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ST-2A
Mains-powered RF bug · 1957

ST-2A was a mains-powered hybrid 1 covert listening device (bug), also known as a Surveillance Transmitter (ST), developed around 1957 by the Technical Services Division (TSD, later OTS) of the US Central Intelligence Agency (CIA). It was an improved version of the ST-2 transmitter [A]. The device operates in the VHF-L band (56 to 84 MHz) and uses Frequency Modulation (FM).

The transmitter is housed in a black metal case that measures 150 x 83 x 33 mm and weights 386 grams. It is powered directly from the AC mains, for which a fixed cable is present. It can be wired internally for 115V or 230V AC, so that it can be used virtually anywhere in the world.

Sound in the room is picked up by a Shure MC-11 dynamic microphone that was supplied with the set, and transmitted in the VHF-L band via a short piece of wire that acts as the antenna. The device was usually hidden behind a ceiling or under a floor, where (switched) AC was available.
  
ST-2A surveillance transmitter (blue)

The device has an output power of approx. 30 mW, 2 which gives it an operational range of 100 to 200 metres, depending on the position of the antenna and the receiver. This means that the listening post (LP) had to be in the vicinity of the target area. A suitable receiver was the SRR-4.

ST-2A was the successor to the ST-2 of 1956. It is not exactly clear when the ST-2A was made, but given the state-of-the-art and the date codes on the valves (tubes), it is likely that this was in 1957, at a time when valves were gradually being replaced by transistors. In this device, the microphone amplifier is built with transistors, whilst the oscillator and the RF amplifier are still built with valves. The ST-2A was succeeded in the early 1960s, by the solid state RT-3 (SRT-3). 3

  1. In this context, hybrid means that the device contains transistors as well as valves (tubes).
  2. Initially the power output was specified at 50 mW, but this caused the valves to die prematurely [3].
  3. At this point the CIA switched from 2-letter prefixes to 3-letter prefixes, so the nomenclature is not clear.

ST-2A surveillance transmitter (blue)
ST-2A surveillance transmitter
Antenna socket, and colour-coding (triangle)
Microphone sockets and tuning section
Colour code indicating frequency range
Shure MC-11 microphone
Complete set
Blue- and white-coded ST-2A surveillance transmitters with accessories
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ST-2A surveillance transmitter (blue)
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ST-2A surveillance transmitter
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Antenna socket, and colour-coding (triangle)
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Microphone sockets and tuning section
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Colour code indicating frequency range
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Shure MC-11 microphone
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Complete set
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Blue- and white-coded ST-2A surveillance transmitters with accessories

Features
The diagram below gives an overview of the connections of the ST-2A, and the accessories that came with it. At one of the short sides is a fixed cable for connection to the local AC mains, which can be 115V or 230V (internally configurable). At the opposite side is a socket for connection of the antenna, which was usually a piece of insulated wire of 101 cm (depending on the selected frequency). A coloured triangle in the corner of the same side, specifies the frequency range.

Click to see more

At the front is an SMA-socket for connection of the microphone, which was usually a SHURE MC-11, connected via a long cable. A small removable lid at the front right – held in place by a single screw – gives access to two tuning points: one for the oscillator and one of the doubler/amplfier stage. Both tuning coils can be adjusted with a special tuning tool that was supplied with the kit.

Frequencies
The ST-2A units are colour-coded with a triangle at the side of the antenna socket. The center frequencies, as listed in the manual, are given below. Each unit has a tuning range of ± 4 MHz. 1 The actual frequencies of the blue- and white-coded units in our collection are shown in red.

  • Green
    60 MHz
    ± 4 MHz
  • Blue
    70 MHz
    ± 4 MHz
    ← 74.9 MHz
  • White
    80 MHz
    ± 4 MHz
    ← 78.4 MHz
  • Green
    ?
  1. The manual erroneously states that the tuning range is ±4 kHz, which does not make sense for a free-running wideband FM transmitter. It should read: ±4 MHz. In reality the tuning range is ±4-5 MHz.

