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Paraset   Mark VII
WWII clandestine spy radio set

The Whaddon Mark VII (Mk7), commonly known as the Paraset or Paracette (French), is a valve-based, lightweight, short-wave, CW-only clandestine radio transceiver or spy radio set, for the 40 and 80 metre radio bands, developed in 1941 by the Royal Signals Section VIII at Whaddon Hall and Little Horwood (UK). It was used throughout WWII by agents of the SIS (MI6) and the SOE, who were dropped over occupied territory, and also by resistance organisations in the countries under occupation of Nazi Germany, in particular in Belgium, France, The Netherlands and Norway [2].

The Mk-VII (Mk7) was one of the first successful miniaturised radio sets of WWII. It was originally developed for the SIS (MI6) but was later adopted by the Special Operations Executive (SOE), who used it for operations behind enemy lines. It was often dropped by parachute for field agents and resistance groups — hence the name Paraset [6].

The device is built with three thermionic valves (vacuum tubes) and is housed in a metal case. It was supplied with a pair of headphones and was powered either by a mains power supply unit (PSU), or from a battery, using a vibrator pack.
  
Paraset (Mk. VII) with headphones

The Paraset was small, lightweight and very easy to operate, but had several disadvantages. Its valves are not permanently installed, and have to be removed – and stowed inside the top cover – before the case can be closed. Furthermore, it can only be operated in CW (morse) with the small internal morse key at the front right. There is no provision for an external morse key. 1 As the device has no sidetone provision either, you can't hear your own morse code when transmitting.

But the major – and potentionally lethal – issue during WWII, is the fact that the self-regenerative receiver unknowingly becomes a transmitter when the regenerate control is used incorrectly. This unwanted RF signal can be traced from some distance by means of radio direction finding (RDF), even when the device is not in transmit mode. Not knowing that the Germans actively exploited this mis-feature, many agents lost their lives as a result of it. This was particularly the case in major urban areas, where the Germans deployed many direction finding vehicles, and less so in distant rural areas. In the latter part of the war, the Paraset was therefore mainly used in Norway.

After WWII, in the early stages of the Cold War, some Parasets were briefly (re)used by the newly established Stay-Behind Organisations of several European countries, including The Netherlands, until they were replaced by better modern alternatives of domestic or American manufacture.

 Operating instructions

  1. There are Parasets with a socket for an external morse key – located to the right of the internal key – such as S/N 10313, but it is unknown whether this is an original feature or an aftermarket modification [5].

WARNING — The Paraset is one of the rarest spy radio sets of WWII. Very few have survived. It is also one of the most popular designs for replicas, probably because of its aestetics and the simplicity of its circuits [4]. With so many reproductions and so few real devices, collectors and museums should be careful when obtaining one.
Paraset with valves stowed inside top lid
Paraset with valves stowed inside top lid
Paraset with open lid - valves and crystal installed
Paraset (Mk. VII) with headphones
Paraset with original power supply unit (PSU)
Mark VII (Paraset)
Paraset with open lid and valves installed
Mark VII (Paraset) with PSU and accessories
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Paraset with valves stowed inside top lid
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Paraset with valves stowed inside top lid
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Paraset with open lid - valves and crystal installed
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Paraset (Mk. VII) with headphones
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Paraset with original power supply unit (PSU)
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Mark VII (Paraset)
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Paraset with open lid and valves installed
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Mark VII (Paraset) with PSU and accessories

Features
The diagram below provides a quick overview of the controls and connections of the Paraset, all of which are located on the top panel. At the upper edge of this panel are three 8-pin sockets in which the valves should be installed. The valves have to be removed – and stowed inside the top cover – before the case can be closed. The PSU should be connected to the socket at the top left.

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The mode of operation (receive or transmit) is determined by the position of the MODE-selector at the centre. The receiver can freely be tuned over the entire 3-7.6 MHz range by means of the large and small knobs at the bottom left, using the calibration chart inside the case lid as a guide.

The frequency of the transmitter is determined by a quartz crystal that should be inserted into the socket at the top right. After connecting a suitable wire antenna and counterpoise (ground), the oscillator and antenna adjustments at the lower edge have to be tuned for maximum output — whilst holding down the morse key — using the tuning indicators (lamps) to find the optimum.

