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(c) 2000,2017 Peter McCollum

Surveillance and Bugging Devices

While one part of the Agency handled the traditional communications role, including the design, construction, and use of radio equipment; there was a separate group known as the Technical Services Division (TSD), which was responsible for providing a wide range of devices to operatives; including equipment for audio surveillance, clandestine photography, and lock bypass. Later, the equivalent of TSD became the Office of Technical Service (OTS).

Info on one or more non-U.S. surveillance sets (such as the Czech Tesla-made PR-35) can be found in the “Selected Foreign Sets” section.

The SRR-4 Surveillance Receiver

This receiver is for general-purpose surveillance work. Based on tube date codes, it was likely made in 1961 or 1962. It receives FM, AM, and CW signals in a single band from 50 to 200 MC, with calibration points every 5 MC. The design includes 13 subminiature tubes, plus several germanium diodes, and 3 transistors (in the power supply module).

The SRR-4 would be ideal for monitoring "bug" transmitters, such as the ST-2A (described in the next sub-section) and SRT-5 [ref 147]. It was used in an operation in Denmark 1965 to attempt to eavesdrop on a meeting in a hotel [ref 146]. The set is featured in H. Keith Melton's book "CIA Special Weapons & Equipment" on page 52.

The SRR-4 was likely based on the military R-744/PRD receiver, which is very similar. CIA memos begin mentioning the R-744 in November 1958. By April 1959, an R-744 had been evaluated, saying “the performance of this receiver is excellent”. So, it is likely that CIA began designing some modifications to the design in mid-1959. Perhaps the first unit was available in 1960 or 1961.

More pictures and details of the SRR-4 can be found here: . There is also a table detailing the differences between the SRR-4 and the R-744.


The SRR-4 receiver, serial # A59. The whip antenna can be mounted as shown, or perpendicular to the front panel when the receiver is positioned face-up. The antenna extends to a length of 53 inches. Since the set has a BNC connector for the main audio output, an included accessory is an adaptor to allow a 1/4-inch phone jack to be used with standard headphones. The power cord shown is for 220 VAC operation. This particular unit was part of a "stay-behind" program in Norway. Author's collection.


The 10-pin power input connector mates with a U-77 plug, the same as the audio connector on many 1950's-vintage military sets. The DETector AUDIO output is suitable for connection to a recorder (wire or tape). The CAL setting enables a 5 MC harmonic oscillator.


Inside the rear of the case is a "PP-1A/ASR" power supply (the module in the middle), and battery compartments on either side. The batteries (1.5V and 45V) are accessed from panels on each end of the case.


The circuitry includes 9 plug-in modules (the gray objects with colored arrows), plus 4 additional subminiature tubes. The plug-in modules perform functions such as IF amps, BFO, CAL oscillator, and audio. Main chassis components are silver-plated, while rotary switch contacts and mini coax connectors are gold-plated.


An upper view of the chassis. Note the 1N21B cartridge-type diode, under the screwdriver cap at bottom-center.


Inside the PP-1A/ASR power supply. Note the two unusual dome-shaped power transistors used in the DC-DC converter. Directly below the transistors is a toroidal inverter transformer, and another early germanium transistor. Two of the original electrolytic capacitors have been replaced with newer Japanese-made units.


The ST-2A Surveillance Transmitter

The ST-2A is one of the earlier models in a long series of equipment. The purpose of a surveillance transmitter (ST) is to transmit the sounds (conversations) from within a room to a person or recording device monitoring a receiver nearby. For example, an ST may be hidden in a hotel room before the targeted person arrives, and the opposition can set up equipment on an adjacent floor to monitor and record any conversations that take place.


The ST-2A as shown in the manual. The mic is a Shure MC-11.


A view of an ST-2A with the cover removed. One of the 1AD4 tubes is visible on the far right. The two gold connectors are for RF out and audio in. On the bottom right corner is an access panel for tuning. The AC power cord comes in from the left. Author's collection.


