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GPO Receiver
RDF Receiver WL-53400

Tester WL.53400 was the official name of a compact portable radio direction finder (RDF) that was intended for intercepting and locating clandestine transmitters operating on British territory. The receiver was developed around 1939 by the Wireless Branch (W. Branch) of the General Post Office (GPO, now British Telecom, or BT) at Dollis Hill (UK), and is commonly known as the GPO-receiver.
The receiver was developed and built by the GPO and was used intensively during and after WWII. It is housed in a Bakelite enclosure and is fully self-contained. The circuit is built around just two HIVAC 1.5V miniature valves (tubes).

The most special feature of this receiver is the fact that the top lid of the bakelite case, acts as the frequency range selector (plug-in), but also as the direction-finding (DF) loop antenna. The image on the right shows a typical GPO receiver, with lid number 10 in place. The controls, read-out and connections are all at the front.
GPO Receiver (Tester WL.53400)

Only a small number of GPO receivers were ever produced at Dollis Hill, making them into desired collector's items. The unit shown here has serial number 94 and was part of the 1st batch of 100 units that were produced in 1939. In the period following WWII, it was used by the Department of Trade and Industry (DTI) to locate Cold War related and clandestine transmitters and 'pirates'.
GPO Receiver (Tester WL.53400) Frequency coil/antenna on top of the GPO receiver GPO receiver front panel Connecting the headphones Tuning to the desired frequency Signal-strength indicator Close-up of the on/off switch and the meter push-buttons GPO receiver in front of its storage case

The receiver is also described in Louis Meulstee's Wireless for the Warrier. Part 4 [1], but his description is somewhat different from our device. First of all there are 10 frequency-range coils (lids) rather than 9. Secondly, our own GPO receiver has two different valves (HIVAC XL and XP), rather than two identical ones (XP). The XL valve was made especially for LF applications.
The GPO receiver is a very compact device that has all controls and connections nicely aligned at the front panel. The unit is powered by internal LT and HT batteries, and is turned on by flipping the metal ON/OFF switch to the left. A pair of headphones or an earpiece should be connected to the two sockets at the top left. When in operation, the large meter at the right is used as a field-strength indicator. When operating the black and red push-buttons (above the ON/OFF switch), the meter is used for checking the LT and HT voltages, which are 1.5V and 30V DC respectively.

GPO Tester WL.53400 control panel

A suitable coil lid (i.e. frequency plug-in) should be clipped on top of the receiver before switching it on. The receiver can be used in oscillating and non-oscillating mode, with the REACTION knob controlling the level of self-oscillation. Tuning into a station is best done with the receiver oscillating. The case can then be rotated in the horizontal pane in order to find the maximum signal strength. When homing in on the signal the sensitively of the receiver can be reduced by turning the REACTION knob to the MIN position. If the signal is still too strong, the 'meter adjustment' screw at the botton left can be used to further reduce the sensitivity.

When in close proximity of a (strong) radio station, it might not be possible to reduce the sensitivity enough to get a useful reading on the meter. In such cases it is best to search for the minimum field strength when rotating the receiver, rather than the maximum.
Storage case
When unused, the GPO-receiver, the frequency modules and the accessories were stored in a qubic leather transit case. The case has three compartments: (1) headphones and spares, (2) the receiver with one frequency module (lid) and (3) the remaining 9 frequency modules.
The image on the right shows a typical war-time leather storage case that roughly measures 23 x 21 x 23 cm. It has a leather strap that allows the case to be carried around by hand, and a smaller strap with a buckle to keep the top lid closed. The corners at the bottom of the case have been strengthened with extra thick leather.

Inside the case are three compartments. A narrow one at the left where the headphones and the spare valves are stored, a slightly larger compartment where the actual receiver is stored (complete with one frequency plug-in) and the largest area that holds the remaining nine frequency coils. The interior is coated with dark green felt, except for the receiver's space which is coated with green velvet to avoid scratches.

The case shown here has seen quite some action. After its war-time use, it served with the DTI (UK), that was mandated for locating and charging clandestine radio stations. Initially, such stations were Cold War spies operating on British territory, but later the attention shifted to 'pirate' stations. Once the receiver was replaced with more modern equipment in the early 1960s, it was dusting away on a shelf in a damp shed where it sat for more than 40 years.
GPO receiver, frequency coils and accessories stored in leather case

When the receiver was rediscovered in 2011, the case was in deteriorating state. Much of the leather had dried out and parts of it had desintegrated completely. Part of the felt on the inside had become infected with fungus and pieces of the interior coating had come off. However, with a bit of patience and some old fabrics, the case was restored to much of its former glory.
Leather case with carrying strap up Leather storage case GPO receiver, frequency coils and accessories stored in leather case Taking the GPO receiver out of the leather case Leather storage case with 9 spare frequency coils Leather storage case, front view Leather storage case, side view GPO receiver, frequency coils and accessories stored in leather case Empty storage case
Spare HIVAC vacuum tubes (valves) Spare valves Storage case before restoration Storage case before restoration