Advantages
  • Small dimensions (probably the smallest transceiver at the time)
  • Lightweight design (easy to transport and hide)
  • Lightweight PSU
  • Relatively high power output (5W)
  • Valves accessible from the outside
  • Easy to operate
  • Service-friendly
Disadvantages
  • Valves have to be removed prior to closing the case
  • No provision for external morse key
  • No sidetone during transmission
  • Potentially strong RF signal emitted from the receiver
  • Under-dimensioned power supply unit
  • 240V setting is actually the same as 225V setting
  • Dangerous interconnection cable (the pins carry a high voltage) 1
  1. This was corrected in later versions.

Mark VII (Paraset) with PSU and accessories
Front panel
Valves stowed inside top lid
Front panel
Morse key at the bottom right
Antenna and ground (earth) terminals
Power socket
Internal morse key
Operating the internal morse key
Rectangular crystal installed in socket
Circular crystal installed in socket
Antenna and counterpoise connected
Receiver adjusted to 3.4 MHz (84 on the linear scale)
Front panel and cover
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Mark VII (Paraset) with PSU and accessories
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Front panel
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Valves stowed inside top lid
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Front panel
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Morse key at the bottom right
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Antenna and ground (earth) terminals
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Power socket
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Internal morse key
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Operating the internal morse key
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Rectangular crystal installed in socket
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Circular crystal installed in socket
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Antenna and counterpoise connected
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Receiver adjusted to 3.4 MHz (84 on the linear scale)
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Front panel and cover




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Parts
Metal or wooden enclosure
Mk.VII (Paraset) in metal container
Headphoners
Quartz crystals
Power supply unit
PSU
Power interconnection cable
Antenna and counterpoise wires
Operating instructions
Enclosure
Like most Parasets, the one featured here is housed in a steel enclosure, although some early ones were supplied in a wooden case. It has a hinged cover with clips to hold the valves when the device is in transit, plus a calibration chart to convert the linear receiver tuning scale to kHz.

Some Parasets were supplied in an unobtrusive travel suitcase that also had space for the PSU, spare parts, wiring and code material.

  
Paraset in closed metal container

Paraset
The actual transceiver is housed inside the metal (or wooden) enclosure shown above, and is held in place by four screws (see below). When the device is in transit, the three valves have to be removed and should be stowed inside the cover, as shown in the image on the right.

 Look inside the transceiver

  
Paraset with valves stowed inside top lid

Headphones
The Paraset contains an Audio Frequency (AF) amplifier that amplifies the sound from the self-regenerative detector/receiver to a level that is sufficient for high-impedance headphones.

The image on the right shows the original pair of 2000Ω headphones that was supplied with the Paraset during WWII.
  
Headphones

Crystals
The transmitter is crystal-operated and requires a suitable quartz crystal to be installed in the crystal socket in the upper right corner.

The socket accepts virtually any USA/UK crystal with 3 mm pins that are spaced 19 mm apart, such as the ones shown on the right.

Crystals with 4 mm pins, such as the DC-34 and DC-35, will probably fit, but are likely to damage the sockets (unless used with a 3 mm adapter).

 More about crystals

  
Suitable crystals

Power supply unit
When the Paraset was to be used in an urban environment, it was usually supplied with the mains power supply unit (PSU) shown in the image on the right. At the front is a voltage selector that makes the PSU suitable for virtually any AC mains network in the world.

The LT and HT voltages are available on the 4-pin Howard & Jones socket at the left. A short cable is needed for connection to the Paraset.

 Connector pinout

  
Paraset PSU

Power cable
The power input socket of the Paraset is a 3-pin Howard & Jones socket, whilst the output of the power supply unit is available on a 4-pin Howard & Jones socket. The short interconnection cable shown in the image on the right was used to connect the two devices.

 Pinout of the connectors

  
Interconnection cable

Antenna
The tranceiver was operated with a long wire antenna which should be connected to the terminal marked 'A' at the upper edge. As a general rule: the longer the wire the better. In addition, a suitable counterpoise wire should be connected to the 'E' terminal (earth or ground).

Suitable wires are shown in the image on the right. When possible, the ground wire should be connected to the waterpipe or the central heating system for improved performance.
  
