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PLR
Power line bug monitor - not in collection

PLR 1 is a LF power line receiver, developed around 1960 by Her Majesty's Government Communications Centre HMGCC at Hanslope Park (UK). 2 The device was intended for detecting and monitoring low frequency covert listing devices (bugs) in the 25-150 kHz frequency range – with Amplitude Modulation (AM) – that use the mains power lines as their transport medium.

In most cases, the bug picks up room audio and (AM) modulates it onto a subcarrier in the 100 kHz range. This signal is then injected into the mains network and picked up by the PLR receiver elsewhere in the building or nearby, as long as it is connected to the same mains phase.

The device is housed in a grey hammer painted metal enclosure that measures 200 × 140 × 45 mm and weights 1440 grams. All connections plus the volume control are at one of the short sides. At the top is a tuning scale marked RF TUNE (although it probably should be LF TUNE).
  

It is likely that the top surface also contained a table wich converts the numbers of the linear tuning scale to the actual frequencies. The tuning unit is identical to the RF tuning unit of the Mk.301 spy radio receiver that was developed in 1954 by SG Hart at HMGCC in Borehamwood (UK). This is why we think it is likely that the PLR receiver was also made at HMGCC. Unlike the Mk.301 however, which is built with miniature valves, the PLR receiver is fully transistorised.

The receiver can be used for two purposes: (1) as a counter measure device (TSCM), to detect unwanted power line bugs and (2) in surveillance operations, as the receiver for a planted power line bug. The device shown here is from the personal collection of Reinhard Glogowski in Germany [1]. It is currently not in working order, so we've been unable to determine the exact frequency range and verify its operation. At present, we have no further information about this device. If you have any information that can help us to improve this page, please contact us.

  1. As the name and/or model of this device is currently unknown, we have nicknamed it PLR, which is short for Power Line Receiver.
  2. Not certain but very likely given the use of the tuning unit of the Mk.301 and the fact that HMGCC/Hanslope Park is responsible for bug finding.

HMGCC LF receiver/bug detector
Tuning knob and scale (identical to that of the Mk. 301 spy radio receiver)
Serial number
Front panel
A
×
A
1 / 4
HMGCC LF receiver/bug detector
A
2 / 4
Tuning knob and scale (identical to that of the Mk. 301 spy radio receiver)
A
3 / 4
Serial number
A
4 / 4
Front panel

Features
The image below shows the connections and controls of the receiver. The mains AC power is connected to the 3-pin Bulgin connector at the right. To its left is the ON/OFF switch. At the left is the volume control knob. To its right is a 6 mm jack socket for connection of a pair of (mono) headphones. At the center is a 3-contact screw terminal on which a galvanically separated line signal (L) is available with an impedance of 600Ω. The centre contact (CT) is at 300Ω. At the top is the tuner, which consists of a linear tuning scale and a tuning knob with gearbox mechanism.



Block diagram
Below is a much simplified block diagram of the PLR receiver. At the bottom left is the internal power supply unit (PSU) which feeds the electronic circuits. The two blue capacitors at the top left supply the mains line – via an isolation transformer – to a tuned circuit. This is the tuning unit from the Mk.301 receiver. The signal is then filtered (to get rid of the 50 Hz hum) and fed to the receiver with an AM detector. The resulting AF signal is then amplified to headphones/line level.







Interior
The device is housed in a grey hammer painted metal enclosure that consists of two shells. The upper case shell holds the tuning unit, the controls and all electronics. The lower case shell acts as a cover. It is held in place by eight screws: four at the long sides and four at the short sides.

After removing the screws and the lower case shell, the interior can be observed as shown in the image above. At the right are the connec­tions and the mains power supply unit (PSU). At the left is the tuning unit, which is identical to that of the 1954 Mk.301 spy radio receiver.

The electronic circuits are built around OC44, OC139, OC201 and OC 202 transistors. They are divided over three vertically mounted boards at the centre: receiver (left), amplifier (centre) and PSU/PA (right). The output of the power amplifier (PA) is suitable for a pair of 600Ω headphones.
  

The output of the PA is also available – via a 1:1 isolation transformer – as a 600Ω line signal, that can be fed to a recording device or a leased analogue tele­phone line (POTS). This way, the signal can be monitored from a greater distance, which is particularly useful in surveillance ope­rations. 300Ω equipment can be connected by using the centre contact (CT) of the transformer.

Bottom view
Case shell removed
Interior - left angle view
Interior - rear right angle view
Top view
Tuning unit
Power supply unit with miniature transformer
Two capacitors to 'steal' the LF signal from the mains
B
×
B
1 / 8
Bottom view
B
2 / 8
Case shell removed
B
3 / 8
Interior - left angle view
B
4 / 8
Interior - rear right angle view
B
5 / 8
Top view
B
6 / 8
Tuning unit
B
7 / 8
Power supply unit with miniature transformer
B
8 / 8
Two capacitors to 'steal' the LF signal from the mains

Specifications
  • Device
    Power line AM receiver
  • Purpose
    Detecting and monitoring power line bugs
  • Manufacturer
    HMGCC
  • Model
    ?
  • Country
    UK
  • Year
    1960 (est.)
  • Frequency
    25-150 kHz (est.)
  • Output
    300/600Ω, screw terminals
    600Ω headphones, 6 mm jack
  • Power
    200-240V AC
  • Dimensions
    200 × 140 × 45 mm
  • Weight
    1440 g
References
  1. Reinhard Glogowski, HMGCC PLR receiver
    Crypto Museum, March 2024.
Further information
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Crypto Museum. Created: Tuesday 19 March 2024. Last changed: Tuesday, 19 March 2024 - 16:31 CET.
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