Parts
Surveillance Transmitter ST-2A
Wire antenna
SHURE MC-11 dynamic microphone
Mic
Tuning tool
Instruction manual
VHF-L band WBFM communications receiver
Transmitter
The actual transmitter is housed in a black metal enclosure and has a fixed power lead. There was usually no plug at the end of the power cable, as the device was commonly connected directly to the wiring behind a ceiling or under a floor. It is shown here with regular banana plugs.

The transmitter was available in three frequency ranges, each identified by a coloured triangle on the side of the antenna socket. The one shown here is marked with a blue triangle (70 MHz).
  
ST-2A surveiullance transmitter with power lead

Antenna
A piece of wire of approx. 101 cm, acts as a ¼λ antenna. According to the manual, Amphenol No. 14-500 cable should be used for this, but any other type of wire will work just as well.

The wire antenna should be connected to the Amphenol Sub-Minax socket at the right side of the transmitter. The RF output power of the transmitter is approx. 30 mW into a 50 Ω load.
  
Wire antenna

Microphone
Each transmitter was supplied with a Shure MC-11 dynamic microphone, that has an impedance of 1000 Ω and a frequency response of 400 to 4000 Hz. It is shown here with a short lead, but was usually supplied with a much longer one, to allow the microphone being placed at a convient location, away from the surveillance transmitter.

It should be connected to the SMA socket at the front of the transmitter. When placed in the vicinity of the transmitter, it picks up hum from the mains transformer's stray magnetic field.

 More about Shure microphones

  
Shure MC-11 microphone with short lead

Tuning tool
The ST-2A transmitter was available in three frequency bands (roughly at 60, 70 and 80 MHz), that could be fine-tuned with the cores of the coils in the tuned circuits of the oscillator and the doubler/amplifier. These tuning coils are accessible through two holes in the transmitter's case, that are covered by a small metal panel.

A special tool, such as the one shown in the image on the right, was supplied for adjusting the cores. After removing the panel, The tool can be inserted through the holes in the case and the frequency can be adjusted over an 8 MHz range.

  
Tuning tool

Instruction manual
Each ST-2A transmitter came with a numbered copy of the operating and instruction manual, shown in the image on the right.

The manual contains instructions for setting up the device, adjusting the frequency and carrying out regular maintenance. For repair, detailed circuit descriptions and a full circuit diagram are present as well.

 Download the manual

  
Operating and maintenance manual

Receiver
The signal from the ST-2A can be picked up by any Wide-Band Frequency Modulation (WBFM) communications receiver that is suitable for reception of the VHF-L band (56 - 84 MHz).

A good example of such a receiver, which might actually have been used by the CIA in this case, is the SRR-4 surveillance receiver, shown in the image on the right.

 More information

  
Telescopic antenna mounted on the SRR-4

Complete set
ST-2A surveiullance transmitter with power lead
ST-2A surveillance transmitter
Wire antenna
Shure MC-11 microphone with short lead
Shure MC-11 microphone
Shure MC-11 microphone
Tuning tool
Microphone sockets and tuning section
Access to the tuning points uncovered
Operating and maintenance manual
Operating and maintenance manual for Transmitter ST-2A
Circuit diagram from the ST-2A manual
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Complete set
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ST-2A surveiullance transmitter with power lead
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ST-2A surveillance transmitter
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Wire antenna
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Shure MC-11 microphone with short lead
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Shure MC-11 microphone
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Shure MC-11 microphone
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Tuning tool
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Microphone sockets and tuning section
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Access to the tuning points uncovered
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Operating and maintenance manual
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Operating and maintenance manual for Transmitter ST-2A
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Circuit diagram from the ST-2A manual




Click to see more

Interior
Accessing the interior of the ST-2A is straightforward. The device is housed in a black welded aluminium enclosure. It has a lid at the top, which is held in place by two screws at the short sides of the transmitter. After removing these two screws, the top panel can be lifted off.

