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VHF Surveillance Radio Receiver

The SRR-4 was a VHF surveillance receiver, developed by the Technical Services Division (TSD) 1 of the US Central Intelligence Agency (CIA) around 1958. It covers 50 - 200 MHz in a single band and can demodulate AM, FM, CW and modulated CW signals. Although the receiver was intended for general communications applications, it was typically used for intercept, surveillance, band monitoring, direction finding and in particular for the reception of covert listening devices (bugs).

The receiver is built with 12 subminiature valves (tubes) and has an internal Power Supply Unit (PSU) that is built with 3 transistors. The device can be powered by a 12V DC source, or directly from the 110/220V AC mains, by connecting the appropriate power cable to the power socket.

In addition, the SRR-4 can be powered by LT and HT batteries that are fitted internally. A complete set consists of the SSR-4 receiver, a telescopic antenna, various power cables, a spares case and a small instruction manual, all nicely packed inside a leather catalogue-type carrying case.
Telescopic antenna mounted on the SRR-4

For actual (field) use, some additional items may be required, such as batteries, headphones, and perhaps external recording equipment. The receiver is continuously tunable over the entire 50 - 200 MHz frequency band, and the tuning cursor of the drum-type frequency scale at the top left can be calibrated by means of the built-in calibration oscillator at 5 MHz intervals.

The design of the radio is based on the military R-744 receiver which covers a frequency range of 20 to 100 MHz and is housed in a nearly identical enclosure. It is currently unknown where the SRR-4 was manufactured, but as the R-744 was made in the US by Arvin Industries and by Mallory, it seems likely that it was made by one of these companies. The SRR-4 was succeeded in the early 1960s by the fully transistorized SRR-5, which offered an expanded frequency range of 50 to 400 MHz. The receiver was also used by the Norwegian Stay-Behind Organisation (SBO) [1].

  1. The Technical Services Division (TSD) of the CIA was renamed Office of Technical Service (OTS) in 1974 [3]. The SRR-4 is not completely developed from scratch by the TSD, but is based on the same design as the US Army Signal Corps R-744 receiver.

Telescopic antenna mounted on the SRR-4 SRR-4 seen from front top SRR-4 on its back Front panel Headphones cable Mains power cable Contents of the accessory box SRR-4 with telescopic antenna
All controls and connections are located at the front panel of the SRR-4, as shown in the diagram below. At the top left is the film-type frequency scale that convers the entire 50-200 MHz frequency band. A bulged glass cover provides some level of magnification, whilst a small lamp, mounted at the left, illuminates the scale in the dark. The frequency is adjusted with the large knob to the right of the frequency scale and can be locked with a small lever underneath it.

Front panel of the SRR-4

At the bottom center is a selector marked OPERATE. It is used to select the modulation type (FM, AM or CW) or the built-in calibrator. When enabled, the calibrator produces a signal at 5 MHz intervals (e.g. 120 MHz, 125 MHz, 130 MHz, etc.). The scale calibration knob, at the top edge of the front panel, allows the horizontal hairline of the scale to be adjusted to the nearest 5 MHz.

A suitable antenna should be connected to the ANT socket. When the supplied telescopic antenna is used, it should be fitted to the antenna mount at the top right. The S-meter at the bottom left shows the signal strength of the received station. The meter can also be used to test the internal batteries, by momentarily setting the meter-function-switch to 1.5V (LT) or 45V (HT).

As the receiver does not have a built-in speaker, 600 ohms headphones should be connected to the socket marked PA AUDIO and the AF GAIN knob is used to adjust the volume. As the audio output is provided on a BNC-type socket, a special conversion cable is supplied for connection of a standard pair of headphones, with a 6.3 mm jack. Recording equipment (such as a tape or wire recorder) can be connected to the socket marked DET AUDIO, which has a fixed audio level.

