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OD Receiver
WWII Clandestine Receiver

OD Receiver was a three-valve receiver for reception of CW signals in the shortwave (SW) radio band between 2.85 and 3.15 MHz, 1 developed during World War II (WWII) with help from Philips by the Ordedienst (OD) – one of three important resistance organisations in the Netherlands – for a secret national communications network. The OD Receiver was part of a complete radio set that included a transmitter and ancillaries. At least two design variants are known to have existed [3].

As far as we know, there were two generations of the receiver, which differ in circuit design and enclosure. The first generation was based on three EF6 valves, built on an aluminium chassis and housed in a wooden cabinet, disguised as a commercial LUXOR medical diathermy device [7].

It covered the 2.85-3.15 MHz (100 m) frequency range, which is about the same as that of the OD Transmitter. A later version, developed by Dutch radio amateur Jan Lourens (PA0BN), was housed in a home-made aluminium enclosure of which the lid could easily be removed. It is shown here.
  
OD Receiver

The later version had removable plug-in coils, which made it suitable for reception of the 31, 41 and 49-metre broadcast bands as well. About 34 of this variant were built [3]. The receivers were series-manufactured by the OD — probably in Zaandam and Eindhoven — with components and engineering supplied by Philips in Eindhoven (Netherlands). In addition, the circuit diagrams were distributed to the various OD Regions allowing them to be built with locally sourced parts as well.

  1. This is the frequency range of the initial design. A later variant covered the 31, 41 and 49-metre broadcast bands as well, by using plug-in coils. Furthermore, variations in the basic frequency range are known.

OD Receiver
OD Receiver
Front view
Rear view
Rear view
Cover and bottom panels removed
Cover removed (rear view)
Detector coil removed
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OD Receiver
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OD Receiver
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Front view
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Rear view
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Rear view
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Cover and bottom panels removed
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Cover removed (rear view)
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Detector coil removed

Features
The diagram below shows the front panel of the second generation of the OD Receiver. It is housed in a simple homemade aluminium enclosure that measures 185 x 135 x 90 mm, and has bakelite knobs at three sides. At the right is the volume knob with integrated ON/OFF switch. At the left is the antenna tuning knob. The frequency tuning knob is at the centre of the front panel, allowing adjustment over a fairly narrow frequency range of approx. 300 kHz around 3 MHz.

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The precise centre frequency of the unit, can be adjusted with an internal trimmer, that can be reached through a hole in the front panel, just above the tuning knob. As the receiver is free-running, it had to be calibrated regularly by means of an external source, such as a crystal-based transmitter, or an OD Transmitter that had been calibrated with an external wavemeter.

Note that, unlike many other receivers that use the same principle, this version of the receiver does not have an external oscillator/detector reaction control. Instead the feedback loop can be adjusted with an internal trimmer. An external reaction control was probably omitted, because the receiver can only be adjusted over a limited range for which a single reaction setting can be used. It can still be adjusted easily, by removing the cover and setting the trimmer by hand.

Related items
Complete OD radio set
OD transmitter
TX
OD Wavemeter
Morse key
Key
Poem Code used by the Ordedienst (OD)
History
Within the OD, engineer Jan Thijssen (1908-1945) was charged with building a national radio network. Anton van Schendel, an employee of the Radio Monitoring Service (RCD) of the PTT, became responsible for training the operators, which he recruted from radio amateurs (HAMs).

As secrecy of the messages was of the utmost importance, B.J. Suermondt was tasked with the establishment of an encryption bureau. He produced the coding instructions and trained the crypto-officers of the region commanders. Each radio station was given common code material.

After the reorganisation of the OD in 1942, the OD transmitters and rx.htm were developed by ir. J.P. Heyboer (1912-1945), who had been made available by Philips in Eindhoven, and built in the workshop of Jan Hendrik (Henk) Op den Velde 1 in Zaandam, and probably also somewhere in Eindhoven. Philips also supplied the required components, under control of ir. G.H. Thal Larsen (1899-1963) and radio technician H.A. Hoekstra [4]. The design of the OD Receiver was later improved by Jan Lourens (PA0BN) in Oosterbeek (Netherlands), who built at least 34 of them.

