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Polish WWII spy radio set - this page is a stub

The BP-5 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 1944 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 design of the BP-5 is based on the earlier BP-3 and covers the same frequency range of 2-8 MHz. It contains some improvements however, the most notable of which is the addition of an AM (voice) modulator that is marked by the presence of a 4-pin socket along the bottom edge of the control panel.
Control panel of the BP-5

The BP-5 was developed during the final stages of WWII and was introduced in mid-1944. Apart from limited use in Europe, the BP-5 was heavily used in the Pacific War Theatre, where the AM (voice) option was of great benefit to reconnaissance and surveillance patrol missions [1].

This page is a stub and acts as a placeholder for future information about the BP-5. The images of the BP-5 shown here were taken at Museum Jan Cover in The Netherlands [3]. For further details about the Polish BP-series of spy radio sets, please refer to our page about the BP-3.
Control panel of the BP-5 Extra 4-pin socket for modulator Mode selector Polish BP-3 (left) aside a BP-5 BP-5 control panel

The BP-5 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) has to be set to the desired frequency range (2-4 or 4-8 MHz). A rotary switch at the centre is used to select the desired mode of operation: F for Fonia (AM, voice), S for Strojenie (Tune) and Gr for Grafia (CW, telegraphy) [1]. 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 (S, tune) or 210 mA when at full power (Gr).
BP-3 versus BP-5
The image below shows a BP-5 (right) aside the earlier BP-3 on which its design is based. Aside from a few minor changes, the control panel of the BP-5 is largely identical to that of the BP-3. The most prominent feature of the BP-5 is the presence of the 4-pin modulator socket at the front edge, just below the oscillator range selector. Click the image below for a larger view.

 More information

Polish BP-3 (left) aside a BP-5

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 this BP-5), 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-5

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-5. 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-5. 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.
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.

 More about the Polish history
  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. Museum Jan Corver, BP-5 spy radio set featured on this page
    Crypto Museum, December 2014.

Further information

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