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Wired Easy Chair

WEC, or WEC Mark I, was a wired covert listening device, developed in 1960 by the Dutch Radar Laboratory (NRP) for the US Central Intelligence Agency (CIA), as part of a long-term development contract under the name Easy Chair. The device was intended for intercepting a conversation in a room, by using the microphone of an existing telephone set, whilst the handset was on-hook.
The device consists of three units: a transmitter, a line interface and a receiver. The transmitter impresses a carrier signal well above the audible range onto the telephone line. Part of this signal reaches the telephone set where, by the virtue of coupling between the internal parts, some of it reaches the terminals of the carbon microphone.

Some of the carrier energy will be absorbed by the carbon microphone and the remainder will be reflected, subject to the instantaneous value of the microphone's resistance, which changes as a result of acoustic signals in the target area.

A fraction of the reflected signal travels back through the telephone line, where it is picked up at the tapping point by a sensitive receiver inside the WEC. In order to separate the received signal from the (strong) transmitted carrier signal, a hybrid duplexing unit is present in the WEC. The line interface is responsible for an optimum impedance matching between the WEC and the line. This is done to avoid losses and to reduce the chance of detection by means of TDR equipment.

In 1963, the WEC system was upgraded with several extensions, most of which were intended for a specific CIA target. A line-switching unit was added to allow the monitoring equipment to switch smoothly between any of 10 telephone lines, using an optical LDR-based switcher. In addition, the ability was added to control and monitor the system, via a regular telephone line.

In 1961, a new type of Easy Chair Passive Element (PE) was introduced, that could be connected to the telephone line inside the target building. It was powered by the RF signal from the WEC unit, and delivered its reflected signal via the telephone line back to the unmodified receiver of the WEC, where the original audio was recovered. This project was known as Rocking Chair (RC) [D].
Suitcase WEC Mk I in green suitcase WEC Mk I in green suitcase WEC Mk I Front view Mains cable Telephone line cable (tap) Audio output cable

The diagram below gives a good view of the controls and connections. The device consists of three removable units that are mounted in a frame: a transmitter (left), a line interface (centre) and a receiver (right). The transmitter has a frequency range of 20 to 300 kHz, divided over three bands, and has an adjustable power output. The device has a built-in mains power supply unit (PSU) that is part of the receiver (right). It is suitable for 110 and 220V AC mains networks.

A socket for connection to the AC mains is located at the right. Also at the right, is a socket on which the receiver's audio output is available. At the top of the device is a three-pin socket for connection to the telephone line. It connects to the (twisted) wire pair, commonly known as the A and B lines, and to ground. Once connected to the phone line, the line interface (centre) should be tuned for the best possible impedance matching to the line, in order to maximise the amount of energie that is coupled into the line, and minimise the chance of detection by a sweep team.
The application of the WEC is limited to those situations were a regular (analogue) telephone set is present in the target area, which is connected to the exchange of a Public Switched Telephone Network (PSTN), somewhere outside of the target area. Furthermore, it is necessary that a tapping point can be made inconspicuously on the line between the target area and the exchange [B].

The diagram above shows how this works. The tapping point can be made at an existing junction box, in a building where the cable passes through, at the exchange, at an underground cable joint, or by digging up the cable and cutting into it. For situations where it was difficult to access and operate the WEC at the tapping point unobtrusively, a remote control unit was developed.

The diagram above shows the additional remote control unit (RCU) at the bottom left. It is connected to an extra telephone line that is connected to a remote listening post (LP). The WEC unit can be controlled via this telephone line which also delivers the intercepted audio at the LP.
Block diagram
The block diagram below shows how the WEC unit works. At the left is the transmitter of which the frequency can be adjusted between 20 and 300 kHz, divided over three bands. Furthermore the output power of the transmitter is adjustable in order to minimise the chance of detection.