Front panel Frequency scale Adjusting the frequency Meter Power socket Opening the LT battery compartment HT battery compartment
The SRR-4 can be powered by a variety of sources:

  • Internal batteries
    For portable use and operation in the field, the SRR-4 can be powered by internal batteries. As it is a valve-based device, it needs a high 45V anode voltage for the valves (tubes) and a low 1.5V voltage for the filaments of the valves. These voltages are generally known as HT and LT respectively. The SRR-4 has two battery compartments that can be accessed via a removable panel at each of the sides. The one at the left is for the HT battery. The compartment at the right allows two LT batteries to be used in parallel.

  • 12V DC
    The receiver can also be powered by a 12V DC source, such as the battery of a normal car. In this case, the HT and LT voltages are supplied by the internal power supply unit (PSU). Note that the positive battery terminal (+) should be connected to the chassis (ground) and the -12V DC should be supplied to the radio via the connector at the front panel.

  • 110V or 220V AC
    In a fixed setup, e.g. in a hotel room, the receiver can also be powered from the local mains. The SRR-4 is suitable for the standard 110V and 220V AC networks, which should be connected to the U-77/U connector at the front panel. Suitable cables were supplied with the unit. Note that the mains cables are potentially dangerous, as the contacts of the U-77/U plug carry live voltage and can easily be touched. These voltages can be lethal. Always remove the wall plug before attaching or detaching the radio.
Opening the LT battery compartment LT battery compartment HT battery compartment HT battery compartment Mains power cable U-77/U plug on the mains power cable
SRR-4 in use
The first official confirmation that the CIA used the military-supplied SRR-4 receiver, was in the book Inside the Company: CIA Diary, by Philip Agee who worked for the CIA from 1957 to 1968 [4]. According to Agee it was often used as part of a Listening Post (LP), for the reception of a covert listening device (bug) in a nearby room, often in combination with a recording device.

Another documented case of the SRR-4 is the operation of the West-German intelligence agency (BND) against double agent 'GEIGE' in 1965. On 20 March 1965 at 12:00, a meeting was about to take place between 'GEIGE' and a BND operative, named 'LENSKY', in Copenhagen (Denmark) in the Skovriderkroen Hotel [5].

The CIA had been briefed on the subject by the BND through their liaison office in München, but did not take part in the operation. Unknown to the Germans however, CIA operatives managed to book a room above the hotel's restaurant [7].
Skovriderkroen Inn, just north of Copenhagen at the time of the operation.

A day before the meeting, a CIA operative arrived at the hotel and set up a Listening Post (LP) inside the reserved room. During the meeting, he used an SRR-4 receiver to 'scan' the entire band from 50 to 200 MHz several times for any hidden bugs. As he did not speak or understand Danish, the wife of another CIA operative was present in order to identify any intercepted signals.

The LP was situated in the room just above the rightmost table in the image on the right, with the SRR-4 receiver in such a position that it was never more than 40 feet away from any table in the room. This allowed the LP to pick up any bug in the room, even a very weak one. The receiver had been tested the previous day in situ, by a TSD technician with a standard CIA SRT-5 bug.

For the duration of the meeting, from 12:00 to 14:00, the CIA operative constantly monitored the restaurant and scanned the entire frequency band at least five times for any listening devices.
The restaurant of the Skovriderkroen Inn. At the front right the table above which the LP was located.

Particular attention was payed to the 130-140 MHz frequency band, as this is the band in which the SRT-5 bugs operate. Existing communications from taxis, aircraft, radio stations, etc. were positively identified by the interpreter. At no point was any 'restaurant sound' or conversation picked up, so it was assumed that the room was clear. This was later confirmed in a report [8][9].

According to several sources, including [1], the SRR-4 was also used during the Cold War by Stay-Behind Organisations (SBOs), such as the Norwegian one, and by other US Government agencies, such as the Federal Bureau of Investigation (FBI) and the Drug Enforcement Administratie (DEA).