Inside the enclosure are two pertinax side panels – along the long sides – with slots for the four PCBs, each of which can be removed vertically.

At the left is the mains transformer, which can be hard-wired for either 115V or 230V AC. Instructions on how to change the wiring are given in the manual [A]. The transformer has secondary windings for the filaments of the valves (2.5V), the transistor circuits (5V) and the anodes of the valves (65V). The filaments of the two valves (1.25V each) are connected in series.
  
Interior seen from the front

All internal wiring, including the wires to and from the mains transformer, have teflon insulation, which makes them heat resistent. From left to right, the four printed circuit boards (PCBs) are assigned as follows: lower-voltage board (5V), high-voltage board (65V), AF board and RF board.

The AF board contains a three-stage microphone amplifier, built with Philco 2N207B transistors. It amplifies the sound from the MC-11 dynamic 1000Ω microphone to a level that can be applied directly to the tuned circuit of the RF oscillator.

The rightmost PCB holds the RF circuits. The oscillator – built around an 1AD4 valve – is at the left side of the PCB, whilst the other side holds the frequency doubler/RF amplifier, also built around an 1AD4 valve. For the first production batch, valves from Raytheon were used, whilst TUNG-SOL valves were used in later batches.
  
1AD4 valve removed from the PA stage

The 1AD4 valves have 5 contact pins that are fitted in suitable contact sockets on the PCB, with a bended metal spring-clip to hold the valve in place. The image above shows a TUNG-SOL 1AD4 valve that has been removed from the frequency doubler/amplifier. From eye-witness accounts, it is known that early production batches of the 1AD4 valve had problems with the filaments, that would lose emission after 50 to 100 hours of operation, but still measured as conducting [2].

ST-2A interior seen from the top
Interior seen from the front
Interior seen from the rear
Transformer and PSU boards
AF amplifier and RF boards
All PCBs partially removed from the pertinax frame
3 x 20uF electrolytic capacitor
1AD4 valve from the PA stage
Low-voltage board
Low-voltage board, seen from the side
AF board
Oscillator with 1AD4 valve
Frequency double/RF amplifier with 1AD4 valve
1AD4 valve removed from the PA stage
Serial number 227
Serial number 299
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ST-2A interior seen from the top
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Interior seen from the front
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Interior seen from the rear
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Transformer and PSU boards
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AF amplifier and RF boards
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All PCBs partially removed from the pertinax frame
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3 x 20uF electrolytic capacitor
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1AD4 valve from the PA stage
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Low-voltage board
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Low-voltage board, seen from the side
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AF board
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Oscillator with 1AD4 valve
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Frequency double/RF amplifier with 1AD4 valve
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1AD4 valve removed from the PA stage
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Serial number 227
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Serial number 299

Restoration
When we received the two ST-2A transmitters featured on this page, we carefully checked them before connecting them to the mains power, as they had probably not been operated for several years. First of all, the primary side of the transformer was rewired for the European 230V mains.

This was done by desoldering the primary trans­former wires of the two units from the ceramic solder points at the side panel. As one of the mains cables had become brittle over the years (and was missing from the other unit), it was decided to replace them at the same time.

The image on the right shows the transformer wiring of the unit with the highest serial number (299), in which twisted red teflon wires are used. It should be noted that the earlier unit (with serial number 227) had grey untwisted wiring. This indicates a small manfufacturing change.
  
Transformer wired for 230V AV

As we wanted to connect the transmitters to a local wall socket for testing, two banana plugs were fitted at the end of each of the power leads. Before switching the units on, the circuits were carefully removed from the pertinax frame and inspected for broken wires and shorted contacts.

No short circuits were found, but in one unit the black teflon ground wire from the RF board was broken at the case edge. As the other end of the wire was about to break as well, it was decided to replace it with a 'new' era-correct teflon wire.

