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Sweetheart   Type 31/1
Clandestine radio receiver

The type 31/1, also known as Sweetheart, is one of the most beautiful clandestine receivers of WWII. It was developed for use by the SOE in 1943 by the Norwegian Willy Simonsen. It consists of a small body-wearable receiver, a separate battery unit and a tobacco tin that contains the earphones. Receiver and battery case are painted in blue/grey wrinkle paint.

In 1942, the Norwegian graduate electronics engineer Willy Simonsen escaped to England where he started working for the Inter Services Research Bureau (ISRB). He used his knownledge and experience with Norwegian resistance work, to design a small pocket-size receiver with low power consumption, that could be driven for a long time by standard domestic batteries.

For the design of the Sweetheart, he was not allowed to use military-grade components. As a result, the receiver had to be built from standard domestical (unpreferential) components.
  
Type 31/1 (Sweetheart)

Furthermore, the receiver could not be built by the highly skilled craftsmen that assembled the other spy radio sets, so they had to revert to a 'standard' manufacturer. As it was built under supervision of the SOE, the receiver was given the designation Type 31/1. It was nicknamed Sweetheart, probably because of a nice young lady who worked on the project with Simonsen.

Approximately 50,000 Sweethearts were built by Hale Electric Co. Ltd., at a price of just 8 GBP each. About 5000 units went to the Norwegian government in exile, and were subsequently dropped over occupied Norwegian territory, intended for use by the resistance.

The image on the right shows three young men in a hole in a forest near Hvarnes, using the Sweetheart receiver. The image was taken in the summer of 1944. From left to right: Josef Haraldsen (district leader), Erling Slorvik (radio telegrapher) and Hans Lien (chief arms officer).
  
Three men using the Sweetheart in a Norwegian forest in the summer of 1944. Copyright unknown.

Together they were the leaders of MILORG D-15 (Vestfold). Hidden safely in the wide forest near Hvarneskollen, south-east of Oslo, they exchanged hundreds of messages with London [3]. More images of the D-15 Vestfold group below (copyright unknown).

MILORG (short for: Military Organisation) was the organised Norwegian Resistance that served directly under the Norwegian Armed Forces High Command (FOR) at Kingston House in London, where also the King and the Norwegian Government in exile were located [4]. During WWII, MILORG executed a range of resistance and sabotage activities. By spring 1945, MILORG had about 40,000 men under arms in Norway.

Type 31/1 (Sweetheart) The receiver The frequency dial Norwegian instructions Battery box
Sweetheart, complete set Sweetheart receiver, battery box and earphones Sweetheart, seen from the rear Rear side Front panel knobs Front Rear Rear panel wiring
Earphones
One way of keeping power consumption as low as possible was to use crystal earphones instead of normal headphones and transformers. Such crystal earphones were not a comodity in the UK in 1943, so Simonsen had to order them from the USA. They were manufactured by Brush (USA) for use in hearing aids, another type of devices that require low power consumption.

These earphones used Rochelle salt for the piezo-crystal element and could not withstand the low pressure in an airplane.

To allow the Sweetheart units to be dropped over occupied territory, they were therefore packed in hermetically sealed tobacco tins.

A warning in the user instructions says that the receiver should not be flown over 5000 meters, unless the headphones were hermetically sealed (please check the user instructions in the download section below).
  

Tobacco tin with earphones Earphones stored inside tobacco tin Earphones Y-junction in earphone cable Ear piece seen from the side Ear piece Connector - contact side Cable removed from ear piece
Interior
The Type 31/1 Sweetheart receiver is extremely small for its time, and was designed for low power consumption. It was powered by two batteries: one standard 4.5 Volt torch battery for the filament and a small 30 Volt HT battery, commonly used in hearing aids, for the anode voltage.

The Sweetheart would still work when the HT voltage dropped to 20 Volt, allowing the HT battery to last for 150 to 200 hours, whilst the LT battery (4.5 Volt) had to be replaced approx. every 50 hours.

The image on the right show the interior of the Sweetheart after removing its cover. It contains only three valves of the type 1T4 that, like the earphones, were manufactured in the USA. The three miniature valves are mounted in a small subframe with a range of colourful passive compontents soldered to their sockets.
  
Interior seen from the front

The receiver covers a frequency range from 6 to 12 MHz in a single band, with the small dial calibrated in metres. As they were ideal for the reception of BBC newscasts in Western Europe, Sweetheart receivers were also dropped by the SOE over other parts of Europe during WWII. They are therefore also known as the Propaganda Set.

Opening the Sweetheart Case opened Interior Interior Top view Tuning scale Interior seen from the front Components below the valves
Coil - top view Close-up of the coil Coil detail Coil detail Tuning Antenna tuning Case lid (inside) Top lid detail
Documentation
  1. Original Norwegian User Instructions
    2 pages, user instructions and circuit diagram.

  2. English User Instructions
    2 pages, user instructions and circuit diagram (poor quality copy).

  3. User manual (Norwegian)
    Hi-res scan of the single page user instructions.

  4. Circuit diagram
    Hi-res scan of the circuit diagram (reverse side of user manual).
References
  1. Original Norwegian User Instructions
    Packed with Sweetheart receivers dropped over occupied Norwegian territory.

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

  3. Website: Majorstua (Norwegian)
    Head office of Milorg D-15 (Vestfold) in the winter of 1944-45.

  4. Website: Motstandsbevegelsen - Hjemmelsfronten - Milorg - XU (Norwegian)
    Resistance Movement - Resistance - Milorg - XU, etc.

  5. English User Instructions and circuit diagram
    Packed with Sweetheart receivers dropped over European territory (poor quality copy).

  6. R.C.D. Receiver, Type 31/1, Propaganda Set
    Source unknown.
Further information
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Crypto Museum. Created: Monday 21 September 2009. Last changed: Monday, 22 May 2017 - 17:02 CET.
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