Antenna and counterpoise wires

Mark VII (Paraset) with PSU and accessories
Paraset in closed metal container
Paraset with valves stowed inside top lid
Mark VII (Paraset)
Headphones
Headphones
Paraset PSU
Paraset PSU
Suitable crystals
Crystal
Rectangular crystal installed in socket
Circular crystal installed in socket
Interconnection cable
Antenna and counterpoise wires
Paraset with original power supply unit (PSU)
Paraset with headphones and [pwer supply unit
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Mark VII (Paraset) with PSU and accessories
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Paraset in closed metal container
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Paraset with valves stowed inside top lid
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Mark VII (Paraset)
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Headphones
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Headphones
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Paraset PSU
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Paraset PSU
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Suitable crystals
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Crystal
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Rectangular crystal installed in socket
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Circular crystal installed in socket
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Interconnection cable
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Antenna and counterpoise wires
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Paraset with original power supply unit (PSU)
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Paraset with headphones and [pwer supply unit

Operating instructions
During WWII, the Paraset was supplied to a number of agents and resistance groups in occupied European countries like France, Belgium, Norway and The Netherlands. From surviving accounts it is known that these devices were usually supplied with simplified operating instructions, but to our knownledge none of these documents appear to have survived the war. Until now that is...

In March 2021, Dutch amateur radio operator Bas Levering (PE4BAS) published two barely readable paper sheets – titled: Instructions for working a Paracette – which he had discovered in the legacy of Dirk Rustema, obtained in 2019 [7].

During WWII, Rustema – amateur callsign PA0DR – was a clandestine radio operator of the Orde­dienst (OD) — one of three important resistance organisations in The Netherlands. The Paraset was amoung the first radio sets used by the OD, and had been smuggled into the country via the Swedish route: by boat to Delfzijl (Netherlands).
  
Documents from the legacy of WWII radio operator Dirk Rustema, as obtained by Bas Levering [7].

The operating instructions for the Paraset came on two very thin folio paper sheets – presumably carbon copies (CC) – that were folded and stapled together. Apart from these instructions, the operator also needed a secret code for encrypting and decrypting messages. It consisted of the title of a poem (or song) and a 5-digit number to mask the position of the selected key words.

Part of the original Paraset instructions, as kept by Dirk Rustema and published by Bas Levering in 2021 [7].

Rustema probably retained the instructions that came with his Paraset, and rediscovered them years later, by which time much of the text had faded [A]. It prompted him to retype the text verbatim on his company's stationary, and save it for the future [B]. Unfortunately how­ever, he skipped the paragraphs that had meanwhile become unreadable. In May 2021, Crypto Museum obtained high-quality scans of the document from Bas Levering [7]. Using digital enhancement, we were able to reconstruct most of the text and make a nearly complete transcription [C].

 Download the reconstructed transcript

Handwritten notes on the original document [7]. Click to view the digitally enhanced version of the document.

Note that a few comments are handwritten at the top of the original document, probably at two different moments in time. First of all, the pencil-written marking S. 2441 in the top left corner. This could refer to the release date of the radio set: week 24 of 1941. The prefix 'S' could refer to Sweden, the country to which the radio was originally assigned. The other comments are somewhat darker and were probably added at a later moment; most likely well after the war.

"TLK de JCD" clearly refers to the call signs used for the station (JCD) and its counterpart (TLK). The handwritten text "Toen onze mop een mopje was" is the title of a famous Dutch poem that was used for several years during the war by the British Special Operations Executive (SOE) and the Dutch Ordedienst (OD) for exchanging encrypted messages. It is part of a poem code [10].

 Poem code used by the OD


About the authenticity of the document

After carefully studying the scans of the original document, there is no doubt at Crypto Museum that these are genuine instructions of British origin, that date back to WWII, possibly 1941. The typeface and the paper type is correct for the era. The language suggest that it was written by a native English speaker. The documents are hand-typeded carbon copies (CC), which means that typos (of which there are several) could not be corrected.

It was clearly supplied with a Paraset, which was probably at some point in the possession of Dirk Rustema, a Dutch amateur radio operator (PA0DR) who also acted as a clandestine radio operator for the Dutch resistance (OD) during the war. It is known that during the course of the war, several Parasets were supplied by the British SOE to the Dutch resistance via the so-called Swedish route: from the UK to Sweden and then by ship to The Netherlands, where they arrived in the harbour of Delfzijl, just 20 km east of Middelstum where Rustema lived.