Once both units had been inspected thoroughly, it was decided to power the first one up. This was done gradually by means of a VARIAC, to allow the electrolytic capacitors to reform them­selves. Once the mains voltage was raised to the desired 230V, reception of the estimated TX frequency was checked on a WBFM VHF receiver.
  
Broken ground wire from the RF board

After some searching, the signal was found. Initially there was a strong 50 Hz hum on the signal, but this soon turned out to be the stray magenetic field of the mains transformer that was picked up by the microphone. It was solved by moving the microphone away from the transmitter. The transmitter produces a clear signal and has an adjustable deviation, allowing any sound in a room to be picked up easily. The second unit worked as well, after gradually raising the mains voltage.

It was discovered that the transmitter suffers slighly from the so-called hand-effect, which means that the transmission frequency changes slightly when the antenna is placed near an object. This is to be expected from a transmitter that is based on a free running oscillator. The RF amplifer somewhat reduces this effect, but can not eliminate it completely. Furthermore, the transmission frequency is ~ 300 kHz lower whilst the top lid is removed. This should be considered normal.

The following restoration work was carried out:

Rewiring the transformer
Transformer wired for 230V AV
Broken ground wire from the RF board
Transformer and PSU boards
Serial number 227
Serial number 299
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Rewiring the transformer
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Transformer wired for 230V AV
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Broken ground wire from the RF board
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Transformer and PSU boards
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Serial number 227
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Serial number 299

Connections
1AD4 valve
1AD4 is a subminiature filament type, fully-shielded sharp-cutoff pentode valve (tube), designed for RF and AF applications in portable equipment. It has five wire leads that can be soldered onto a PCB directly, but can also be cut and inserted into a suitable socket, as is the case in the ST-2A.

 1AD4 datasheet

Specifications
  • Frequency
    VHF-L band 56 to 84 MHz (3 channels, see above)
  • Modulation
    FM
  • Deviation
    5-50 kHz/100µV signal input (adjustable)
  • Output
    ≥ 30 mW into 50Ω
  • Power
    115V or 230V AC, 50-60 Hz
  • Consumption
    1.3W at 190V AC, 2W at 230V AC, 2.5W at 250V AC
  • Microphone
    Shure MC-11, or equivalent 1000Ω reluctance microphone
  • Response
    400 to 3500 Hz
  • Antenna
    101 cm
  • Dimensions
    150 × 83 × 33 mm
  • Weight
    386 g
Checklist
Separately required
  • WBFM receiver 56-84 MHz, e.g. SRR-4
Serial numbers
The serial number can found on a label inside the enclosure, close to the transformer's mains connection. It is currently unknown how many ST-2A transmitters were made, but it is clear that the two units in our collection (227 and 299) are from different production batches or from different manufacturing, as the wiring from the transformer and the S/N labels are different. It is certain however, that at least 300 units were made, as our manual is 'COPY 35 of 300' [A].

Please report additional serial numbers and colour-code. The following numbers are known:

  • 227
    White
    Crypto Museum
  • 299
    Blue
    Crypto Museum
Documentation
  1. Operating and Maintenance Manual for Transmitter ST-2A
    CIA, Date unknown, but probably 1957. Copy 35 of 300.

  2. ST-2A Circuit Diagram — extracted from [A]
    CIA, Date unknown, but probably 1957.
References
  1. Pete McCollum, CIA bug ST-2A - THANKS !
    Received September 2020.

  2. Anonymous former CIA/TSD member, personal communication
    April 2000. Obtained via [1].

  3. CIA, Memorandum for the record, Report of Meeting on 22, 23 and 24 October 1957
    About short-living tubes in ST-2A. 1 November 1957
Further information
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Crypto Museum. Created: Monday 21 September 2020. Last changed: Thursday, 01 October 2020 - 12:01 CET.
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