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TM 104
Wireless contact microphone

TM-104 was a covert listening device (bug) with built-in contact microphone, supplied in the early 1970s by TMS Electronic in Köln (Germany). The miniature transmitter worked in the FM radio broadcast band (90-110 MHz) and was intended for listening through walls and windows.

The actual transmitter is housed in a black plastic enclosure that measures 58 x 38 x 18 mm and weights 50 grams. It is powered by three internal Mallory button cells that last – depending on the version – for up to 2600 hours. The range of the device, under optimal conditions, is between 400 and 800 metres.

The device is attached to a metal bracket with two suction cups 1 that allow it to be fitted to a window, using the glass as a membrane for the internal contact microphone. According to the brochure, the microphone is very sensitive [A].
TM-104 bug with 2 suction cups for fixating to a window

The transmitter is enabled by inserting a small shorting plug into the 2.5 mm jack socket at the top. This socket can also be used for connection of a switching receiver, allowing the bug to be activated remotely. At the other side of the device is the antenna socket, which accepts a regular miniature banana plug – such as the ones used with model railways – with a 75 cm piece of wire.

The enclosure consists of two black plastic case shells, that are simply pressed together. The two halfs can easily be separated, so that the battery compartment can be accessed. The image on the right shows the interior of the device, with the rectangular battery compartment at the centre.

The only component that is visible, is the white ceramic trimmer that is used for adjusting the transmission frequency. The other electronic parts are all cast in white artificial rubber. This was done to improve the stability, but also to protect the circuit design from being copied.
Battery compartment and frequency adjustment

Like all TMS transmitters, the TM-104 operates by default in the FM radio broadcast band, and can be adjusted to any frequency between 90 and 110 MHz, allowing a regular domestic FM radio to be used for reception. This has the disadvantage that the transmitter can also be picked up (accidentally) by anyone who searches a station. For this reason, the transmitter could also be ordered for frequencies in the 2 meter (150 MHz) or 4 meter (75 MHz) band, but this required a special receiver, which was also available from TMS. Note that neither of these transmitters could be operated legally within Germany, so they were listed as 'Nur für export' (for export only) [A].

  1. These are regular domestic suction cups that are normally fitted to the kitchen tiles, with a plastic hook for hanging the towel. The hinged black hooks also function as a handle when fitting the cups.

TM-104 bug with 2 suction cups for fixating to a window Top view Inserting the shorting plug Connecting the antenna Microphone side Contact microphone Interior, cast in white rubber Battery compartment and frequency adjustment
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TM-104 bug with 2 suction cups for fixating to a window
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Top view
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Inserting the shorting plug
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Connecting the antenna
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Microphone side
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Contact microphone
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Interior, cast in white rubber
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Battery compartment and frequency adjustment

According to the brochure [A], the TM-104 was available in two versions – S and L – both of which were cast in white artificial rubber to make it shock and tropical proof. In addition, both version could be ordered for other frequency ranges as well.

  • TM-104 S
    This is the standard version of the device, that had a specified range of approx. 800 metres. It was powered by three Mallory MN-625-G button cells, which lasted for 40 hours, or by three RM-625-H for 120 hours of operation.

  • TM-104 L
    This is the long-duty variant with has a range of 400 metres. When powered by a Mallory MN-1500 battery, it could transmit for 1500 hours, and with an RM-502-H battery even as long as 2600 hours.
Štěnice x
It is known that in the early 1970s, the TM-104 attracted the attention of the Ministerium für Staatssicherheit (MfS) – also known as the Stasi – of the former DDR (East Germany). The device featured above was purchased by the Stasi directly from TMS, sometime between 1972 and 1975.

Apparently, the TM-104 was also used during the Cold War by the Statni Bezpecnost (StB) — the repressive secret civil intelligence service of Czechoslovakia — as it is on public display at the Police Museum in Prague as Štěnice x (bug x). 1

According to the note (in German) in front of the device, it was remote-controlled by means of a switching receiver named SOVA. 2 This was done by connecting the switching receiver to the white mini-jack at the top. The photograph on the right was taken at an exhibition in Furth im Wald by Detlev Vreisleben on 22 January 2010 [2].

Whether the device was actually used by the East German Stasi, remains to be seen. Unlike other TMS bugs, like the TM-105, TM-106 and TM-109, it does not appear in any Stasi reports that have been found to date. It is therefore likely that the Stasi obtained it for evaluation purposes.

  1. According to the note in front of the device, it was known in Czechoslovakia as Štěnice IX, but this is likely to be incorrect. According to the database of the Czech Police Museum, Štěnice IX was a combination of devices TRM 500 and 501, but the device shown in the exhibition is almost certainly a TRM 104. As the correct Czech name for the device is currently unknown, we will call it Štěnice 'x' for now.
  2. This is probably also incorrect.

The interior of the TM-104 is fully cast in white artificial rubber, making it very difficult to inspect its design. It is clear however, that it is basically a simple free-running FM transmitter, as it exhibits the so-called hand-effect: the frequency changes when the hand is moved close to it.

This is confirmed by the x-ray 1 above, in which the circuit of a simple one-transistor transmitter can easily be recognised in the right half. The left part only contains the microphone and the jack socket for the activation plug. From this image we also learn that the upper battery contact is the plus (+) terminal, and that the activation socket simply disconnects the (-) terminal from the circuit. This means that it can be controlled by an open-collector output from, say, a switching receiver. It allows the transmitter to be controlled remotely. Further x-ray images below.

  1. Images made with Creative Electron TruView Prime.  More

Full x-ray of the TM-104 bug Close-up of the circuit Microphone and activation socket Battery compartment Circuit seen from the side
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Full x-ray of the TM-104 bug
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Close-up of the circuit
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Microphone and activation socket
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Battery compartment
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Circuit seen from the side

  1. TMS catalogue
    Date unknown, but probably early 1972/73. 1
  1. Document kindly supplied by Detlev Vreisleben [1].

  1. Detlev Vreisleben, TM-104 transmitter
    Retrieved May 2019.

  2. Detlev Vreisleben, Photograph of Štěnice IX
    Prague, 22 January 2010.
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Crypto Museum. Created: Wednesday 12 June 2019. Last changed: Saturday, 15 June 2019 - 20:00 CET.
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