Click for homepage
← UK
RX
  
Sweetheart   Type 31/1
Clandestine radio receiver

Type 31/1 — also known as Sweetheart — was a miniature valve-based clandestine receiver, developed in 1943 by the Norwegian Willy Simonsen, for use by the British Special Operations Executive (SOE) during WWII. Approx. 50,000 units were manufactured by Hale Electric Co. Ltd in London (UK), that were dropped over occupied Europe, in particular in France and Norway [2].

The device consists of a small body wearable unit — the actual receiver — a separate battery pack, and a special pair of earphones that came in a hermetically sealed tobacco tin, as it could not withstand the low pressure in an airplane.

The receiver is small enough to be transported unobtrusively in the pocket of, say, a coat, and is powered by two batteries that supply 4.5 V (LT) and 30 V (HT) respectively. Due to the use of crystal earphones 1 it uses very little power. The HT battery lasted for up to 200 hours, whilst the LT battery had to be swapped every 50 hours.
  
Type 31/1 (Sweetheart)

The receiver does not have a power switch, and can only be switched off by disconnecting the battery pack. Production of the receiver started at the Hale Electric Co. Ltd factory in London (UK) in 1943, and by the end of the war, approx. 50,000 units had been manufactured. About 5000 of these were dropped over occupied Norwegian territory. It allowed the resistance to listen to the coded messages, broadcast by the
BBC
in London. For this reason, Sweetheart is also known as the Propaganda Set. The operating instructions are printed on the body in English or Norwegian.

  1. Also because the valve-stages are resistance-capacitance coupled (rather than by means of transformers).

Sweetheart receiver in original carton packaging Sweetheart, complete set Type 31/1 (Sweetheart) Sweetheart receiver, battery box and earphones Sweetheart receiver Sweetheart, seen from the rear Front view Earphones
A
×
A
1 / 8
Sweetheart receiver in original carton packaging
A
2 / 8
Sweetheart, complete set
A
3 / 8
Type 31/1 (Sweetheart)
A
4 / 8
Sweetheart receiver, battery box and earphones
A
5 / 8
Sweetheart receiver
A
6 / 8
Sweetheart, seen from the rear
A
7 / 8
Front view
A
8 / 8
Earphones

Features
The diagram below provides a quick overview of the controls and connections on the body of the Sweetheart receiver, when looking at the device from the rear. The red/black/yellow wiring is fixed to the body of the receiver, and ends in a 3-pin plug that mates with the 3-pin socket on the battery box. The antenna and ground wires should be connected to the 'A' and 'E' sockets.

Overview of Sweetheart's features. Click to enlarge.

At the left of the rear panel is a socket with slide-contacts, that accepts a Brush hearing-aid earphones plug. The same plug is used to fit the twisted cable to the earbuds. The receiver is turned ON by connecting it to the battery box. The tuning controls are at the front panel.

Sweetheart receiver with batteries and earphones Front Rear Front panel knobs Rear side Rear panel wiring Tuning window Top
B
×
B
1 / 8
Sweetheart receiver with batteries and earphones
B
2 / 8
Front
B
3 / 8
Rear
B
4 / 8
Front panel knobs
B
5 / 8
Rear side
B
6 / 8
Rear panel wiring
B
7 / 8
Tuning window
B
8 / 8
Top

History
In 1942, the Norwegian graduate electronics engineer Willy Simonsen escaped to England where he was put to work at the Inter Services Research Bureau (ISRB). He used his knownledge and experience with Norwegian resistance work, to design a small pocket-size receiver with very low power consumption, that could be driven for a long time from 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 – electronic parts.

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 regular civil 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.
  
Interior

Approximately 50,000 Sweetheart receivers were built by Hale Electric Co. Ltd., at a price of just 8 GBP each. About 5000 of them were intended for the Norwegian government in exile, and were subsequently dropped over occupied Norwegian territory where they were used by the resistance.

The image on the right shows three young men, sitting 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).

Together they formed the leaders of the secret organisation 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).
  