At the right is the receiver, which consists of a band-pass filter, several amplifier stages, a detector and an audio amplifier. Transmitter and receiver are both coupled to the same line, via the duplexer at the centre. This is a bridge-type circuit with adjustable resistors and capacitors, to achieve an optimum matching to the line. This is necessary, as in practice the line impedance can vary wildly under different circumstances, such as line-length and carrier frequency.
Frequency ranges
  1. 20 - 50 kHz
  2. 50 - 120 kHz
  3. 120 - 300 kHz

Unobtrusive travel suitcase Complete WEC Mark I device
Mains power cable Line interface cable Audio output cable

The WEC Mark I was usually carried around in a common unobtrusive green Skyway travel suitcase that measures approx. 62 x 43 x 19 cm. The WEC unit fitted nicely inside this suitcase and was held in place by four wooden blocks in the corners. The remaining space was used for storing the cables, whilst the manual was stowed in the pocket inside the top lid.   

Mains power cable
At the right side of the device is a two-pin (3-contact) socket for connection of the mains power cable, shown in the image on the right.

The plug at the device side of this cable is typical for the era and was commonly used on domestic appliences such as vacuum cleaners and electric clothes irons. The plug at the other side of the cable has to fit a wall socket. It is shown here with a typical European (Dutch) wall plug. Note that the correct mains voltage (110 or 220) has to be set with the voltage selector first.
Mains cable

Line interface cable
This 3-wire cable was used to connect the WEC to a telephone line. The wires are twisted in order to maintain the balanced nature of telephone lines. The 3-pin plug should be connected to the socket on top of the device.

At the other side of the cable are three crocodile clips for connection to the wires of the telephone line. The black clip is for connection to ground. The two red clips are for the A and B wires.
Telephone line cable (tap)

Audio output cable
Audio output from the receiver is available on two 6.3 mm jack busses at the right front of the device. The adapter cable shown in the image on the right is provided for connection to an amplifier or a recorder.   
Audio output cable

Suitcase WEC Mk I in green suitcase Audio output cable Telephone line cable (tap) Telephone line plug Telephone cable crocodile clips

Suitable telephone sets
Early tests with prototype WEC equipment in the early 1960s had shown that the performance of the system depends largely on the targeted telephone set. At the time, all analogue telephones were built around a transformer, several capacitors and a handset with a carbon microphone.
The actual circuit itself varied between brands and models. In some phones, at least one of the terminals of the carbon element was connected directly to one of the line wires, whilst in others they were not. Furthermore, the microphone signal was often cut-off by the dial interruptor.

WEC was first tested with two phones that were commonly used in the Netherlands: Heemaf and Ericsson. The Ericsson one gave the best results, as it could be used without any modifications to the set. Its performance could be improved by adding a capacitor, but this was not mandatory.
Ericsson telephone set model 1951

This was due to the fact that one of the terminals of the carbon microphone was directly coupled to one of the outside lines. With the Heemaf set this was not the case. For the best results one, or preferably two, capacitors had to be installed covertly inside the telephone to get the best result.
When miniature ceramic capacitors were used, this could be done inconspicuously as part of a staged repair job, for example. The image on the right shows the Heemaf model 1955 tele­phone set that was used at the NRP for the initial tests.

The reason for using this model for the first tests, was probably because it was used on the local PABX at the NRP. In this photograph that was taken around 1961, we see the two NRP directors Gerhard Prins and At Admiraal at work in one of their offices. In front of them, on the desk at the right, is a Heemaf telephone set.
Heemaf Model 1955

The PABX was connected to a remote NRP-site via a 6 km long isolated telephone line, on which the WEC was tested. Although there was a wide variety in specifications between telephone sets and exchanges in the various countries in the world, the operating principle was always identical. In most countries, only a couple of telephone sets were approved for connection to the public exchange, and these were nearly always supplied by the state-owned telecom monopolist (PTT). Likewise, the PABX was also supplied by the PTT, making it easier to covertly install components.

 Photograph of Heemaf telephone at the NRP

Rocking Chair   RC
In 1961, at the request of the CIA, the NRP developed a variant of the WEC system, under the codename Rocking Chair (RC). The principle of the RC was to connect a Passive Element (PE), similar to the one used with Easy Chair systems, directly to the telephone line in the target area.

At a tapping point outside the target area, a WEC unit was used to active the PE and to receive the intercepted audio.