This model is a hybrid design, using two 1AD4 tubes and 3 type 2N207B transistors. The three transistors are a basic audio amplifier, one tube is the RF oscillator, and the second tube functions as a doubler. The unit operates from 115 or 230 VAC, 50-60 cps, unswitched. The output frequency is in the range of 56 to 84 mc, FM, and produces at least 30 mW of RF power into a 50 ohm load. It measures 5-5/8" X 3" X 1-1/4", and weighs 19 oz. The suggested microphone is a Shure MC-11 (1K ohm dynamic, 400-4000 Hz), and the antenna is a 40" length of Amphenol No. 14-500 cable (Amphenol lists this as a "Sub-Minax" cable). The mic input connector is a type SMC, and the RF out is an Amphenol "Sub-Minax" connector.

Referencing the picture above, the major components in the unit, from left to right, are: power transformer, low-voltage rectifiers and filters board, high-voltage rectifiers and filters board, audio board (3 transistors plus a deviation adjustment pot), and RF board.

ST-2A schematic

The only controls or adjustments are an oscillator tuning coil, a doubler tuning coil, and an audio FM deviation control pot. There is an access panel that allows adjustment of the tuning while the cover is in place.

The overall tuning range for the ST-2A is 56 to 84 mc, but each individual unit was pre-tuned at the factory to operate on one of three frequencies: 59 mc, 68 mc, or 77 mc. Each unit has a colored marking on the corner of the case to indicate which of the factory tunings has been applied; the colors are green, blue, and white respectively. According to the manual, each unit can be re-tuned by the user over an 8 mc range, which implies that there are some component differences between the green, blue, and white units.

The power supply provides 2.5 VDC for the tube filaments (1.25 V each, in series); 65 VDC for the tube plates; and 5 VDC for the transistors. Rectifiers are germanium 1N91 and 1N93 types.

The apparent predecessor of the ST-2A, the ST-2, was available before April 1956. The ST-2A was introduced in 1957, undergoing “operational evaluation” in July of that year. There were reliability problems with the 1AD4 tubes - they would often lose filament emission after 50-100 hours of operation. The ST-2A was replaced by the 'Surveillance Radio Transmitter' SRT-3, which was all solid-state, and about 1/3 the size. At that time the nomenclature changed from ST-x to SRT-x. The SRT-1 and SRT-2 were prototype models that were never generally used. The SRT-3 was a well-used item for a number of years, and was eventually followed by a long series of SRT-x units, up to about SRT-44.


American Microphone Company model DD4

This mic measures 1-5/8" in diameter and is 7/8" thick. It is a dynamic type, and seems to contain a fairly large magnet. It was used by CIA as well as commercial users – see Popular Science magazine, October 1957.


The type DD4 microphone. Author's collection.


RT-3R Transmitter

Very similar to ST-2A, but all solid-state and runs from DC power.

Surveillance Transmitter from 1958; improved version of RT-3. Battery power, or separate AC supply. Size 3-13/16” x 1-3/4” x ¾”. 360 hours on battery power. FM, 55-63 MC or 64-72 MC or 73-81 MC. Supplied with Shure MC-30 mic; also tested with RCA BK-6B. Transistors 2N500 x2, 2N207B x2. Manual mentions use with receivers ASR-1 and PFR-5; and Nems-Clarke 1501 for testing.

RT-3PS supply is 110/220 VAC 50/60 CPS input. Output: 6.5V 15 mA, with a simple Zener diode regulator.

Note that “RT-3” nomenclature matches the RS-1 transmitter (also “RT-3”). This overlap is likely because they were produced by separate divisions in the Agency: Office of Communications, vs Technical Services Division.

Small Microphones

These tiny mics were made for early behind-the-ear hearing aids. In the early 1960's, they represent a technology that was valuable to OTS/TSD for various types of listening devices. Hearing-aid components are ideal for clandestine audio monitoring, because they are optimized for receiving the human voice.

Three examples of early hearing-aid mics. The “circle-K” logo is for Knowles Electronics, the manufacturer. The type 1751 has a built-in preamp (probably a single transistor), so it has 3 contacts for Power, Output, and Common. The other two are simple dynamic mics, roughly 2.5K impedance. These parts were surplus from the former Vicon hearing aid factory. Author's collection.


HRS-23M Beacon Locating System

This system is apparently intended for tracking or locating a small transmitter such as a “wire” worn by an informant or agent, or a beacon attached to a vehicle. General description from the manual, which is dated January 1978:

The HRS-23M (or HRS-23) is a portable, battery operated, beacon receiver system operating in the 220-240 MHz frequency range. Through proper application, it will allow an operator to locate a compatible beacon transmitter by providing him with Left-Right direction and signal strength information. System operation is flexible since three modes of operations are provided – Airborne, Auto, and Portable. Most of the equipment necessary for each mode of operation is contained in two carrying cases provided.”