It seems likely that the pencil-written marking 'S. 2441' was already on the document when Rustema received it, and might have been added by the SOE when the device was assigned to Sweden. In that case it might be a date code: Sweden, week 24 of 1941.

The handwritten text "Toen onze mop een mopje was", is in a different handwriting, and appears to be in ink. It is possible, if not likely, that this was added by Rustema after the war, as a reminder for the encryption method they had used at that stage. According to the post-war written account of chief marconist Ton van Schendel, the text — a famous Dutch poem — was used as part of a secret code by the SOE and the OD. That report also confirms the use of the Paraset by the OD and the involvement of Dirk Rustema in this context [9].

The marking "TLK de JCD" is somewhat mysterious. It is in a format that was (and still is) commonly used in radio communication. At the time, French was the common postal language and the word 'de' is French for 'from'. It is certain that TLK and JCD are callsigns, but it is currently unknown to whom they belonged. Also peculiar is the word Paracette in the title of the document. This name was given to the device by the SOE, who initially adopted the device for the French resistance, but later also deployed it in other countries [6].

Known users
Below is a list of confirmed users 1 of the Paraset, during and after WWII. The device was initially developed in 1941 for the British Secret Intelligence Service (SIS), where it was known as Mk-VII (Mk-7). Shortly afterwards it was adopted by the British Special Operations Executive (SOE) for operations behind enemy lines. As the SOE commonly dropped the supplies for its agents by parachute over occupied territory, the Mk-VII became known as the Paraset or Paracette. 2

The device was also supplied to resistance organisations in various occupied European countries, either directly or via the SOE. In the Netherlands for example, the Paraset was supplied to the Ordedienst via the so-called Swedish route. They were delivered by boat from Sweden to Delfzijl in the north/east of The Netherlands, from where they were distributed throughout the country.

World War II
Cold War
  1. This list is by no means complete. If you know of any further users of the Paraset, please let us know.
  2. Derived from French, as the SOE initially adopted the device for use in France [6].


Circuit diagram
Paraset   Mk-VII
Below is the circuit diagram of the Paraset. At the left is the transmitter, which is built around a metal 6V6 valve. At the bottom left is the crystal. Note the twisted wire between the g1 and the anode, which acts as a small capacitor (C3) that gets the oscillator going. At the top left is the output transformer, that comprises a tank coil (L1a) and an antenna coil (L1b). Each of these coils has its own pick-up loop that is connected to a filament light bulb that acts as tuning indicator.


The receiver is at the right. It is built around two 6SK7 valves and consists of a regenerative detector (V2) and an AF section (V3) that amplifies the audio signal enough to drive a pair of 2000Ω headphones. The frequency is adjusted with C10, whilst R7 is used to find the optimum point of regeneration. The MODE-switch (S1) selects between receiver and transmitter. It switches the HT power (S1a) and the antenna (S1b). In the middle position it is off. The circuit in the grey section at the top right, derives the +245V voltage for the receiver from the +360V HT rail.

Power supply unit
Below is the circuit diagram of the original mains power supply unit (PSU). It consists of a large transformer with a selection of taps on the primary side, that makes the device compatible with virtually any AC mains network in the world. The double capacitor (C2, C3) acts as a mains filter.


At the secondary side is a double HT winding and a separate 6V winding for the filaments of the valves. The HT voltage is rectified in a 6X5 rectifier valve (V1) and smoothened by capacitor C1. The output is available on a 4-pin Jones socket, whilst the Paraset itself has a 3-pin Jones socket. A short interconnection cable is needed to connect the two devices together. In the initial version, the interconnection cable has plugs at both ends, which makes it dangerous and potentially lethal. In later versions this was solved by swapping the 3-pin plug for a 3-pin socket [6].


Interior
The paraset is housed in a steel rectangular container that measures 222 x 142 x 110 mm and weights 2580 grams; valves and one crystal included. All parts are mounted to the reverse side of the control panel, which is bolted to the metal enclosure by means of four screws at the edges.

After removing the four screws, the interior can be lifted from the enclosure, as shown in the image on the right. All components are soldered directly to the bottom of the valve sockets and the connectors, or to mounting stubs or special solder strips that are bolted to the front panel.