Leather carrying case Spare parts case Telescopic antenna Antenna mounting block Power cables Service cable Headset adapter cable Instruction and Technical Manual
Carrying case
The complete SSR-4 receiver set was originally supplied in a large brown leather carrying case that was adapted for this purpose. It has a rigid frame that keeps the receiver at the center. The remaining space is used to store the manual, the power cables, the spares box and the antenna.   

Spare parts case
The RSS-4 was supplied with a slimline metal box, roughly the size of the top surface of the receiver, in which a range of spare parts were stored. It also contains the antenna mounting block (when unused) and the service cable.

 Contents of the spare parts case
Contents of accessory box

An external antenna can be connected to the BNC socket at the front. Alternatively, a long telescopic antenna with a threaded base can be used for short-range reception.

When the original telescopic antenna is used, the antenna mounting block should be fitted to the front panel first (see below).
Base of telescopic antenna

Antenna mount
When using the original telescopic antenna, the antenna mounting block (usually stored inside the spares box) should be fitted to the mounting holes at the top right of the front panel.

The mounting block is made of pertinax and can be fitted in two ways, allowing the receiver to be used with the front panel in horizontal or vertical position. It has a short coaxial cable by which it should be fitted to the antenna input socket.
Antenna mount

Power cables
As the SRR-4 is suitable for a variety of power sources, three power cables are supplied with the receiver, all of which can be connected to the power socket at the right of the front panel.

Two mains power cables were supplied, one for 110C AC and one for 220V AC, plus a 12V DC cable for connection to a car battery.
Mains power cable

Service cable
All intermal parts of the receiver, with the exception of the PSU, are mounted to the front panel. When servicing the unit, the front panel can be removed from the case, by loosening six large bolts at the edges of the front panel.

As this breaks the connection between the PSU and the receiver, the 9-way service cable (usually stored inside the spares box) can be installed to restore the connection. It is also possible to make your own improvised reproduction cable.
Original service cable

Headset adapter
For unknown reasons, a BNC socket is provided at the receiver's front panel, for connection of a pair of headphones. As most headphones are supplied with a 6.3 mm jack however, this short adapter cable was supplied with the set.

As in practice this cable was often lost, after-market adapters are commonly found with the surviving receivers. For connection of a recording device, a separate output with a fixed audio level is also available at the front panel.
Headphones cable

The SRR-4 was supplied with a 25-page A4-size handbook with full operating instructions [A]. The manual was usually stored inside the leather carrying case, in one of the side pockets.

Alternatively, a 52-page service manual was available, with full operating instructions, circuit descriptions and circuit diagrams [B]. Despite its age, the circuit diagrams are of outstanding quality as they are professionally printed, probably due to the fact that the SSR-4 was manufactured in reasonable quantities.

 Download the operating instructions
 Download the technical manual
SRR-4 circuit diagram

Accessory box Contents of accessory box Telescopic antenna Top of telescopic antenna Base of telescopic antenna Antenna mount - close-up Original service cable Improvised service cable
Service cable installed Headphones cable Headphones cable SRR-4 service manual
The diagram below shows the interior of the SRR-4, with its front panel facing down and the helix-type frequency scale on the left. The location of the various parts and components is clearly visible, and can be used as a guide when reading the description below.

The interior of the SRR-4 can be accessed by loosening the six large bolts at the edges of the front panel and extracting the front from the outer case shell. The actual receiver is completely mounted to the rear of the front panel, whilst the case shell contains the batteries and the PSU.

The image on the right shows the receiver after it has been removed from the outer case shell, with the front panel facing downwards. At the right is the film-type frequency scale which is driven by a small gear box. At the bottom right is a blue 8-pin plug that connects the radio to the outer case shell and, hence, the AC/DC PSU.

Along the bottom edge of the front panel are the connections and controls of which the wiring is clearly visible in the image on the right. First-grade gold-plated plugs and sockets are used to connect the various parts of the radio together.

The actual receiver is visible in the left half of the image above, and consists of two sections. A large metal unit that contains the RF stages and the local oscillator, and a smaller section with 9 grey cylindrical plug in units (with coloured arrows on top), that represent the IF and AF stages.

