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Polish WWII spy radio set

The BP-3 was a valve-based spy radio transceiver, developed during WWII by Tadeusz Heftman of the Polish Military Wireless Unit (Polski Wojskowy Warsztat Radiowy) in Stanmore (UK) [1]. It was introduced in 1943 and was intended for use by Agents and Resistance Organisations in Europe.
The 'B'-series of radio sets (from 1943 onwards: 'BP') were produced alongside the 'A'-series and featured increased power output. The sets were housed in a black wrinkle-finished metal box with a lid, and were labelled in Polish or English.

The BP-3 covers 2-8 MHz and is built around 6 valves, all of which are mounted internally. It the field it was powered by a 12V DC converter with rotary transformer, but it could also be powered by the external 120/220V AC Power Supply Unit (PSU) that was connected to the 5-pin socket on the front panel, just like on the other 'BP' models.
Polish BP-3 spy radio set

The BP-3 measures just 28 x 21 x 11 cm and weights less than 6 kg. It uses a long wire or dipole antenna and comes with a set of accessories, such as power cables, crystals, antenna and counterpoise wires, external morse key, a mains power supply unit (PSU) and a 12V DC converter. The separate exernal PSU measures 9.5 x 21 x 28.5 cm and weights just over 10 kg.

The transmitter produces an output power of 50 Watts in CW, which is more than its much larger and havier British counterpart the B2. Apart from the Polish Resistance in occupied Europe, the BP-3 was also used by the SOE (both in Europe and Asia), and the French, Czech and Yugoslavian Resistance. For a long time, the Polish spy radio sets were superior to their British counterparts. The BP-3 was followed by the BP-4, which had a different frequency range (4-16MHz) and finally in 1944 by the BP-5, which covered the same 2-8 MHz but had a built-in AM (voice) modulator.
Closed BP-3 case, resembling a toolbox Polish BP-3 spy radio set Polish BP-3 spy radio set BP-3 with connected power cord and crystal Built-in morse key Close-up of the antenna terminals Close-up of the power socket Placing a crystal

The BP-3 has a clear and well-organised control panel which is shown below. The area left of the yellow dotted line is used by the receiver, whilst the remaining space is taken by the transmitter. The receiver has a small circular tuning dial at the center, with two knobs (course and fine) for adjusting the frequency. The other knob is for adjusting the volume. The full frequency span is divided over two ranges (2-5 MHz and 5-8 MHz) selectable with the black knob at the top left. The receiver is suitable for phone (F) and CW (Gr), selectable with the switch at the bottom left.

The transmitter is more complex and has a large number of adjustments and indicators. First of all, a suitable crystal has to be installed in the socket marked 'Q' along the bottom edge. Next the Oscillator (OSC) and the Power Amplifier (PA) both have to be set to the desired frequency range (2-4 or 4-8 MHz). A toggle switch at the centre is used to set the required ouput power: PÓŁ MOCY (half power) or CAŁA MOC (full power). Once the transmitter is enabled, the three tuning knobs have to be adjusted for maximum brightness of their indicators. At this point the current meter at the top centre should read approx. 110 mA (or 210 mA when at full power is selected).

The front panel of the later BP-5 (mid-1944) is nearly identical to that of the BP-3, with the main difference being an extra 4-pin socket along the bottom edge of the front panel of the BP-5. This socket was used for the AM (phone) modulator extension. The intermediate BP-4 (1943/44) is also nearly identical to the BP-3 but has a different frequency span (4-16 MHz).
The BP-3 is housed inside a wrinkle-finished black metal case that resembles a trademan's toolbox of the era. The external Power Supply Unit (PSU) is housed in a similar albeit smaller case. The Poles believed that, when carried around in occupied Europe, such toolboxes attracted far less attention than the leather suitcases in which most British spy radios were transported.
All components inside the BP-3 are mounted to the control panel, which is held in place by four bolts: two at the front and the at the rear. After removing these bolts, the control panel can be lifted from the metal box, revealing the interior.