The large coil at the centre is the combined oscillator/antenna coil – in fact a transformer – which is wound onto a large paxolin cylinder. Inside the cylinder are the pickup coils for the two tuning indicators; one at each end. Their filament light bulbs protrude the control panel.
  
Interior and empty case

Despite its age and the stress under which it was developed, the Paraset is a service-friendly device. It is mechanically well constructed and all components are easily accessible. Considering the fact that the device shown here is 80+ years old (in 2021), it appears to be well preserved.

The fact that the valves are fitted at the front panel rather than inside the enclosure, can be seen as a disadvantage (they must be removed to close the case) but also as a feature, as they can be replaced without dismantling the Paraset.

The power supply unit (PSU) is housed in a metal enclosure that consists of a U-shaped base — to which all parts are mounted — and a U-shaped cover. The cover has ventilation slots at the top and is held in place by 5 screws at the front and 5 at the rear. After removing all 10 screws, the cover can be lifted and the interior is exposed.
  
PSU interior

The interior is shown in the image above, but please note that the transformer has been replaced by a modern alternative that is lower than the original one. The are very few other components, and the only user serviceable part of the PSU is the 6X5 rectifier valve, which is fitted in a socket to the left of the transformer. Despite the fact that the PSU is a very simple device, it is also very vulnerable. The 6X5 is notorious for shorting out and can completely destroy the transformer.

WARNING — Note that the interconnection cable of the initial Parasets can be dangerous, as it has plugs (with pins sticking out) at both ends. When the cable is plugged into a live PSU, the pins at the other end carry the potentially lethal voltage of +360V. Always install both ends of the cable - before - plugging the PSU into the mains. The gender of the plug and socket at the transceiver end was swapped on later models [6].
Interior and empty case
Paraset removed from the enclosure
Interior - seen from the lower edge
Interior
Interior
Interior
Tank coil
Oscillator coil detail (note: pickup coil inside core)
Antenna coil detail (note: pickup coil inside core)
Interior after restoration
Internal morse key
Internal morse key (seen from the bottom)
PSU interior
PSU interior seen from the left
PSU interior - top view
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Interior and empty case
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Paraset removed from the enclosure
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Interior - seen from the lower edge
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Interior
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Interior
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Interior
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Tank coil
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Oscillator coil detail (note: pickup coil inside core)
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Antenna coil detail (note: pickup coil inside core)
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Interior after restoration
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Internal morse key
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Internal morse key (seen from the bottom)
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PSU interior
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PSU interior seen from the left
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PSU interior - top view

Restoration
PSU
When we received our Paraset in September 2020, it was in an unknown state, but we knew that the power supply unit (PSU) was broken. A first investigation revealed that the rubber of the original mains cable had become brittle as a result of which the two mains wires were shorted.

We noticed that the two mains filter capacitors had exploded and that one of the secondary HT windings had a short circuit. This means that the mains transformer was lost. In addition, the smoothing capacitor and the bleeding resistor had both died as well, which means that not a single component inside the PSU had survived.

To make matters worse, a previous owner had drilled four holes in the front panel, to make the various voltages available on banana sockets. Surprisingly, the device does not have a mains fuse, so it may easily have been overloaded.
  
Exploded capacitors

Although it is possible to have the transformer rewired, we decided to replace it with a same-size modern alternative for safety reasons. We also knew that the original transformer was under-dimensioned, as wartime users frequently complained that it was always running hot, and that in several cases they had to interrupt the transmission in order to give it a chance to cool down.

After replacing the passive components, the new transformer was fitted in the existing mounting holes and was wired to the rectifier socket, the output connector and the voltage selector. In addition, a new 6X5 rectifier valve was installed.

It was decided to leave the non-original banana sockets in place, as otherwise it would be very difficult to cover the four holes in the front panel. Although these banana sockets were not present when the device was released, they are left as a witness of an aftermarket modification. The result is visible in the image on the right.
  