In order to save space and weight, the receiver is completely built with subminiature valves, four of which are visible at one side of the front-end, as shown in the image on the right. From left to right we seen the 1st, the 2nd and the 3rd RF amplifier and at the far right the local oscillator.

These four valves are the only ones that are directly accessible. They are socketed, so that they are easily replaced when necessary. All other valves are mounted inside the cylindrical grey modules and can not be accessed. In case of a defect, the entire module must be replaced.
Four subminiature valves

In order to make servicing of the receiver even easier, a set of spare modules was supplied in the spares kit, along with several other spare parts. Although this is very convenient from a service point of view at the time the receiver was introduced, it makes it very difficult to repair a vintage SRR-4 today, especially when the spares kit is missing and no replacement modules are available.

The case shell contains the PSU, which is described in more detail in the chapter Restoration.

Removing the radio from the case shell Receiver removed from the case shell Outer case shell Bare receiver Interior Interior Interior Interior
Wiring of the MODE selector Film scale and inter-unit connector Internal connectors Receiver with circuit modules Four subminiature valves Close-up of subminiature valve Circuit modules Scale driving gear
Block diagram
Below is the simplified block diagram of the SRR-4. At the top left is the antenna input. After filtering, the antenna signal is amplified in a three-stage amplifier before mixing it with the signal from the Local Oscillator (LO). The IF signal is then amplified by five identical stages before it is fed to the AM detector and the FM discriminator. It is then amplified to headphones level.

At the bottom left is the Power Supply Unit (PSU). It converts the AC mains voltage to 12DC and then converts the 12V DC into 1.5V DC (LT) and 40.5V DC (HT). The unit can also be powered directly by -12V DC. In addition, it can be driven by LT and HT batteries. In the centre position of the meter selector switch, the meter shows the current through the last three IF stages.

Restoration   PP-1A/ASR
When we received our SRR-4 receiver, it appeared to be dead and the 1A fuse in the 12V DC lines was blown. After studing the circuit diagram of the receiver [C] and the separate circuit diagram of the power supply unit (PSU) [D], it was decided that the problem was most likely inside the PSU.

Getting access to the PSU is rather easy. Remove the outer case shell by loosening the six large bolts at the edges of the front panel. Inside the case are the two battery compartments (LT/HT) and a small metal box with the PSU at the centre.

The PSU is held in place by 4 recessed screws at the bottom of the case shell. After removing these 4 screws and removing the the 7-pin plug from socket J401 on top of the PSU, the entire PSU can be lifted from the case shell. Note this plug may be locked. Now take off the top lid of the PSU by removing the 4 screws at the top.
Front panel removed

Inside the die-cast aluminium enclosure of the PSU are two compartments. The one closest to the external connections is the mains transformer that converts 110V or 220V AC into -12V DC. The other compartment contains a separate toroidal transformer that converts the -12V DC input voltage into +40.5V (HT) and +1.5V (LT). On top is a small PCB to which the wires are soldered.

Make a note or a photograph of the wiring and then desolder the wires. Now remove the secondary transformer by removing 4 recessed screws from the bottom of the enclosure. The secondary converter consists of two PCBs, one with the transformer and one with additional components. The two can be separated by removing two screws from the upper board.

In our case, it was immediately obvious that someone had attempted to repair the PSU before, but had probably not been successful. Two capacitors had already been replaced.
Bottom view

After wiring up the boards and measuring the ouput voltages everthing seemed to be normal. However, as soon as a load was connected, the 1.5V LT rail completely collapsed. This led us to the conclusion that the two diodes in the LT power section (CR405 and CR406) were probably gone. After replacing them, and swapping the earlier replaced cheap Japanese capacitors for proper ones, everthing worked as expected, and we were soon able to receive our first station.

