agency, the Bundesnachrichtendienst (BND), in the early 1960s as the successor to the 1957 KSG-Sender. In the early years, the burst encoder of the KSG-Sender, the so-called Kurzsignalgeber (KSG) was used with the FS-7. In the late 1970s and early 1980s, the FS-7 was replaced by the fully transistorised SP-20 spy radio set.

The FS-7 transmitter Front view Front panel The crystal socket at the font left Interior of the FS-7 transmitter The oscillator valve (EL95) The PA valve (EL81) Circuit diagram
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The FS-7 transmitter
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Front view
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Front panel
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The crystal socket at the font left
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Interior of the FS-7 transmitter
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The oscillator valve (EL95)
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The PA valve (EL81)
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Circuit diagram

Frequency ranges
The transmitter covers 2.5-24MHz in 6 colour-coded ranges. The 4 knobs at the front of the unit each have a differently coloured arrow and a table on top of the unit is used to determine the settings for each frequency range:

  • 2.5 - 3 MHz (orange)
  • 3 - 4 MHz (blue)
  • 4 - 5 MHz (yellow)
  • 5 - 8 MHz (green)
  • 8 - 14 MHz (red)
  • 14 - 24 MHz (brown)
A morse key (or keying device) can be connected to the transmitter in two different ways. The standard method is shown in the drawing below. It is suitable for speeds up to 100 baud. In this mode a transistor (OC450 or BC458) is used for switching the cathode of the oscillator (EL95), whilst the PA (EL81) is kept running. Note that pins 1 and 2 of the 5-pin DIN plug are shorted.

Connecting a manual key to the FS-7

When using the RT-3, or indeed any other type of burst encoder, a different connection should be made inside the 5-pin DIN plug. As a result, the oscillator is kept running, whilst the screen-grid voltage of the PA (EL81) is switched by the RT-3. This allows speeds up to approx. 800 baud.

Connecting a burst encoder to the FS-7

As the FS-7 does not provide an acoustical feedback when sending morse code, a special cable with a T-adapter was supplied. It allowed the morse key to be connected simultaneously to the FS-7 transmitter and to the side-tone input of the FE-8 receiver (i.e. in parallel with the morse key or the burst transmitter).

A third method for keying the morse code even faster, e.g. at 1200 baud, was developed later, when the MMP burst encoder appeared. As both methods described above are not suitable for sending morse code at such high speeds, a small FSK modulator was added. It was inserted between the crystal socket of the FS-7 and the crystal and connected to the key output of the MMP. By switching an adjustable capacitor in parallel to the crystal, the transmission frequency was changed in the rythm of the morse signal.

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The original morse key that was supplied with the FS-7
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More T-connector with cable
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Kurzsignalgeber (KSG) burst encoder, originally developed for the KSG-Sender
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RT-3 mechanical burst encoder
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The first fully electronic burst encoder that was used with the SP-15
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An improved electronic burst encoder
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FSK modulator for allowing the SP-15 to be used at 1200 baud

Circuit Diagram
Below is the circuit diagram of the FS-7 transmitter, as it was recreated from the original one by Karsten Hansky [5] in May 2015. Click the image for the hi-res version. At the left is the EL95 oscillator valve. Note that the kathode is keyed by means of an OC450 silicium transistor. The right half of the diagram is the Power Amplifier (PA) built around an EL81 valve. Note the two neon lights and the antenna current meter, all of which are used for maximum power tuning.

FS-7 circuit diagram. Click to download a hi-res version.

In the mid-1970s, the Dutch Stay-Behind Organization O&I (Operatiën en Inlichtingen, eng.: Operations and Intelligence) ordered the development of an external synthesizer for the FS-7. The synthesizer was probably built by RACAL and the existing FS-7 units were modified for this.

The modified transmitter became known as the FSS-7. As far as we currently know, The Netherlands was the only country where a synthesizer was added to the FS-15. Other countries, such as Germany, moved to the synthesized FS-20 spy radio set at this stage.

The synthesizer was implemented as a seperate box that was connected to the other modules by means of a purpose-built black box. The crystal socket was subsequently removed from the FS-7 and the hole was closed with a small aluminium panel, as shown in the image on the right.
The blocked crystal socket

According to documents found in the Dutch National Archives [3], 160 synthesizers were ordered (100 for the I-department and 60 for the O-department) in 1974 for a total amount of 2 million Dutch Guilders (more than 900,000 Euro). This breaks down to 5600 Euro for a single unit.

Although the name of the manufacturer is not revealed in the document [3] (parts of it are still classified) it is likely that the synthesizer was developed and built by RACAL in the UK.

The image on the right shows the synthesizer that was found inside the SP-15 container that is now in the collection of Museum Jan Corver [4]. It is constructed in such a way that it can be plugged straight into the black box that combines all modules, by means of a 9-pin male sub-D connector (DB9) at the back. At the front are four recessed dials with 10 positions each.
The FSS-7 synthesizer

This way, the frequency of the FSS-7 transmitter can be set in kHz. A few years later, approx. 1976, the SP-15 and hence the FS-7 was replaced by a fully digital spy radio station, also manufactured by Racal, known as the PRM-4150. This suitcase-based transceiver was used as a gap-fill solution until it was finally replaced by the FS-5000 (Harpoon) in the early 1990s.

 More about the Dutch version

All units unpacked from the container The complete radio ready for use The modified FSS-2 transmitter, showing the covered crystal socket. The Racal synthesizer unit Close-up of the controls of the Racal synthesizer unit The IFS-7 interace unit
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All units unpacked from the container
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The complete radio ready for use
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The modified FSS-2 transmitter, showing the covered crystal socket.
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The Racal synthesizer unit
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Close-up of the controls of the Racal synthesizer unit
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The IFS-7 interace unit

  1. FS-7 Circuit Diagram 1
    Original circuit diagram from H. Pfitzner

  2. FS-7 Circuit Diagram (repro)
    Recreated by Karsten Hansky. May 2015.

  3. FS-7 Prüfvorschrift (test protocol) 1
    Date unknown.
  1. Document kindly provided by Jim Meyer [1]

  1. Helmut 'Jim' Meyer, HS0ZHK, My way to Ham - Radio and beyond
    Website QRZ.COM. Personal correspondence. Retrieved April 2013.

  2. H. Pfitzner, FS-7 Circuit Diagram
    Date unknown.

  3. Geschiedenis van de Sectie Algemene Zaken, Hoofdstuk VI, Consolidatie
    History of the Section General Affairs, Chapter 6, Consolidation (Dutch).
    Dutch National Archives. Top Secret. Partly declassified in 2007 under the FOI Act. p. 80.

  4. Museum Jan Corver, Complete SP-15 set in water-tight container
    Photographs taken during the exhibition Secret Messages.
    Crypto Museum, October 2008.

  5. Karsten Hansky, FS-7 Circuit Diagram
    Recreated from original diagram. May 2015.
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Crypto Museum. Created: Sunday 20 September 2015. Last changed: Sunday, 20 January 2019 - 17:49 CET.
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