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).
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 .
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.
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
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.
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.
The Scanlock Mark 3 is suitable for reception of the following types
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).
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
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
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.
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 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
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
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.
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.
Any links shown in red are currently unavailable.
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© Crypto Museum. Created: Tuesday 10 September 2013. Last changed: Wednesday, 12 April 2017 - 09:45 CET.