Outer case shell Outer case shell Front panel of PSU removed Front panel removed Conventional transformer that converts the mains voltage to 12V DC Secondary PSU Secondary PSU

Spare parts case
  • Tube 6051
  • Tube 6611
  • Tube 6612
  • Diode 1N21B
  • Pilot lamp
  • IF amplifier subassembly 473-216 (red)
  • IF amplifier/AM detector subassembly 473-217 (blue)
  • FM discriminator subassembly 473-218 (black)
  • Calibration oscillator subassembly 473-223 (brown)
  • BFO subassembly 473-220 (green)
  • AF amplifier subassembly 473-221 (yellow)
  • Fuse 3AG 0.25A (2x)
  • Fuse 3AG 1A (2x)
Additional equipment
The following items are not supplied with the radio, but may be required for full use:

  • Batteries
  • Headphones
  • Recording equipment
  • Line plugs
  • Car antenna input adapter

It is sometimes assumed that the SRR-4 was also used by the US Navy in combination with the KY-71/UPX video decoder. However, the name (SRR-4) should not be confused with that of the Naval AN/SRR-4 Radio Receiving Set. The latter is a complete radar and IFF decoding setup, used aboard US Navy ships from 1952 onwards, whilst 'SRR-4' is a CIA designator. The double use of model designators is quite common and often leads to confusion. In the same vein, KY-71/UPX was a video decoder, whilst KY-71/D was the designator for the STU-II crypto phone.

The SRR-4 is based on the same design as the R-744 surveillance receiver that was developed by the US Army Signal Corps in 1957/58 for intercepting enemy radio signals, direction finding and surveillance. It was used by the US Army in Vietnam and was also used in Canada and Australia.

The R-744 is housed in a similar enclosure and has a nearly identical layout of its front panel, but there are some notable differences, which are listed in the table below.

It is currently unclear which of the two receivers was introduced first and whether the SRR-4 is based on the R-744 or the other way around. If you have additional information that may help answering this question, please contact us.

 More about the R-744

Differences between the SRR-4 and the R-744
Description SRR-4 R-744
Colour Black Green
Developed ~1958 ~1957
Frequency range 50-200 MHz 20-100 MHz
Audio socket(s) BNC Jack 6.3 mm
Power supply 110/220V AC, 12V DC, batteries 24V DC, batteries
Power socket U79/U 4-pin 24V vehicle socket
Scale illumination ON/OFF Adjustable
IF stages 5 4
Local oscillator 1 tube 6051 2 tubes 6012 (parallel)
Mixer 1N21B (diode) 6611 (tube)
Inter-unit power socket   J303
In order to allow the outer case shell (that contains the PSU) to be separated from the radio without having to disconnect any wires, an inter-unit connection is present, consisting of a plug (P303) and a socket (J303). To allow the receiver to be operated outside the case, a service cable was supplied in the spares kit. Both are wired as follows (the frame is connected to the chassis):

  1. 1.5V DC (from battery)
  2. 1.5V DC (from PSU)
  3. 40V DC (from PSU)
  4. 45V DC (from battery)
  5. 12V DC (to PSU)
  6. AC common (to PSU)
  7. 110V AC (to PSU)
  8. 220V AC (to PSU)
Internal power socket   J401
The internal PP-1A/ASR power supply unit is fully separated from the rest of the radio, and is housed in a small metal enclosure that is mounted at the bottom of the outer case shell. The PSU converts 110/220V AC to 12V DC and then converts the 12V DC to 40.5V DC (HT) and 1.25V DC (LT) for the filaments. This 2-stage power conversion was necessary to allow the radio to be powered by an external 12V DC source as well. The socket on the PSU is wired as follows:

  1. 0V DC (GND) (in)
  2. 12V DC (in)
  3. 1.25V (out)
  4. 40.5V (out)
  5. 110V AC (in)
  6. 220V AC (in)
  7. 0V AC (AC common) (in)
    Looking into the socket on the PSU
This connector is a 7-way 126-series hexagonal connector, made by Winchester Electronics, Amphenol, Continental, and probably others.