The manual includes instructions for mounting the system in a car or small aircraft, using a pair of vertical rod antennas. The receiver demodulates AM, and includes a BFO. The manual does not include schematics, repair info, or images of the inside of the unit. The “HRS” designation is likely an acronym for Homing Receiver Set/System.


Some of the main components of the HRS-23 system. The main unit in the center is a 6-channel HRR-23 receiver, with the HRD-23 homing device above. The 3 small rectangular objects on the upper-right are frequency selection modules. A vertical antenna is visible on the left, the second antenna is hidden under the foam. The black box on the right is the HRK-23 Test Fixture, which generates a simulated signal for setup and calibration. The button for opening the case on the left side is actually the receiver's gain control. Image courtesy of Bill Adams.

In a second case are various accessories that can be used in an auto or aircraft installation, including a pair of vertical antennas (HRN-23) and bases which are mounted on the roof of a car, or on the front hood of an aircraft. Image courtesy of Bill Adams.


A DF indicator is mounted under the handle. The false rivet in the upper-right is a push-button which illuminates the meter, while the rivet in the upper-left is an earphone jack. The meter is covered with a polarizing filter so that it is only visible when viewed directly from above, so that someone standing nearby would not be able to see the meter. Image courtesy of Bill Adams.


Induction Coil (Wire Tapping Device)

This device, marked "F-371 IndCoil", is an audio wire tap, most likely intended for telephone lines. It is an inductive pickup, so does not require any direct connection to the signal wire. This makes it much more difficult to detect, and does not interfere with telephone operation in any way.

When clipped over the red wire on a traditional telephone line, and connected to a suitable preamp, it can efficiently monitor a conversation. Note that it is necessary to tap only one of the two signal wires - if both wires pass through the device, the signal is cancelled. The tap includes a square, closed armature that is opened by pushing a spring-loaded button. Two sides of the square have fine wire coils wound on them, connected in series.

Although this item formerly belonged to a TSD employee, veterans of TSD that were contacted do not recognize this particular model as a 'stock' item. It may be a sample or prototype.

The wire-tapping device. Pushing the black button on the right side causes the armature to open so that it can be clipped over a wire. The oblong aluminum portion is a separate impedance matching transformer, marked "3.2" (ohms) on the input, and "1200" on the output. It has a 1/8" plug on the input side, and a matching jack on the output side. Author's collection.

Unidentified Subminiature Audio Amp

The exact purpose of this device is unknown, however it is extremely similar to an early transistor hearing aid, but without a built-in microphone. It is a small aluminum clamshell box, about 2.5" long, containing a 3-transistor audio amplifier. It uses early Raytheon germanium transistors (one is a CK721 with a mid-1953 date code), and the stages are transformer-coupled. The required battery is about the size of an 'N' cell - there is a hand-written note inside the case lid saying that the battery is 2.6 volts. The output connector is a 2-pin hearing aid earphone connector. The input is a tiny 7-pin Winchester connector. It is unknown what was intended to attach to the input connector.


Three views of the submini amplifier. The black component in the lower left, marked "3-21", is one of the 3 transistors. The color-coded resistors are 1/2 watt units (for size comparison). Author's collection.


Japanese-made Audio Amplifier

This device is another simple audio amplifier, but in this case it was made in Japan. The dimensions are 5-1/4" X 1-3/8" X 1-1/4", in a steel box with gray enamel paint. It uses two socketed Sony 2T65 transistors (similar to 2N214's), with R-C coupling between the stages and a transformer-coupled output. The electrolytic caps are made by Nippon Chemicon. The input connector is an unidentified type, about 1/4" in diameter with a center pin. The output is a 3-pin male mic connector, like those used with older C.B. and mobile radios. The output impedance is 600 ohms, and DC power input is 9V. The phenolic circuit board is hand-wired (not printed).

The inside of the lid and the inside of the box each are stamped "1". Since before WWII, Japanese manufacturing was in the habit of marking pieces with an 'assembly number', since many pieces were hand-fitted to each other. So, it is possible that this unit is a prototype.