Frequency ranges
The receiver covers all frequencies between 150 kHz and 28 MHz, divided over 10 ranges. For each range, a separate coil (top lid) is supplied. Each coil has four in-line contacts that mate with four spring-contacts in the receiver. The lid is held in place by four metal clips that are positioned asymmetrically in order to prevent the lid from being mounted the wrong way around.
The following ranges are available:
  1. 150 - 230 kHz
  2. 230 - 360 kHz
  3. 360 - 600 kHz
  4. 600 kHz - 1 MHz
  5. 1 - 1.6 MHz
  6. 1.6 - 2.7 MHz
  7. 2.7 - 4.6 MHz
  8. 4.6 - 9 MHz
  9. 9 - 16 MHz
  10. 16 - 28 MHz
Frequency coil/antenna on top of the GPO receiver

The image above shows the GPO Receiver with lid number 10 (16-28 MHz) lying on top of it (upside down). De lid is rectangular and has a circular hollow space inside it, in which the actual frequency coil is located. The coil can be opened by removing the 7 bolts that keep it in place.
Frequency coil/antenna on top of the GPO receiver Placing the frequency coil/antenna Four spring contacts inside the receiver See how the contacts of the lid mate with the spring-contacts inside the receiver Removing the coil Removing the coil The coil removed from the top lid Frequency range coil

The GPO receiver has a high-impedance audio output. It was used in combination with a high-impedance headset or with crystal earphones, such as the one shown here. It is connected to the receiver's PHONES-socket by means of the 2-pin plug shown in the picture.

Although the unit was originally supplied with a pair of black headphones, crystal earphones were often preferred as they could be worn inconspicuously.
Crystal earphone

Storage case with receiver and headphones Crystal earphone Crystal earphone Close-up of the earphone GPO receiver with earphone in front of leather case Storing the earphone in the side-bag

The GPO Tester WL.53400 is a beautifully built simple receiver that can be used in the near field of a (clandestine) transmitter. Its electronic circuit constists of only two minuature valves, made by UK manufacturer HIVAC. According to [1], both valves are identical (XP), but in our sample we found two different ones (XP and XL). Furthermore, one of each is supplied as spares with this receiver. It is therefore possible that a HIVAC XL was used for the LF amplifier.
The image on the right gives a clear view at all the electronics that are present inside the box. At the center are the two minature valves and the four spring-contacts for the frequency coil. At the left are three adjustable capacitors.

Below the visible items, at the bottom of the case, are only a couple of passive components (resitors and capacitors). The rest of the case is filled with the indicator and the batteries. The indicator is a mA-meter that is also used for testing the LT and HT voltages. This is done by pressing the black and red buttons at the front.
Close-up of the electronics

Apparently our unit was modified at some point during its lifetime. Traces inside the case reveal that leaking batteries have caused problems in the past. The original batteries (LT 1.5V and HT 30/60V) were installed in two empty spaces inside the case [2]. In the modified version [3], two battery holders (made of pertinax) are installed. A separate 1.5V battery is used for the filament of each valve, whilst two 30V batteries are connected in series to produce 30V and 60V HT.
GPO Receiver interior Close-up of the electronics Close-up of the two valves Four spring contacts inside the receiver 30/60V battery compartment 2 x 1.5V battery compartment Close-up of the on/off switch and the meter push-buttons Original, unmodified case

The circuit diagram below was sent to us in November 2013 [4] and clearly shows the operation of the receiver. It consists of a tuned reacting detector (V1) followed by a DC amplifier (V2). The set is tuned by means of two adjustable capacitors: one marked TUNE (C2) for selecting one of five 'click-in' tuning ranges, and a separate one marked TRIMMER (C3) for fine tuning. A separate capacitor marked REACTION (C1) controls the level of feedback or self-oscillation. It is used to control the sensitivity of the receiver. This adjustment has 24 click-in positions.

GPO Tester WL.53400 circuit diagram [4]

Potentiometer P1 at the top right can be used to further reduce the sensitivity when in close proximity of a transmitter. The second stage (V2) amplifies the detected signal to headphones level and also controls the reading of the field-strength meter. For testing the condition of the batteries, two switches, S1 and S2, can be used to measure the LT and HT voltages respectively. The measuring circuit is dimensioned in such as way that LT Test (S1) produces a full-scale reading at 1.5V, whilst HT Test (S2) gives a full-scale reading at 30V (i.e. 1 mA on the scale).
  1. Louis Meulstee, RDF Receiver GPO
    Wireless for the Warrior. Part 4. September 2004. ISBN 0952063-36-0.

  2. Museum Jan Corver, GPO Receiver with serial number 203
    Images made during exhibition Secret Messages. Crypto Museum, November 2008.

  3. Crypto Museum, GPO Receiver with serial number 94
    Modified battery compartment. Crypto Museum, March 2012.

  4. General Post Office (GPO), Tester WL 53400
    Description, Operation and Circuit Diagram. 30 November 1939.
    Kindly supplied by Ray Henville, UK, November 2013.

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Crypto Museum. Last changed: Saturday, 24 December 2016 - 19:21 CET.
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