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OP-3   Type 30/1
Polish clandestine receiver

OP-3 was a portable clandestine receiver, developed and built during WW-II by the Polish Military Wireless Unit (Polski Wojskowy Warsztat Radiowy) in Stanmore (UK). The receiver was introduced in 1943 and was intended for the reception of the BBC and Polish Radio Broadcasts from London. It is also known as Type 30/1 and was sometimes used as backup for clandestine radio sets [1].
Although the receiver is valve-based, it measures only 4 x 12.5 x 17.5 cm and weights just under 1 kg. It is housed in grey metal 'slip-on' case with rounded sides. A similar case is used for the high and low voltage batteries.

The image on the right shows the two units. The OP-3 is at the front and has a fixed power cable that is plugged into the battery pack at the rear. The units are small enough to be concealed under the clothing of the operator. Each unit has a leather strap at the center of the control panel, allowing it to be attached to, say, a button.
OP-3 with battery case

At the front of the unit is a metal plate with operating instructions. Like the text that is engraved on the control panel, it was available in Polish, Danish and English. The unit shown here is the English version. The leather straps were replaced by metal rings on some models (see below).

In total, 287 OP-3 receivers were built, 105 of which were dropped over occupied Poland in 1944, together with similar receivers such as the British MCR-1. They were mainly used by the Polish clandestine radio networks, and allowed the resistance groups to listen to the coded messages that were distributed by the BBC in London. Such coded messages indicated, for example, the time and place of a scheduled air-dropping. The OP-3 was used in several other countries [1].
OP-3 with battery case OP-3 with battery case OP-3 front panel Tuning to the desired frequency Adjusting the volume Selecting the frequency band Leather strap for attachment to clothing Close-up of the tuning dial

All controls and connections of the OP-3 are on the top panel of the radio. At the far left is the volume adjustement with the band selector to its right. In between these two knobs are the antenna (top) and ground (bottom) connections. The right half contains the tuning dial and the frequency scale with lense. A pair of low-impedance headphones is connected on the right.

The radio is normally used for listening to AM (voice) signals, but can also be used to for the reception of morse signals (CW) by pushing-in a knob at the top center (Gr). The radio is powered by two voltages: HT 60V DC for the valves and LT 1.4V DC for their filaments. A fixed power cable enters the radio on the right. At the end of the cable is a typical British 4-pin plug that mates with the socket on top of the battery unit. The batteries lasted approximately 35 to 50 hours.

The drawing above shows how the batteries are organised and how they are connected to the 4-pin socket (looking into the socket on the battery pack). Inside the case are four compartments. The largest area is taken by the 60V battery. The two smaller compartments can each hold a single A-size 1.5V battery. The 4th compartment holds the socket and a decoupling capacitor.
Opening the case Removing the battery pack from the shell Interior of the battery pack Interior of the battery pack

The interior of the OP-3 can easily be accessed by removing two recessed screws from the bottom and sliding-off the case shell. This reveals the interior as shown below. Opening the battery case can be a bit harder, as the case might be stuck due to battery leakage in the past. In any case: never pull the leather straps when opening the case. Use a metal hook instead.
Inside the case shell of the receiver are four springs mounted to the bottom. When the case is closed, they prevent the valves from falling out of their sockets when dropping the radio.

The circuit is built around four valves: a 1R5 for the mixer/oscillator, a 1T4 for the IF amplifier, a 1S5 for the detector and finally another 1T4 for the AF amplifier. For the reception of morse signals (CW), the IF stage (1T4) can be brought into oscillation by closing the Gr-switch. A full circuit diagram is available in Louis Meulstee's excellent Wireless for the Warrier Volume 4 [1].
OP-3 interior

The image above clearly shows that the receiver can roughly be divided into three parts. The valves and the adjustable coils are all mounted in the bottom section. The rest of the circuitry is located in the other two sections. The main tuning capacitor is visible on the right. In the left half a series of trimming capacitors allow calibration of the receiver on each of the three bands.
Opening the case Empty case shell showing the valve-retaining springs at the bottom OP-3 interior, rear view OP-3 interior Close-up of the valves inside the OP-3 Close-up of an adjustment inside the OP-3 Tuning capacitor inside the OP-3 Adjustments inside the OP-3

Polish version
The image on the right shows a version of the OP-3 (Type 30/1) with Polish inscriptions on the knobs and Polish user instructions at the front. SILA is the Polish word for Volume and STR. is the abbreviation of Strojenie (Tuning).

Another difference with the OP-3 shown at the top of this page, is the metal ring on the side of the control panel, instead of the leather carrying strap. It was probably a later modification to make it more robust. The metal ring is clearly visible in the image on the right. The radio is otherwise identical.
Polish control panel

Operating instructions in English Polish operating instructions Polish operating instructions Control panel of the Polish version metal ring for carrying the OP-3 Polish version of the OP-3 Polish version of the OP-3 Polish control panel

Frequency ranges
  • 200-530 m (0.6-1.5MHz)
  • 2-5MHz
  • 5-12MHz
  • English
  • Polish
  • Danish
  • OP-3
  • OP-3A
  • OP-3G (with speaker)
  1. Louis Meulstee, Wireless for the Warrior, volume 4
    ISBN 0952063-36-0, September 2004.

Further information

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Crypto Museum. Created: Wednesday 26 September 2012. Last changed: Wednesday, 11 November 2015 - 09:24 CET.
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