The "Japanese amplifier", with the cover removed. The two gray objects are the Sony transistors. The input connector is on the right (not visible), and the output is on the left. The red and blue leads on the left are the power input. The white, blue and brown wires connect to the output connector, but are not attached on the other end. Author's collection.


Maico Hearing Aid

Maico is a commercial hearing aid brand that dates from the late 30's. The "Transist-ear" is one of their earliest solid-state models, using three CK718 transistors.

This one has had its mic removed, and a 1/8" phone jack added in its place. It was obviously being used as a subminiature mic preamp, similar to the custom-made unit described above.


Front and back views of the modified Maico hearing aid. The added 1/8" phone jack is visible at the top. Author's collection.


Unidentified Battery Box

This unidentified battery box was machined from translucent black plastic, with phenolic inserts mounted from behind with small screws. It appears to accept two 30 volt batteries, and six 'N' cells. The box measures 5-5/8" X 2-1/2" X 7/8".

Assuming that the two large batteries are 30V each, and that each small cell is 1.5V; then the box could provide any of the following outputs: +60V (with or without 300 ohm resistor in series), +30V, -1.5V (two cells in parallel), -6V, and -3V.


The battery box with the cover removed. In the center is a pair of contacts that are normally shorted, but when a plug is inserted through a hole in the case, the contacts open and are shunted by 300 ohms. Author's collection.

Home-made Surveillance Devices

Shown here are some devices constructed by the author, as an exercise in building various surveillance devices using 50's or 60's vintage components wherever possible.


This is a "bug" transmitter, built as a working copy of the "book-spine transmitter" shown in Keith Melton's "Ultimate Spy" book (page 111 of the 3rd edition, page 103 of the 2nd edition, page 83 in the 1st edition). From the bottom upward, the major components are: mic element, 1st audio stage, 2nd audio stage, RF oscillator, and 3 batteries. The transistors are 2N591 (two, audio) and 2N499. The antenna is a PC trace that goes around the perimeter of the board, thus forming a loop. This loop is also the "L" in the RF tank circuit. The "C" in the tank is the distributed capacitance of the components. The frequency is in the 80 MC range, making it similar to the ST-2A described above. RF power is estimated to be 3 to 4 mW.

Schematic of the "book-spine transmitter"


Shown here is another home-made surveillance transmitter, with features that include: AC power (like the ST-2A), tunable from about 70 to 90 MC FM, all solid-state with early 1960's transistors, and a built-in microphone. The mic is a tiny hearing-aid element in the top center, glued to the brass tube which provides a coupling to a length of translucent plastic tubing. The antenna connector is a pin jack, seen on the right. The three transistors just to the right of the transformer are the audio stages; while the two near the antenna jack are the RF oscillator and buffer stages.

Schematic of the home-made transmitter


A general-purpose 3-transistor audio amplifier, assembled in a small tin box. On the right is a gain control and a power switch. At the bottom are the input and output jacks.


The schematic of the amplifier. Note that the input provides a source of DC power, which can be used to operate transducers that contain an active preamp. The use of germanium transistors allows the design to be simple, and to work effectively from a single 1.5 volt cell. The 0.01 uF cap at the input is to shunt any RF signals to ground, which is necessary when the amplifier is used to drive a transmitter.


A tiny hearing aid microphone is hidden inside this vintage fountain pen cap, along with a 1-transistor preamp. The small hole near the top is the sound inlet.


The schematic of the fountain pen microphone. DC power is drawn from the output connection. As with the amplifier above, there is an RF bypass cap included.


This unit is an AM transmitter assembled in a small tin box. It is designed to connect to the audio amplifier above. On the right, the power switch and antenna wire can be seen. The transmitter is tunable from 1500 to 2000 KC.


The schematic of the AM transmitter. Although it works as designed, it is effective only with a long antenna wire, or a very sensitive receiver. Note the two 3.3K resistors - if one is jumpered, then the power output is reduced. The 2N43 directly modulates the RF oscillator, so there is likely some FM component introduced as well.


A Commercial Surveillance Receiver

A Swintek Q-6R surveillance receiver from the 1980's, as used by police and private detectives. The receiver itself supports up to 6 crystal-controlled channels, and includes a relay output for controlling a tape recorder. The antenna is mounted inside the lid of the case. The two smaller black boxes on the right are transmitter units, one with a lapel mic connected. Author's collection.


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