 More information
WEC Mk I in green suitcase

The WEC Mk I is housed in grey hamerite metal enclosure that measures approx. 52 x 38 x 16 cm. It consists of a meetal frame in which three modules or inserts are mounted from the front. The interior can be accessed by removing the screws at the edges of the top panel, after which the top panel can be removed. The three modules are interconnected via internal cables.
Each of the three modules can be extracted after removing the screws around the edges of their front panel and disconnecting the internal wiring. Below, each module will be discussed further. The full circuit diagram of each section, complete with a technical description, can be downloaded at the bottom of this page.

 Technical manual
WEC with top panel removed

WEC with top panel removed WEC with top panel removed

The leftmost unit is the transmitter. It consists of a Hartley type oscillator that operates directly on the desired carrier frequency (20-300 kHz). It is followed by a power amplifier (PA) with negative feedback, in order to minimise distortion and noise. The transmitter also has an Automatic Voltage Control circuit (AVC), that keeps the output (line) voltage at a constant level.

The image on the right shows the transmitter. The large rectangular grey block at the right is the oscillator's tuning capacitor.

Transmitter Transmitter - left side 8-pin socket on the transmitter chassis Valves and BNC sockets Filters Transmitter - bottom view Transmitter - band selector Transmitter detail

The line interface is the smallest of the three modules, and provides a proper and balanced connection to the telephone line, to ensure the best possible energy transfer.

The interface also acts as a duplexer, which allows transmitter and receiver to share the same (intercepted) line, using the same carrier frequency. This means that the duplexer has to cancel-out most of the (strong) transmission signal at the receiver's input, in order to avoid receiver overloading. This is also known as spillover cancellation.
Line interface

Line interface Duplexer - rear view Duplexer - rear view Duplexer - line matching Bottom view of duplexer Resistive matching array Capacitive matching array Multi-turn potentiometer

The rightmost module is the receiver, which consists of two pre-amplifiers followed by a detector, an audio band-pass filter and several amplifier stages with Automatic Gain Control (AGC). The meter is used to check the carrier amplitude or the duplexer balance.

The receiver module also carries the mains Power Supply Unit (PSU). At the lower edge of the right side, the mains socket and the mains fuse are located. When the module is installed in the case, these connections protrude the holes in the right side of the case.
Receiver and PSU

Receiver and PSU Receiver - left side Left side of receiver module 8-pin socket on the receiver module Receiver detail (valves) Receiver - bottom view Receiver detail Power Supply Unit (PSU)

Technical specifications
  • Frequency
    20 - 300 kHz (3 bands)
  • Power
    110 or 220V AC (switch-selectable)
  • Size
    52 x 387 x 16 cm
  • Weight
CIA   Central Intelligence Agency
United Status intelligence agency.  More

EC   Easy Chair
CIA codename for the super-secret project to develop covert listening devices based on the principle of the Russian resonant cavity microphone, also known as The Thing. The name EASY CHAIR is also written as EASYCHAIR.  More

LP   Listening Post
Common expression for the monitoring station. In the case of a PE, this is also the position of the activation transmitter.

NRP   Nederlands Radar Proefstation
Dutch Radar Laboratory, at the time located in Noordwijk (Netherlands).  More

PE   Passive Element
NRP name for derivatives of the resonant cavity microphone. In this context, passive means that it does not have its own local power source.

RC   Rocking Chair
CIA code name for a project to develop a passive covert listening device that is powered by RF energy transmitted to it via a regular analogue telephone line, and delivering its intercepted audio via the same line. Requires WEC equipment at the tapping point.  More

WEC   Wired Easy Chair
Easy Chair bugging equipment that was designed to operate via wires. In this case, an analogue telephone line is used to carry energy and intelligence.

  1. Report on WEC System
    CM302572/A, 30 April 1960.

  2. Technical Manual of WEC MK I Equipment
    CM302572/B, July 1960 (est.).

  3. Report on Auxiliary Equipment for WEC
    CM302572/C, 20 May 1963.

  4. Report on RC-System (R.C. Final Research Report)
    CM302599. NRP/CIA 31 October 1961.

  1. NRP/CIA, Collection of documents related to WEC equipment
    Crypto Museum Archive CM 302572 (see above).

  2. Anonymous, WEC Mk I Wired Easy Chair device
    September 2016.

Further information

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Crypto Museum. Created: Wednesday 04 January 2017. Last changed: Saturday, 01 April 2017 - 16:35 CET.
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