After Op den Velde was arrested on 2 March 1944, Hoekstra took over his work on the roll-out of the national radio network. In the south of the country, Heyboer had meanwhile completed the south part of the network and had taken over the activities of Jan Thijssen on 31 December 1943, after the latter had been expelled from the OD, due to a conflict with the OD management. 2

OD radio station of Region 6 (Apeldoorn). Photograph kindly provided by Louis Meulstee [3].

This image shows the radio station of OD Region 6, when it was located in the city of Apeldoorn. At the left is the first version of the receiver, which is housed in a wooden enclosure. All knobs and the sockets for the headphones are at the front panel. At the right is the OD Transmitter with the two PE06/40 valves and no enclosure. The name of the operator is currently unknown [3].


OD radio station of Region 11 in Zaandam in April 1945. Photograph kindly provided by Louis Meulstee [3].

The image above shows the clandestine radio set of Region 11, when it was located in Zaandam in April 1945 [3]. The receiver is visible at the centre of the picture. It is one of the later versions, which has an output transformer on the chassis and a reaction potentiometer at the front panel. The transmitter is at the left. The one shown here was modified by radio amateur J. Zandbergen (PA0ZY) to a Master Oscillator/Power Amplifier circuit. At the front left is a rotary DC converter.


OD radio station of Region 11 in Alkmaar in late 1944. Photograph kindly provided by Louis Meulstee [3].

The image above shows the same radio station of Region 11, this time photographed in late 1944 when it was located at a hospital in Alkmaar. In this image, the extra knob at the top right of the front panel of the receiver – used for the reaction potentiometer – is clearly visible [3].


OD radio station of Region 15 in Grijpskerke in 1944. Photograph kindly provided by Louis Meulstee [3].

The image above shows the setup of the radio station of OD Region 15 when it was located in Grijpskerke (near Middelburg) in 1944. It is similar to the setup of Region 11 in the hospital of Alkmaar, but with a slightly different receiver that does not have a reaction knob at the front panel. It is unclear which design came first. The receiver in our collection, is of this type.

  1. During the war, Henk op den Velde was known by the codename HEIN [5][6].
  2. Thijssen found the OD too passive and wanted to increase the use of the radio links with the UK. After he left the OD, he formed the new resistance organisation Raad van Verzet (RVV) — the Resistance Council.

Circuit diagram
Below is the circuit diagram of the OD Receiver, as published by Louis Meulstee in the book Wireless for the Warrior, Volume 4 [3]. This circuit was reportedly designed during the war by radio amateur Jan Lourens (PA0BN) in Oosterbeek (Netherlands). It is likely that the diagram was recreated from memory at the time of the publication, as all component values are missing [3].

Circuit diagram as published in [3]

It is a straightforward design, consisting of an RF pre-amplifier, detector and AF amplifier, each built with a single EF6 valve [3]. The RF and AF stages are both transformer coupled. It should be noted however, that this circuit differs significantly from the OD Receiver in our collection [1][2].

It is possible that Jan Lourens (PA0BN) mistakenly supplied the wrong diagram at the time, or that the above is the original circuit diagram of the first generation of the OD Receiver, or that there was more than one design of the receiver. It is known, for example, that a limited number of self-modified receivers were made by radio amateur Piet Neve (PA0PN) in OD Region 15 (Zeeland) [3].

Actual circuit diagram as taken down by Crypto Museum [2]

Above is the actual circuit diagram, as taken down from the OD Receiver in the Crypto Museum collection [1][2]. It is believed that it is the second generation of the receiver, as it has plug-in coils for the RF stage (L1) and for the regenerative detector (L3/L4). The antenna signal is directly fed to the g1 of V1 (no transformer), and the AF output is taken directly from the anode of V3 (via C14). This means that a properly isolated pair of high-impedance headphones had to be used.

Furthermore, the regeneration level of the detector (V2) is not controlled by adjusting the g2 voltage of V2 with a potentiometer, but by adjusting trimmer C8 in the feedback loop (L4). The receiver covers only a small part of the 3 MHz band, with was pre-set with C5 and C6 (reachable through a hole in the font panel) and fine tuned with the big variable capacitor at the centre of the front panel (C7). It is likely that C6 was roughly 'calibrated' with a pre-calibrated transmitter. At the top right is the power switch that is part of the volume control (P1). It switches the LT only.


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Interior
The OD Receiver is designed in such a way that the interior is easily accessible without removing any screws. It is built on an aluminium chassis and is housed in a bended aluminium enclosure. The interior is covered by two removable panels: one at the top/rear and one at the bottom.

