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Cable Checker

The TCM-03 is an advanced unit for testing cables for the presence of microphones and/or carrier-type eavesdropping transmitters (bugs), developed and produced by Audiotel in Corby (UK) around 1992. It is suitable for testing mains power cables as well as other types of cables.

The device is powered by internal batteries that are recharged automatically when the unit is powered from the mains. Protection of the mains power lines is done through the device's own mains socket. As most mains lines consist of three wires (Live, Neutral and Earth), a selector at the rear is used to select the required pair.

The TCM-03 can also be used to check other types of cables and wires, such as telephone lines, intercom systems, PA systems, alarm cables, etc., for eavesdropping devices. For such cables, a special break-out cable is used.
TCM-03 Cable Checker (initial version)

The TCM-03 allows mains power lines and other cable types to be monitored permanently. Some of its functions can also be found in other Audiotel equipment, such as the Scanlock 2000 and the Scanlock ECM. Please note that the version of the TCM-03 shown here is quite different from the later TCM-03 Mark II which is much better built and has significantly more features. Amoung other things, it offers AM and FM demodulation and was available from Audiotel until 2013.

TCM-03 Cable Checker (initial version) TCM-03 front panel TCM-03 rear panel TCM-03 placed under a telephone set TCM-03 interior At the left are 6 holders for 1.5V batteries The PCB with all electronic components Close-up of the mains cabling
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TCM-03 Cable Checker (initial version)
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TCM-03 front panel
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TCM-03 rear panel
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TCM-03 placed under a telephone set
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TCM-03 interior
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At the left are 6 holders for 1.5V batteries
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The PCB with all electronic components
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Close-up of the mains cabling

Controls and connections
The TCM-03 was housed in a non-shielded plastic enclosure with a sloped front panel, that should be placed horizontally an, say, a table. Most of the controls and adjustments are at the front panel, with the exception of the main wire-pair selector and the audio attenuator.

TCM-03 front panel

Also at the front panel is a 6mm jack socket for a pair of headphones. The SELECT switch (2nd from the right) is used to select the required mode of operation. The black rectangluar window at the left, is a digital volt meter that is used to monitor the voltage on an analogue telephone line.

TCM-03 rear panel

All connections to the mains and to any other cables under test are at the rear of the unit. The mains should be connected to the 3-pin Euro socket, wilst the wire-pair to be monitored for carrier-type bugs is selected with the rotary switch marked CARRIER SOUCE to its right. Any other cables should be connected to the 5-pin 270° DIN socket at the left. A break-out cable with crocodile type clips was supplied for this. A recorder can be attached to the sockets at the centre.

Counter measures
The TCM-03 can be used for the following tasks:

  • Check for audio signals
    Setting the SELECT knob at the front panel to 'AF' allows the cable under test to be checked for Audio Frequent (i.e. LF) microphones.

  • Check for audio devices that need power
    Some eavesdropping devices 'steal' a little bit of energy, needed for their operation, from the cable they are connected to. If the regular devices on this cable are switched off, the bug won't operate. By supplying a low voltage to the cable under test, bugs of this type will be activated and, hence, will become detectable.

  • Activate remote controlled devices
    A built-in tone generator can be used to active bugs that are single-tone operated. Once these bugs are activated, they can be detected.

  • Check for carrier bugs
    Rather than using LF signals (audio), many sophisticated bugs modulate their audio signal onto a carrier of, say, 200 kHz. Bugs of this type can normally not be detected without special devices. The TCM-03 can demodulate carrier bugs automatically by setting the SELECT knob on the front panel to CARRIER.

  • Voltage checks
    A built-in digital volt meter can be used to monitor the voltage on a cable, such as a telephone line. Tampering with the line, e.g. when placing a bug, will often show significant changes in voltage.

  • Line jamming
    An automatic tone generator inside the TCM-03 can be used to jam voice-activated telephone taps automatically.

  • Off-hook signalling
    An LED indicator on the TCM-03 front panel will show when the telephone line becomes engaged. This allows the operator to detect unauthorised use of the line, for example by a recording device or a timed device.
TCM-03 Mark II   wanted item
The TCM-03 shown above was clearly a gap-fill solution that was rushed onto the market in the late 1980s in response to an increasing demand for wire-tap detectors. Looking at the interior, reveals that the plastic case is not shielded and that the PCB contains many modifications.

Many of the vital components of the unit are spread around the case and some are just glued together, making it doubtful whether it would withstand any stafety approval test today. Nevertheless, it was a very effective device.

Shortly after the introduction of the TCM-03, Audiotel engineers started the development of an improved version with even more features. In 1992, TCM-03 Mark II was introduced. It was housed in a different case and was much better built. Furthermore, it added the detection of AM and FM radio bugs to the list of features.

The image above shows the improved TCM-03 Mark II as it was pictured on the Audiotel sales datasheet [B]. The Mark II had an extremely long lifespan. Although it was introduced in 1992, it was only discontinued in 2013; a service life of more than 20 years! As analogue telephone lines are less common today, and more and more telephone services are running over digital lines (e.g. ISDN, IP, VoIP, etc.), the device has now become obsolete. The functions of the device have been superceeded by the Scanlock M3, Audiotel's latest bug tracing receiver and cable checker.

  1. TCM-03 Cable Checker (Mark I), Sales leaflet
    Audiotel International Ltd., 1989.

  2. TCM-03 Phone Tap Detector (Mark II), Sales leaflet
    BR 401231, Issue 1.0. Audiotel International Ltd., Date unknown.

  3. TCM-03 Cable Checker (Mark II), Operators Manual
    203-003. Audiotel International Ltd., February 1992.

  4. TCM-03 Cable Checker (Mark II), Operators Manual
    2-197-149. Audiotel International Ltd., TCM-03 Mark II. 1998.
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Crypto Museum. Created: Sunday 19 May 2019. Last changed: Sunday, 17 February 2019 - 15:30 CET.
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