PSU interior seen from the rear right

Although there is no fuse in the original design, it was decided to fit an era-correct one inside the device – in series with the primary winding of the transformer – for safety reasons and to protect the transformer should the 6X5 valve short out unexpectedly. As the four banana sockets would otherwise not have a function, they were wired in parallel with the 4-pin Jones socket, as was the case in the 'original' aftermarket modification. The result is shown in the circuit diagram below:

Circuit diagram of the restored PSU. Note the addition of the fuse (F1).
Restored power supply unit


Paraset
The paraset itself was first tested with two professional adjustable power supply units: one set to 6.3V for the filaments, and one gradually raised from 90V to 340V for the anodes of the valves. Whilst doing this, we used an infrared camera to check if any parts were internally overheating.

Once we were satisfied that this was not the case and that there were no short circuits, it was time to conduct a first test. Wires were connected to the antenna and ground terminals, a 3.518 MHz crystal was installed, the MODE-selector was set to TRANS, and the morse key was held down.

After a bit of playing with the TANK and AREAL tuning controls, the two indicator lamps lit up and the transmitter came to life. It produced a strong and clear signal on the desired frequency. It appeared to be easy to find the optimum setting, which is just below maximum light. 1
  
Operating the internal morse key

Next, the headphones were installed and the MODE-selector was set to RECEIVE, but no sound was heared. After wiggling the valves, it became clear that the socket of V3 was worn out. The contacts were fixed and noise was heared through the headphones, but no signal was received.

Furthermore, the reaction potentiometer (R7) was stuck, as a result of which the reaction point could not be adjusted. After oiling its axle, the resistors and capacitors of the receiver circuit were checked. It appeared that two resistors (R4, R9) were way out of spec, and that all electrolytic capacitors (C13, C16, C18 and C19) were gone.

The resistors were replaced by modern ceramic alternatives, covered in old sleeves, whilst the electrolytic capacitors – with their prominent blue jackets – were carefully disassembled and emptied, so they could house a replacement.
  
partly disassembled electrolytic capacitor

Whilst carrying out the repairs, it was noticed that the coupling capacitor (C14) between V2 and V3 had been replaced in the past. As it looked way too new, it was replaced by a new part that was made to look old. It was also noticed that there were some solder problems on the taps of the receiver coil (L3). Apparantly, the wire to the cathode of V2 had been broken in the past.

This was probably caused by frequent wiggling when installing the valve (V2). As the wire had been replaced by a modern one, and the solder joints on the receiver coil (L3) looked messy, we decided to replace it with a properly sleeved one, and rework the solder joints of the receiver coil.

After carefully checking the restored parts, the receiver was powered up again and a 3.4 MHz signal was applied to the AERIAL input. After turning both tuning knobs, and adjusting the REACTION control knob as per instructions in the manual, the signal came through loud and clear.
  
Another view of the restored capacitors

Surprisingly, the index on the linear tuning scale still corresponds exactly with the graph inside the case lid — 80 years after the device was manufactured and calibrated. The restored Parsaset was then closed, and its operation was checked in combination with the original PSU, of which the restoration had meanwhile also been completed. It appears that transmitter and receiver are very stable and easy to operate. No wonder that the Paraset has become a popular repro subject.

  1. This corresponds to the instructions in the manual, where it is suggested to adjust the TANK and AERIAL controls until the TANK indicator burns at approx. 7/8th of its maximum strength [A][C].

Short circuit in the mains wiring
Exploded capacitors
Broken transformer
Dead smoothing capacitor and bleeding resistor
Broken capacitor and resistor in the PSU
Exploded mains filter capacitors and replacements
PSU interior seen from the rear right
Open PSU seen from the left
Internal wiring of the transformer, the voltage selector, the power output socket and the (added) fuse holder
Mains cable connections
Interior before restoration
Interior after restoration
partly disassembled electrolytic capacitor
Capacitors in the receiver
Capacitors in the power circuit of the receiver
Another view of the restored capacitors
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Short circuit in the mains wiring
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Exploded capacitors
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Broken transformer
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Dead smoothing capacitor and bleeding resistor
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Broken capacitor and resistor in the PSU
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Exploded mains filter capacitors and replacements
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PSU interior seen from the rear right
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Open PSU seen from the left
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Internal wiring of the transformer, the voltage selector, the power output socket and the (added) fuse holder
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Mains cable connections
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Interior before restoration
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Interior after restoration
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partly disassembled electrolytic capacitor
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Capacitors in the receiver
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Capacitors in the power circuit of the receiver
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Another view of the restored capacitors

Problems
  • Short circuit in brittle mains cable (PSU)
  • One of the secondary HT windings of the mains transformer shorted out (PSU)
  • Both mains filter capacitors blown (PSU)
  • Smoothing capacitor gone (PSU)
  • Bleeder resistor has become infinite (PSU)
  • Rectifier valve (6X5) missing (PSU)
  • Four extra holes in front panel (PSU)
  • Interconnection cable stiff and too modern 1
  • Both Howard & Jones plugs unstable
  • Reaction potentiometer stuck
  • Receiver very insensitive
  1. The original braided cable had been replaced by a modern rubber cable by a previous owner.