These two panels are simply clamped onto the case, and can easily be taken off, as shown in the image above. This reveals the aluminium chassis that holds all components plus the front and side panels. The image on the right shows the enclosure as seen from the rear. Towards the bottom edge is a contact strip with the sockets.

The receiver is built around three identical EF6 valves. The one at the right is used as an RF pre-amplifier. It has a removable antenna coil and a tuning capacitor that is accessible from the side. The antenna/ground sockets are right below it.
  
Cover removed (rear view)

At the center is the oscillator/detector valve. Like the RF pre-amplifier it has a removable coil, that can be swapped to make the receiver suitable for the reception of the 31, 41 and 49 metre broadcast bands. Unfortunately, the additional coils are missing from the device shown here.

The oscillator is tuned with the adjustable capacitor at the front panel. It has fixed and adjustable capacitors connected in parallel, so that the centre of the frequency range can be adjusted as required. Furthermore, the feedback loop (reaction) can be adjusted with a trimmer that is fitted in the left corner of the front panel.

The leftmost valve is used as an AF amplifier. Via the potentiometer in the side panel, it is fed with the signal from the detector stage, which is then amplified to a level that is suitable for a high-impedance pair of headphones (4000-5000 Ω).
  
Detector and AF amplifier (after restoration)

The sockets for the headphones are at the far left. Note that there is no isolation transformer present in the output stage. The headphones are directly connected to the anode voltage of the AF amplifier valve – via a 25 nF capacitor – so it is strongly advised to use a properly isolated one.

The bottom section of the chassis holds most of the passive components, such as resistors, capacitors and choke coils. Like the top section of the chassis, is has two compartments: one for the RF amplifier, and one for the remaining part.

The first two valves (V1, V2) have a choke coil in their anode rail. These coils are mounted to the side panels and shown in the image on the right.

This image also shows the point where the HT voltage from one of the sockets, is distributed throughout the device via a network of resistors.
  
Detector - bottom section

The device is powered by two voltages which have a common ground: +6.3V for the filaments of the valves (LT), and +250V for the anodes (HT). This relatively high HT voltage is chosen to allow the same power supply unit (PSU) to be used as for the OD Transmitter. 0V goes to the chassis.

Cover and bottom panels removed
Cover removed (rear view)
Bottom panel removed (bottom view)
Bottom section
Interior - rear view
RF amplifier (after restoration)
Detector and AF amplifier (after restoration)
Volume potentiometer
RF coil removed
Detector coil removed
Frequency calibration
Regeneration feedback adjustment (C8)
RF amplifier - bottom section
Detector - bottom section
AF amplifier - bottom section
Detector coil socket wiring
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Cover removed (rear view)
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Bottom panel removed (bottom view)
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Bottom section
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Interior - rear view
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RF amplifier (after restoration)
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Detector and AF amplifier (after restoration)
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Volume potentiometer
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Detector coil removed
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Frequency calibration
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Regeneration feedback adjustment (C8)
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RF amplifier - bottom section
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Detector - bottom section
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AF amplifier - bottom section
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Detector coil socket wiring

Restoration
When we acquired our OD Receiver in November 2020 [1], it was uncertain whether it would ever work again. Although it should not be too difficult to bring the simple circuit back to life again, this might involve swapping certain components for modern alternatives. Given the unique nature of the device however, we wanted to keep it in its original wartime condition as much as possible.

A first visual inspection revealed that the device shows severe signs of corrosion. This is partly caused by the poor quality aluminium that was available during the war, but also by the condition under which it was stored after the war. The corrosion is most serious at the points where two different types of metal are in contact with each other. This is known as galvanic corrosion.

After carefully removing the most serious corrosion, some wires that had loosened over time, were re-soldered. This was partly caused by the corrosion, but also by the fact that the antenna tuning capacitor was not properly fixated to the side panel, as a result of which the leads and the first valve (V1) moved when the knob was turned. Next, the valves and the plug-in coils were removed, so that the values of all resistors and capacitors could be checked with an LCR meter.

When we were satisfied that all values were still within their specified range, the valve sockets were cleaned and the valves were re-seated. The +6.3V LT voltage for the filaments of the valves was connected, and the power switch was turned ON. After the current had stabilised at ~ 560 mA (which seems normal for three EF6 valves), the +250V HT voltage was applied. Noise was heared through the headphones, and the 3 MHz signal from an external generator was quickly picked up. The OD Receiver still works after more than 75 years, with its original components!

