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WT-11
Radio interference investigation receiver

GPO Receiver WT No. 11, was a portable radio direction finder, made around 1937 1 by McMichael Radio Ltd. in Slough (UK), 2 for the British General Post Office (GPO). The device was intended for finding sources of radio interference on the Long Wave (LW) and Medium Wave (MW) bands, such as clandestine radio stations and interference caused by domestic equipment and power lines [2].

The device is housed in a black-painted wooden enclosure that measures 300 x 300 x 150 mm and weights 5160 grams (without the batteries). It can be carried by the leather grip at the top.

Volume and frequency adjustments are at the top, at both sides of the frequency tuning scale. At the front is a signal strength indicator and a speaker, with the letters GPO carved out of the wood. At the rear are the sockets for ground, external antenna and two pairs of headphones. The antenna sensitivity can be adjusted freely. It is powered by internal LT and HT batteries.
  
GPO interference investigation receiver WT No. 11

Although the device can be used with an external antenna, it was commonly used with the internal directional window antenna, that is part of the inner wooden frame. It allows the device to be rotated in order to obtain a maximum (or minimum) signal strength reading on the meter. During WWII, it was mainly used for monitoring the LW and MW bands for clandestine stations.

In the 1950s, the receiver was used for locating sources of interference on public broadcasts in the LW and MW bands. A nice account of the latter was given by David Rudram in the June/July 1990 issue of Radio Bygones [2]. According to a label at the side, the device shown here was last calibrated on 10 July 1962. The device is also known by its circuit diagram number WL 28600. In 1939, it was succeeded by GPO Tester WL 53400, that was also suitable for the Short Wave (SW).

  1. Estimated from date codes on some of the components.
  2. In 1956 merged with Sobell Group to become Radio & Allied Industries Ltd. (RAI).
    In 1961, RAI was acquired by General Electric Company (GEC) [4].  Wikipedia

GPO interference investigation receiver WT No. 11
Rear view
Tuning scale at the top
Tuning scale
Speaker with GPO initials, and signal strength meter
Power switch and band selector
Antenna selector and antenna sensitivity tuning
ID tag and calibration label
A
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A
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GPO interference investigation receiver WT No. 11
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Rear view
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Tuning scale at the top
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Tuning scale
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Speaker with GPO initials, and signal strength meter
A
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Power switch and band selector
A
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Antenna selector and antenna sensitivity tuning
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ID tag and calibration label

Features
The images below give an overview of the controls and connections of the WT-11. At the front is a large speaker, protected by a metal grid, with the letters GPO (General Post Office) carved out in a wooden oval. To the right of the speaker is the signal strength meter, which can also be used to check the LT and HT voltages by altering the position of the function switch situated above it.

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At the right side is the power switch, which also acts as the band selector. At the top is a bakelite panel with a large tuning scale at the centre. It has a separate scale for LW and MW. At either side of the scale is a large bakelite knob for adjusting the audio volume and for tuning the frequency.

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At the rear are two 6 mm sockets for connection of the headphones. Also at the rear is a panel with sockets for connection of an external antenna and ground. A rotary switch allows the user to select between the internal frame antenna or an external one. To its right is the sensitivity knob.


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Interior
The interior of the WT-11 can be accessed by removing the panel at the rear. It is held in place by two U-shaped metal clips at the bottom and a shift-lock at the top. By shifting the lock to the right the panel can be tilted towards the rear, after which it can be lifted from the U-shaped clips.

Inside the wooden enclosure is a wooden frame to which the electronic circuits are mounted. The inner frame also acts as a directional antenna. It can be used to narrow down the position of an illegal transmitter or a source of interference.

The actual receiver is mounted at the top right, as shown in the image on the right. It consists of four miniature valves – held in place by 2 spring-loaded metal brackets – and several filters. Below the circuit is a large speaker that is mounted to the front panel. To the left of the speaker are the terminals of the signal-strength/voltage meter.
  
Interior - electronic circuit detail

The antenna selector and the headphones sockets are located on separate panels, mounted in the upper corners of the inner wooden frame. At the bottom is space for the LT and HT batteries. A wooden support bar – stamped with the text WT 11 – is present to keep the batteries in place.

The black & white image on the right was kindly provided by Walter Sanger in the UK [5], and was probably part of the original documentation of the device. It shows the interior, seen from the rear, with the two battery packs installed.

At the left is an Ever Ready Type B117 battery pack, that provides the +90V HT voltage for the anodes of the valves. At the right is the smaller Every Ready All Dry (AD) high capacity battery that provides the +1.5V LT voltage for the valve filaments. They are still available today [6].
  
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WT-11 opened from the rear
Interior
Interior - electronic circuit detail
Interior detail
Interior detail
Battery terminals
B
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B
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WT-11 opened from the rear
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Interior
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Interior - electronic circuit detail
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Interior detail
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Interior detail
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Battery terminals

Similar receivers
GPO Receiver (UK)
GPO
Wilhelm Quante St.Sg.52 (Germany)
 Other direction finders


Help wanted
At present, no further information about this receiver is available. We are particularly interested in finding the original circuit diagrams and stories or anecdotes from former users.  Contact us


Specifications
  • Name
    Radio interference investigation receiver
  • Identification
    WT No. 11, WL 28600, MRL 50/1
  • Manufacturer
    McMichael, Slough (UK)
  • User
    General Post Office (GPO)
  • LW
    150 - 300 kHz
  • MW
    300 - 1510 kHz
  • Valves
    4 (see below)
  • Antenna
    Internal (frame), external (wire)
  • Power
    LT ?, HT ?
Valves
  • CV820
    3S4
  • CV784
    1S5
  • CV782
    1R5
  • CV785
    1T4
References
  1. Cor Moerman, GPO Receiver WT-11 - THANKS !
    Received November 2020.

  2. David Rudram, Radio and Television Interference Work in the 1950s
    Radio Bygones, Issue 6, page 9.

  3. Philips M. Moss, McMichael Company History
    Slough, 1979/ Retrieved November 2020.

  4. Wikipedia, Radio & Allied Industries
    Retrieved November 2020.

  5. Walter Sanger, Personal correspondence
    December 2020.

  6. Classis Radio Shop, More Radio Batteries
    Retrieved December 2020.
Further information
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Crypto Museum. Created: Sunday 29 November 2020. Last changed: Sunday, 06 December 2020 - 15:21 CET.
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