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Scanlock Mark 3
Bug finder

The Scanlock Mark 3, or MK 3, was a bug finding receiver, invented by Lee Tracey and sold by Technical Security Ltd. (later: Audiotel) in London in 1976. The device is a so-called staircase receiver or, harmonic receiver, and scans the radio spectrum up to 1 GHz in around 1 second.
The Scanlock Mark 3 resembles a large transistor radio of the 1970s and measures just 33.5 x 22 x 8 cm. All controls and the telescopic antenna are located at the front panel. The receiver is powered by an internal mains power supply unit (PSU), or by a built-in rechargeable 16.8V NiCd battery, which is located at the bottom.

The receiver is suitable for the detection of radio bugs that transmit in Amplitude Modulation (AM), Frequency Modulation (FM) or that are modulated onto a subcarrier, also known as Phantom bugs (i.e. apparently silent bugs).
Scanlock MK3

The Scanlock Mark 3 is based on the so-called staircase receiver, developed by Lee Tracey in 1962. Tracey, a former RA sergeant and MI6 agent, started selling the first Scanlock receivers in 1975 through the company Technical Security Ltd., at the time a front operation of MI6 [1].

The receiver scans the entire frequency spectrum between 10 MHz and 1 GHz in just a few seconds and locks automatically onto the strongest signal. In areas with strong broadcast transmitters, such as commercial radio stations, it might be necessary to move around the room until a point is found where the bug is stronger than the broadcast station. When a bug is found, the radio stops scanning and shows the signal strength on a meter.

The Scanlock MK 3 was succeeded by the Scanlock MK4 and finally in 1978 by the Scanlock Mark VB. By that time, Technical Security Ltd. no longer existed and sales had been taken over by Lee Tracey's new company Audiotel International Ltd. In the following years, newer versions of the Scanlock appeared. Audiotel is still in business today and Scanlock is still in their product range.
Scanlock MK3 Scanlock MK3 with carrying strap Scanlock MK3 Scanlock MK3 with external antenna Battery compartment and battery Battery compartment and battery BNC adapter mounted over the telescopic antenna Field strength indicator

All controls of the Scanlock MK3 are neatly arranged on the control panel, at the top of the receiver. In normal use, the receiver should be placed upright so that th telescopic antenna can be fully extended. At the left is the POWER selector, that is used to select between the internal batteries, or the built-in mains power supply. It can also be used to recharge the batteries.

To the right of the power selector is the MODE switch. It allows the radio to be used for the detection of radio bugs with Amplitude Modulation (AM), Frequency Modulation (FM) and Subcarrier bugs (Phantom). The rotary switch is also used for selecting Soundwave and Locate mode. In Phantom-mode, the subcarrier frequency can be adjusted with a separate knob.

At the center of the receiver is a large field strength meter, that has three sockets to its left for connecting a remote, a recorder and a pair of headphones. At the bottom right is a BNC socket of which we currently don't know the purpose. It is probably a 2MHz IF output that can be used for connecting an external spectrum display.
Modes of operation
The Scanlock Mark 3 can be used in a variety of modes and with various types of modulation, depending on the type of bug, the method of searching and the time available to do the sweep. The following modes are available:
  • Automatic mode (AM/FM)
    In this mode, the Scanlock searches fully automatically for bugs in the room and locks itself onto the strongest signal that it finds, with a lock sensitivity of 1mV up to 1 GHz. In earas with strong broadcast transmitters, it may be necessary to move the receiver around the room whilst searching.

  • Soundwave mode
    In this mode, the Scanlock emits a continuous (audible) tone, that changes to an intermittent tone when the receiver detects itself (i.e. if it is heared through the bug). This mode should be used as a last resort, as it is likely to alert the eavesdropping party of the fact that a bug-sweep is taking place.

  • Locate mode
    When the LOCATE button is depressed, the Scanlock provides an audible tone with a pitch that is proportional to the strength of the acquired signal. Starting off with a low frequency ticking sound, the pitch gets higher when approaching the bug. In this mode, the use of a pair of headphones is advised, as otherwise the eavesdropping party might become aware of the fact that a bug-sweep is taking place.

Modulation types
The Scanlock Mark 3 is suitable for reception of the following types of modulation:
  • AM
    This is for bugs that use Amplitude Modulation (AM). Not many bugs are of this type. It is generally used with low-frequency transmitters (below 80 MHz).

  • FM
    This is for the most common type of bugs that use Frequency Modulation (FM). These bugs generally operate at frequencies from 80 MHz onwards. Most of the cheaper commercial and homemade bugs are of this type.

  • Phantom (subcarrier)
    With some of the more sophisticated bugs, the audio is modulated onto a non-audible subcarrier. As a result, the bug appears to be sending a silent carrier, whereas in fact it carries all of the sound it picks up in a room. In this mode, the Scanlock is capable of receiving such bugs, whilst a separate control is used to adjust the subcarrier frequency.

