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Poland WWII ← BP-5
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.
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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 broadcast by the BBC in London (UK). 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].
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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.
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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.
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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].
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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.
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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.
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Device Spy radio receiver Purpose Clandestine reception of BBC and Polish broadcasts Model OP-3, OP-3A, OP-3G Designator Type 30/1 Year 1943 Country UK, Poland Developer Polski Wojskowy Warsztat Radiowy in Stanmore (UK) Users Polish resistance Frequency 0.6-1.5 MHz, 2-12 MHz Ranges 3 (see below) Dimensions 175 × 125 × 40 mm Weight 1 kg Quantity 287
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OP-3 Basic model OP-3A ? OP-3G With speaker
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0.6-1.5 MHz 200-530 m - 2-5 MHz
- 5-12 MHz
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- Louis Meulstee, Wireless for the Warrior, volume 4
ISBN 0952063-36-0, September 2004.
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© Crypto Museum. Created: Wednesday 26 September 2012. Last changed: Saturday, 13 July 2024 - 05:51 CET.
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