Three men using the Sweetheart in a Norwegian forest in the summer of 1944. Copyright unknown.

MILORG — short for: Military Organisation — was the organised Norwegian Resistance that served directly under the Norwegian Armed Forces High Command (FOR), which resided during the war at Kingston House in London — the same address as where King Haakon VII 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.

C
×
C
1 / 3
1 / 3
C
2 / 3
2 / 3
C
3 / 3
3 / 3




Complete Sweetheart kit. Click to enlarge.


Parts
Carton packaging
Box
Miniature valve-based receiver Battery box Earphones stored inside tobacco tin Antenna and counterpoise wires Operating instructions
Carton packaging
Sweetheart was distributed in a small carton box that measures 190 × 140 × 80 mm and weights 1800 grams, all accessories included.

At the top of the box is a warning that reads: Not to be flown above 15,000 feet unless hermetically sealed. This was necessary in order to protect the crystal earphones.
  
Sweetheart receiver in original carton packaging

Receiver
The receiver measures 140 x 110 x 30 mm and weights just 500 grams. It has a brass plate on its body on which the operating instructions are printed, either in English or in Norwegian.

At one end are the reaction control and the frequency adjustment. When turning the latter, the frequency is visible (in metres) in a small window at the top surface. At the rear is a fixed 3-wire cable that should be connected to the battery box. Also at the rear is a socket for the hearing-aid crystal earphones.

  
Sweetheart, seen from the rear

Battery pack
The receiver was powered by two batteries that were installed in the battery box shown in the image on the right. A standard 4.5V torch light battery was used for the LT voltage. It had to be replaced after 50 hours of use.

The HT voltage was provided by a 30V hearing-aid battery, and lasted for up to 200 hours. The battery box has a three-pin socket that mates with the power plug of the receiver.
  
Battery pack

Earphones
To keep the power consumption of the receiver as low as possible, crystal earphones were used in stead of regular headphones. As these were not readily available in the UK, they had to be imported from the USA, where they were made by Brush for use in early electronic hearing aids.

The image on the right shows an original Brush ear­piece, in which the crystal element is made of Rochelle salt. As these elements could not with­stand the low pressure in an airplane, they were supplied in a hermetically sealed tobacco tin. A warning on the box informs the user about this.

  

Antenna and counterpoise
A 10-metre long wire was supplied with the kit, for use as antenna. It has a black plug at one end — that should be inserted into the socket marked 'A' — and a crocodile clip at the other end, which can be used to affix it somewhere.

A 3-metre ground wire was supplied as well. One end of this cable should be inserted into the socket marked 'E', whilst the other end should be connected to ground (e.g. a metal water pipe or the central heating system).
  
Antenna and ground wires

Operating instructions
Each Sweetheart kit came with a single piece of paper, with the operating instructions printed on one side, and the circuit diagram at the other. The instructions were in Norwegian or English.

In addition, abbreviated operating instructions were printed on a brass plate that is fitted on the upper case shell of the receiver. Furthermore, the circuit diagram is glued to the inside of the upper case shell of some receivers.

 Norwegian instructions
 English instructions

  
Operating instructions

Sweetheart receiver in original carton packaging printed warning on the packaging Sweetheart, seen from the rear Sweetheart receiver Battery pack Antenna and ground wires Antenna wire (10m) 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 Earphone - rear view
D
×
D
1 / 16
Sweetheart receiver in original carton packaging
D
2 / 16
printed warning on the packaging
D
3 / 16
Sweetheart, seen from the rear
D
4 / 16
Sweetheart receiver
D
5 / 16
Battery pack
D
6 / 16
Antenna and ground wires
D
7 / 16
Antenna wire (10m)
D
8 / 16
Tobacco tin with earphones
D
9 / 16
Earphones stored inside tobacco tin
D
10 / 16
Earphones
D
11 / 16
Y-junction in earphone cable
D
12 / 16
Ear piece seen from the side
D
13 / 16
Ear piece
D
14 / 16
Connector - contact side
D
15 / 16
Cable removed from ear piece
D
16 / 16
Earphone - rear view

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 filaments and a small 30 Volt HT battery, commonly used in hearing aids, for the anode voltage.