So far, the following restorations have been carried out:

OD receiver
OD Receiver on the test bench
Operational OD Receiver in 2020
OD Receiver working again after more than 75 years
Bottom section (before restoration)
Frequency calibration (adjusting C8)
Rear view
Reproduction power cable
C
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OD receiver
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OD Receiver on the test bench
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Operational OD Receiver in 2020
C
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OD Receiver working again after more than 75 years
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Bottom section (before restoration)
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Frequency calibration (adjusting C8)
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Rear view
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Reproduction power cable

Connections
Rear panel
All connections of the OD Receiver are at the rear. Below is the layout of the sockets, as seen from the rear of the device. All sockets accept short 4 mm banana plugs, 1 and are fitted on a pertinax 2 strip. Note that the sockets where a brown pertinax ring is visible, are isolated and carry signals or live voltages. All other sockets are mechanically attached to the chassis (ground).

Sockets as seen from the rear of the OD Receiver

Note that we have modified the socket at the far left, by fitting a 4 mm screw right through it. The screw is held in place with a nut on the inside of the chassis. This is done to keep the pertinax strip in place when inserting banana plugs into the other sockets, and reduces the risk of breaking the pertinax strip. In our case, the leftmost socket was detached due to corrosion.

  1. The sockets are only 10 mm deep. Inserting regular 4 mm banana plugs with a length of 20 mm, all the way in, may damage the components in the bottom section of the receiver, or may cause a short-circuit.
  2. Also known as resopal.

EF6 valve
EF6 is a universal penthode, developed specifically for AF applications. According to the datasheet it is unsuitable for application in RF and IF amplifiers. Nevertheless it was used in all stages of the OD Receiver, probably because no other valves were available at the time. Below is the pinout of the EF6, as seen from the bottom of the valve (i.e. the solder side of the socket).

 EF6 datasheet


Specifications
Original design
  • Design
    Philips, Jan Thijssen
  • Manufacturer
    Philips, Jan Hendrik (Henk) Op den Velde (Zaandam)
  • User
    Binnenlandse Radiodienst (BR) of the Ordedienst (OD)
  • Introduction
    1942
  • Frequency
    2.85 - 3.15 MHz (variations are known)
  • Antenna
    Long-wire
  • Circuits
    RF pre-amplifier, detector, AF amplifier
  • Valves
    3 x EF6
  • Power
    LT +6.3V, HT +250V
  • Dimensions
    200 x 140 x 100 mm
  • Weight
    1100 grams
  • Accessories
    Power Supply Unit (PSU), Headphones
Alternative design
  • Design
    Jan Lourens
  • Manufacturer
    Jan Lourens (Oosterbeek), Piet Neve (Zeeland)
  • User
    Binnenlandse Radiodienst (BR) of the Ordedienst (OD)
  • Frequency
    2.95 - 3.25 MHz (adjustable)
  • Antenna
    Long-wire
  • Circuits
    RF pre-amplifier, detector, AF amplifier
  • Valves
    3 x EF6
  • Dimensions
    240 x 135 x 110 mm (bare case: 185 x 135 x 90 mm)
  • Weight
    922 grams
  • Accessories
    Power Supply Unit (PSU), Headphones
References
  1. Cor Moerman, OD Receiver - THANKS !
    Received November 2020.

  2. Crypto Museum, Internal research of OD Receiver in collection
    CM303549. December 2020.

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

  4. Jan Schulten, De radiopost van de Ordedienst in Rijsbergen
    - achtergronden van het drama op de Vloeiweide - (in Dutch language).
    Jaarboek De Oranjeboom 47, 1994.

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

  6. Wikipedia (Netherlands), Henk op den Velde
    Retrieved December 2020.

  7. Medica Amsterdam, Luxor - Bestralingsapparaat
    1938. In Dutch language. CM301583/3/5.

  8. D.W. (Dick) Rollema (PA0SE), Radioverbindingen van het Verzet in Zeeland
    VERON Electron, May 1987

  9. D.W. (Dick) Rollema (PA0SE), Station G11 van de Binnenlandse Radiodienst
    VERON Electron, May 1988
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Crypto Museum. Last changed: Tuesday, 19 January 2021 - 21:52 CET.
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