The receiver is built around four valves (tubes): a mixer/oscillator (6K8), an IF section (6SK7), a detector/AF amplifier (6SQ7) and finally a Beat Frequency Oscillator (BFO) combined with the AF output stage (6SC7), the latter delivering its output at headphones level. For AM, the BFO can be turned off with a switch at the front panel.
BP-3 interior

The transmitter is built around just two valves: a 6V6 for the crystal oscillator and an 829 double tetrode for the powerful balance power amplifier (PA). Each section has its own tuned circuit that can be adjusted for maximum output power with controls at the front panel. Two neon lamps and one light bulb are used as indicators for maximum power. The transceiver is powered by 3 voltages: +500V for the transmitter, +300V for the receiver and 12V for the filaments.
BP-3 interior BP-3 interior PA valve Transmitter tuning capacitor Transmitter oscillator (right) and PA (left) Restored capacitors in the transmitter section Close-up of the receiver BP-3 interior

Power supply
The BP-3 is powered by an external power supply unit (PSU) that supplies 12V for the filaments, +500V for the anode voltage of the transmitter, and +300V for the anode voltage of the receiver. If the original PSU is missing (as in the case with our BP-3), a suitable alternative with the correct voltages should be connected to the 5-pin socket at the top right, just left of the mA meter.

Power connections when looking into the socket on the BP-3

The image above shows the pin-out of the power connector when looking into the socket at the front panel. Please note that this is mirrored compared to the pin-out in the original circuit diagram, which shows the solder-side of this socket. When in doubt, check the wiring before connecting a PSU to the BP-3. The PSU and the pin-out are the same for the BP-3, BP-4 and BP-5.

Orignal PSU as supplied with the BP-3, BP-4 and BP-5.

The image above shows the circuit diagram of the original valve-based PSU, as it was supplied with the BP-4. It is identical to the PSU of the BP-3. Below is a simplified circuit diagram of a modern replacement PSU, in which the double rectifier valve is replaced with two semiconductor diodes. All other values are identical the original ones. When using the replacement PSU however, please ensure that the filaments are heated before connecting the HT voltages (300 and 500V).

Replacement PSU with 2 semiconductor valves instead of the rectifier valve.

The 12V AC supply is used for the filaments of the valves. In the receiver, the filaments of the valves are connected in pairs, so that each one gets approx. 6V. In the transmitter, the filament of the PA valve (829) is connected directly to the 12V rail, whilst the filament of the oscillator valve (6V6) has a 12 ohm series resistor to bring the voltage down to 6V.
Circuit diagram
Inside the top lid of the BP-3 case is a paper sheet with some operating instructions, the fully circuit diagram and a component list. In practice, the circuit was very helpful when the had to be repaired in the field. Below the somwehat deteriorated original circuit diagram of our BP-3.

BP-3 circuit diagram

When receiving, the PA range selector (95) has to be set to the centre position (ODB.) so that the antenna connected to the input of the receiver. An interesting detail is that only one side of both neon lamps (74 and 97) is connected. This was changed in the later BP-4 and BP-5, probably because the limited energy made it difficult to see the neon indicators in bright daylight.

BP-3 component list

Above: the full list of components used in the BP-3. The numbers in front of each component correspond to the numbers next to each component in the above circuit diagram. The component list was also part of the instruction sheet that was mounted inside the top lid of the metal case.
In the early days of WWII, a group of Poles managed to escape to the United Kingdom. Whilst the Polish soldiers were allowed to setup and train their own Army units within the UK, Polish engineers manned the Polish Military Wireless Unit (Polski Wojskowy Warsztat Radiowy) in Stanmore, just north-west of London, between Edgware and Watford. Here they maintained contact between their government in exile and the Polish Underground Army back in Poland.

The B-series started with the B1 and B2 in 1942, followed a year later by the BP3, BP4 and BP5. At the time of their introduction, the BP radios were superior to the existing British spy sets, both in size and performance. It wasn't until 1943, with the introduction of the (much larger) British B2, that the British were able to match the performance of the Polish sets. And with the introduction of their A3, in 1944, they were finally able to match the size, albeit at a lower RF power output.

 More about the Polish history

British A3 and Polish BP-3

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

  2. Pierre Lorain, Secret Warfare
    1972. English adaption by David Kahn, 1983. ISBN 0-85613-586-0.

  3. Wikipedia, Yalta Conference
    Retrieved December 2014.

  4. RC Church of St Hugh of Lincoln (Letchworth), The Local Polish Community
    Retrieved December 2014.

  5. Wikipedia, Polish Institute and Sikorski Museum
    Retrieved December 2014. → Website.

  6. Website, Generaal Maczek Museum Breda
    Retrieved December 2014.

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