Restored   PSU
  • Mains cable replaced with (secure) 3-wire brown braided cable
  • Mains ground connected to chassis (for safety)
  • Both mains filter capacitors (V2, C3) replaced
  • Smoothing capacitor (C1) replaced (inside original casing)
  • 200k bleader resistor (R1) replaced
  • Transformer (TR1) replaced
  • Era-correct fuse holder added
  • 6X5 rectifier valve (V1) replaced
  • Banana sockets rewired with era-correct wiring
Restored   Paraset
  • Interconnection cable replaced by 3-wire brown braided cable
  • Both Jones connectors restored
  • Valve socket (V3) repaired
  • Reaction potentiometer (R7) repaired
  • Wire between L3 and the cathode of V2 replaced
  • C13 (2µF) replaced (inside original casing)
  • C18 (8µF) and C19 (2µF) replaced (inside original casing)
  • R9 (75k) replaced
  • R4 (1M) replaced
WARNING — Be careful when trying to use an original Paraset power supply unit, as a lot it wrong with its design. First of all, there is no power switch and no fuse whatsoever. This means that the transformer is activated and the valve filaments are heated as soon as the device is plugged into the mains.

Furthermore, the transformer is under-dimensioned, as a result of which it can overheat when the transmitter is used for an extended period of time. After 80+ years, all of the electronic parts are most definitely gone, and the (rubber) wiring is likely to have become brittle over the years. In addition, the 6X5 rectifier tube is notorious for shorting out to ground – especially when driven at its limits – as a result of which it might blow one of the HT windings (as was the case in our device). The latter can be overcome by using a separate 6V winding for the filament of the 6X5 [8], but this was not avaiable on the original transformer.

Also note that the 240V setting of the voltage selector, connects to the same tap on the transformer as the 225V setting. This is particularly important, as the mains voltage in continental Europe has been raised at the beginning of the century from 220V to 230V, and is in many cases even higher that this. Mains voltages of 240V are no exception. As a result, the transformer may saturate and eventually overheat and break down.

Specifications
  • Year
    1941
  • User
    SIS (MI6), SOE
  • Developer
    SIS Section VIII (Whaddon Hall)
  • Purpose
    Agent communication (SIS, SOE), resistance
  • Dimensions
    222 x 142 x 110 mm
  • Weight
    2580 g 1
  • LT
    6.3V, 950mA
  1. Including all three valves and one crystal.

Receiver
  • Modulation
    AM R/T, CW
  • Frequency
    3 - 7.6 MHz
  • Circuits
    Regenerative detector, AF stage
  • Valves
    6SK7 (2x)
  • Output
    High impedance headphones
  • Antenna
    Long wire (and ground wire)
  • HT
    +245V DC (derived from +360V), 10mA
Transmitter
  • Modulation
    CW
  • Frequency
    3.3 - 7.6 MHz
  • Ranges
    (1) 3.3 - 4.5 MHz, (2) 4.5 - 7.6 MHz
  • Output
    4 - 5 W
  • Circuit
    Oscillator/PA
  • Valve
    6V6
  • Morse key
    Internal only
  • HT
    360V, 40-50mA
Power supply unit
  • Input
    110 - 240V AC (6 steps), 40 - 60Hz
  • Current
    LT: 950mA, HT: 10mA (RX), 40-50mA (TX)
  • Valve
    6X5 (rectifier)
  • Dimensions
    114 x 110 x 110 mm
  • Weight
    2700 g
Parts
Connections
Power
Below is the pinout of the Howard & Jones 3-pin socket at the front panel of the Paraset, when looking into the socket. Note that on later versions of the Paraset, the female chassis part was swapped for a male chassis part, for safety reasons. The pinout was kept the same though, and uses the same pin numbering. Equivalent cable parts are also available from Hirose (HRS).