     More information

Searching for bugs
Operation of the Scanlock Mark VB is remarkably simple. All you need to do when first scanning a room for potential bugs (sweeping) is to ensure that all buttons are up (out) when the unit is switched on. Next press the FM button to search for the most common type of FM bugs. The Scanlock will now automatically lock onto the strongest signal in its vicinity, as illustrated below:

With the receiver in location A, the Scanlock will probably lock immediately onto a strong broadcast station that is nearby. Such broadcast stations are generally much stronger than a potential bug in the room. When the Scanlock is moved around the room however, it will continue to scan and lock onto the strongest signal. When the receiver is in location B it will intermittently switch between the broadcast station and the bug, but when it is moved closer to the bug (C) the RF signal from the bug will be stronger than the broadcast station and the bug will be heard.
The Scanlock Mark VB has its own built-in telescopic antenna that is located at the far right of the control panel. When used for automatic bug finding, the telescopic antenna needs to be pulled-up completely. The antenna does not have a knie-joint, so the receiver has to be used upright.
When not in use, the antenna can be pushed all the way in, until only the tip is visible. Inside the receiver, the antenna is completely shielded so that it can not pick up any spurious signals.

It is now possible to use an external antenna, by installing a special cylindrical adapter over the telescopic antenna. One end of the adapter is hollow and has a spring contact inside. At the other end is a common BNC socket. The adapter is installed by placing the hollow end over the tip of the telescopic antenna and screwing the cylinder into the threaded antenna base.
Telescopic antenna partly extended

An external antenna can now be connected to the BNC socket at the top of the adapter. This can be useful for continuously monitoring a room where, say, a meeting is taking place, from an adjacent room without disturbing the attendees. It can also be used to connect a smaller - less sensitive - antenna, such as a magic wand, to search for bugs whilst walking around the room.
Scanlock MK3 with carrying strap Telescopic antenna partly extended Telescopic antenna BNC adapter for external antenna or 'magic wand' BNC adapter mounted over the telescopic antenna Using an external antenna Scanlock MK3 with external antenna Cylinder holding the telescopic antenna

Block diagram
The Scanlock Mark VB is based on Lee Tracey's design of the locking staircase receiver of 1962. As the Mark VB was the first locking receiver of this type that was commercially marketed successfully, it is generally referred to as the original Scanlock, using the following principle:

There is no preselection other than a filter that allows only frequencies above 10 MHz to pass by. The 10 MHz output of the first local oscillator is fed to a so-called comb generator. This is effectively a non-linear junction (i.e. a diode) that causes harmonics of the fundamental frequency to be generated. It produces signals at 10 MHz, 20 MHz, 30 MHz, 40 MHz, etc.

The output of the comb generator is then mixed directly with the antenna signal, resulting in many frequency segments of 10 MHz each, being superimposed on top of each other. The problem of sweeping the entire spectrum between 10 MHz and 4 GHz has now been reduced to sweeping just a single small segement of 10 MHz, by sweeping all superimposed segments simultaneously. By using a sweeping oscillator in the 2nd IF stage, the resulting 10 MHz segments can be swept in less than a second. The receiver locks onto the strongest signal found.

At the output of the 2nd IF stage, a 2 MHz signal is available for further processing. It is fed to the three demodulators (AM, FM and SC), and is also available on a connector at the front of the receiver, to allow the connection of a panoramic display, such as the SM-2 Spectrum Monitor.

In Sound Wave mode (S/W), the speaker produces a continuous 1800 Hz tone. The output of all demodulators (AM, FM, and SC) is checked for this tone by applying it to a sharp 1800 Hz filter. When this particular frequency is detected, the tone is changed into an intermitted one.

Note that frequencies below 10 MHz are not covered by this receiver. Although it is unlikely that bugs operate below 10 MHz, it is possible to use these low frequencies, e.g. when using them as a carrier. The later Scanlock 2000 contained a extra detector to find mains carrier bugs (MC) and the Scanlock ECM even had a separate VLF receiver to cover the 10 kHz to 10 MHz section.
Considering its age - the Scanlock was built in the mid-1970s - it is really well built. The receiver is housed in a strong custom-made extruded aluminium case with plastic side panels. A separate compartment at the bottom houses the long purpose-built 16.8V rechargeable NiCd battery.
The interior of the Scanlock Mark 3 can be accessed by removing the metal panel from the back (or top when the unit is placed horizontally.

When the panel is removed from the case, the interior is immediately exposed. Inside the case is a large single-sided PCB, with all components at the top side. To the left of the large PCB is a smaller one that holds the mains power supply. The transformer can be switched between 110 and 220V mains AC voltages. At the far left is a loudspeaker which is mounted to the side panel. It is switched off when headphones are used.
Interior of the Scanlock MK3

The Scanlock MK3 is built around a so-called staircase receiver, also known as an harmonic receiver, invented by Lee Tracey in the early 1960s. A detailed explanation of this principle and a block diagram can be found on our page about its successor: the Scanlock Mark VB.
Looking inside the Scanlock MK3 Power supply 110/220V AC selector Interior of the Scanlock MK3 Close-up of the front-end Part of the interior MODE selectors Top view of the interior

  1. Crypto Museum, Interview and correspondence with Lee Tracey
    23 May 2013.

Further information

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Crypto Museum. Created: Tuesday 10 September 2013. Last changed: Wednesday, 12 April 2017 - 09:45 CET.
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