The interior of the receiver can be accessed by removing four screws (two at either side) and taking off the case shell. The image on the right shows the interior after removing the case shell.

The circuit is built around 3 identical directly-heated low-power miniature 1T4 valves (tubes) that – like the crystal earphones – were imported from the USA. The valves are installed in ceramic sockets, that are mounted in a vertical subframe, with the passive parts (resistors and capacitors) – fitted in the narrow space between the frame and the rear of the case – soldered to their contacts.
  
Interior

The 3-piece antenna coil is mounted to the rear panel, and is connected to the two variable capacitors that are operated by the two knobs at the front panel. The circuit is dimensioned in such a way, that it still works when the HT voltage has dropped to 20V. As a result, the 30V HT battery lasted between 150 and 200 hours. The LT battery had to be swapped every 50 hours.

The receiver covers a frequency range from 6 to 12 MHz in a single band, with its dial calibrated in metres. This made the receiver ideal for the reception of spoken coded messages, broadcast by the
BBC
in London, but less so for narrowband CW transmissions on the Short Wave (SW) bands.

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 Sweetheart with valves removed
View of the valve sockets Sweetheart with valves removed Wiring detail
E
×
E
1 / 19
Case opened
E
2 / 19
Interior
E
3 / 19
Interior
E
4 / 19
Top view
E
5 / 19
Tuning scale
E
6 / 19
Interior seen from the front
E
7 / 19
Components below the valves
E
8 / 19
Coil - top view
E
9 / 19
Close-up of the coil
E
10 / 19
Coil detail
E
11 / 19
Coil detail
E
12 / 19
Tuning
E
13 / 19
Antenna tuning
E
14 / 19
Case lid (inside)
E
15 / 19
Top lid detail
E
16 / 19
Sweetheart with valves removed
E
17 / 19
View of the valve sockets
E
18 / 19
Sweetheart with valves removed
E
19 / 19
Wiring detail

Circuit diagram
Below is the circuit diagram of the receiver, as it is found on the instruction sheet, and sometimes inside the case shell as well. The circuit is built around three identical directly-heated low-voltage 1T4 valves (tubes) and consists of a regenerative detector (V1) followed by two AF stages (V2 V3).


The final stage (V3) delivers its output to a pair of high-impedance crystal earphones, that are connected to socket (P). The two supply voltages (4.5V LT and 30V HT) are connected at the right. Note that the (-) pole of the 30V battery and the (+) pole of the 4.5V battery are both connected to ground. This is done to provide a slightly negative voltage to the g1 of each of the valves.


Sound sample
The following sound sample of the BBC News broadcast on 12.095 MHz (25 m), was recorded by Karsten Hansky in Germany on 16 August 2018 at 20:00 hours, using the Sweetheart receiver in his collection. It demonstrates the excellent quality of the receiver, even after all these years [7].

Restoration
People who are restoring an old Sweetheart receiver, or who want to build a replica, might be intersted in any or all of the following files:

Specifications
  • Frequency
    6 - 12 MHz (dial calibrated in metres)
  • Valves
    3 × 1T4
  • Output
    High impedance
  • Earphones
    Brush hearing aid crystal earphones
  • Antenna
    10 m insulated wire
  • Counterpoise
    3 m wire
  • Power
    4.5 V (LT) and 30 V (HT)
  • Current
    50 mA (LT) and 0.5 mA (HT)
  • Dimensions
    140 × 110 × 30 mm (battery 100 × 80 × 25 mm)
  • Weight
    500 g (battery box 400 g)
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).

  5. 1T4 miniature battery penthode, datasheet
    General Electric, April 1950.
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.

  7. Karsten Hansky, Sound sample of BBC News
    16 August 2018.
Further information
Any links shown in red are currently unavailable. If you like the information on this website, why not make a donation?
Crypto Museum. Created: Monday 21 September 2009. Last changed: Tuesday, 13 August 2019 - 19:06 CET.
Click for homepage