  1. HT +360V
  2. Ground (common)
  3. LT ~6.3V
    Pinout when looking into the (female) socket on the device
Power supply unit
The LT and HT voltages ae available on a Howard & Jones 4-pin female socket at the front panel of the PSU. Below is the pinout when looking into the socket. A short braided interconnection cable was used to connect the Paraset to the PSU.

  1. HT +360V
  2. not connected
  3. LT ~6.3V
  4. Ground (common)
    Pinout when looking into the (female) socket on the device
6V6 valve
The 6V6 is a beam-tetrode in a metal enclosure, developed in the mid-1930s by RCA for use in the audio stages of broadcast receivers. They are frequently found in single-ended or push-pull amplifiers, and even in today's vintage/retro valve-based amplifiers. Apart from the use in audio amplifiers, the 6V6 can also be found in the oscillator and PA stages of short-wave transmitters. In the Paraset, the 6V6 is used in the single-valve transmitter, as a combined oscillator/power amplifier (PA). In this configuration it produces a maximum power output of 5 Watts.

 6V6 datasheet

Pinout of the 6V6 as seen from the bottom of the valve

6SK7 valve
The 6SK7 is a remote-cutoff pentode in a metal enclosure, developed for use as high-gain radio frequency (RF) and intermediate frequency (IF) amplifiers in radio receivers. Because of its cutoff characteristic, it can handle high signal levels without cross modulation or distortion. It is also suitable for receivers with automatic gain control (AGC). In the Paraset, the 6SK7 is used in both stages of the receiver: the regenerative detector (V2) and the audio frequency (AF) amplifier (V3).

 6SK7 datasheet

Pinout of the 6SK7 as seen from the bottom of the valve

6X5 valve
6X5 is a full wave rectifier. It contains two diodes with a common – indirectly heated – cathode. The valve is notorious for shorting out to ground and destroying one of the windings of the transformer to which it is connected, especially when driven to its limits. This can be avoided by using a separate (floating) 6V supply for the filament of the 6X5 or by adding a fuse.

 6X5 datasheet

Pinout of the 6X5 as seen from the bottom of the valve

Documentation
  1. Instructions for working a Paracette
    Original (partly faded) instruction sheets. Date unknown, but assumed to have been supplied with a Paraset during WWII. Document kindly provided by Bas Levering [7].
     Digitally enhanced version

  2. Retyped (incomplete) transcript by the original owner
    Dirk Rustema. Date unknown, but assumed to be created several years after the war. Document kindly provided by Bas Levering [7].

  3. Reconstructed operating instructions
    Nearly complete transcription of the original instructions.
    Crypto Museum, May 2021.

  4. Frequency calibration chart
    Crypto Museum, May 2021.

  5. 6V6 datasheet
    TUNG-SOL Lamp Works Inc., 6 June 1941.

  6. 6SK7 datasheet
    ET-T818. General Electric, June 1953.

  7. 6X5 datasheet
    TUNG-SOL Lamp Workds Inc., 21 April 1941.
References
  1. Cor Moerman, Mk. VII Paraset - THANKS !
    Received September 2020.

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

  3. Wikipedia, Paraset
    Retrieved May 2021.

  4. Henk van Zwam, The Whaddon Mk-VII, a.k.a. 'The Paraset'
    Visited 6 May 2021.

  5. Johnny Apell, The Whaddon Mk VII - Paraset Clandestine Radio
    SM7UCZ. Visited 6 May 2021.

  6. David White, Personal correspondence
    August 2009, May 2021.

  7. Bas Levering, Instructions for working a Paracette
    Blogspot PE4BAS. 4 March 2021.

  8. Larry, The notorious 6X5 and kin
    Aniques Radio Forum. Retrieved May 2021.

  9. A.S.M. van Schendel, Mijn werkzaamheden als chef-marconist van de OD en mijn belevenissen in de gevangenis
    Organisation of the Internal Radio Service (BR) of the OD and the radio links with the UK.
    Post-war report, in Dutch language. Date unknown.

  10. Poem code 'Toen onze mop een mopje was'
    Ton van Schendel. Date unknown.
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