External power socket U-79/U   J304
An external power source can be connected to the U-79/U socket marked EXT POWER (J304) at the bottom right of the front panel. It accepts a U-77/U plug and allows the receiver to be powered by 12V DC, 110V AC or 220V AC. Note that in the case of 12V DC, the unit has the (+) terminal connected to ground. The pinout of the front panel connector is as follows:

  1. 0V AC (AC common)
  2. 0V DC (GND)
  3. 0V DC (GND)
  4. 12V DC (in)
  5. not connected
  6. 110V AC (in)
  7. 220V AC (in)
  8. not connected
  9. not connected
  10. not connected
    Power socket seen from the front of the receiver
220V AC cable
When powering the SRR-4 from the 220V AC mains, e.g. in Europe, the power cable should be wired as follows. The pinout of the U-79/U socket is as seen when looking into the contacts of the socket. This is identical to the solder side of the plug. Warning: Always connect the U-79 plug to the radio before connecting the other end to the wall socket. When removing the cable, always disconnect the wall socket first. The contact pins of the plug carry a potentially lethal voltage.

110V AC cable
When powering the SRR-4 from the 110V AC mains, e.g. in the USA, the power cable should be wired as follows. The pinout of the U-79/U socket is as seen when looking into the contacts of the socket. This is identical to the solder side of the plug. Warning: Always connect the U-79 plug to the radio before connecting the other end to the wall socket. When removing the cable, always disconnect the wall socket first. The contact pins of the plug carry a potentially lethal voltage.

110V AC cable
When powering the SRR-4 from a 12V DC power source, such as the battery of a car, the power cable should be wired as follows. The pinout of the U-79/U socket is as seen when looking into the contacts of the socket. This is identical to the solder side of the plug. Note that the positive terminal (+) of the battery should be connected to the chassis (ground) of the receiver.

Technical specifications
  • Frequency
    50-200 MHz (single band)
  • Modulation
    AM, FM, CW, MCW
  • Principle
  • Sensitivity
    FM/CW: <1µV, AM: <1.5µV (50-100 MHz), <2µV (100-200 MHz)
  • IF frequency
    4.3 MHz
  • IF bandpass
    80 kHz
  • Calibration
    5 MHz intervals
  • AC input
    80-130V or 160-240V (40-80 Hz)
  • DC input
    11-20 V (neg), 270 mA max.
Known serial numbers
  • A58
    Crypto Museum
  • A59
    Pete McCollum
  • A79
  • ???
    Military Collection Gausdal
The following abbreviations and cryptonyms are used throughout this page and the referenced documents:

AM   Amplitude Modulation
CIA cryptonym for the Technical Services Division (TSD) of the CIA.
FM   Frequency Modulation
CA   West Germany
CIA cryptonym for West Germany (BRD).
CIA cryptonym for the German Bundes Nachrichtendienst BND. Note that the first two letters (CA) indicate West Germany.
CIA   Central Intelligence Agency
Independent US Government civil agency for gathering national security intelligence from around the world. (More)
CW   Continuous Wave
DIZTAG   Chzechoslovakia
CIA cryptonym for the former Replublic of Czechoslovakia.
CIA cryptonym for the Technical Services Division (TSD) of the CIA.
MCW   Modulated Continuous Wave
MK   TSD operation
CIA cryptonym for an operation of the CIA's Technical Services Division (TSD).
Technical Services Division (TSD) of the CIA. Operations include preparation of false documents and identity materials.
TSD   Technical Services Division
The Technical Services Division of the CIA. In 1974 renamed to Office of Technical Service (OTS).
UJRANDOM   BND investigation
CIA cryptonym used as Cable Indicator for BND investigations.
  1. Handbook, Operating Instructions, Radio Receiver SRR-4
    Date unknown, but probably around 1961. 1

  2. Handbook, Operation and Service Instructions, Radio Receiver SRR-4
    Including technical description and circuit diagrams.
    Date unknown, but probably around 1961.

  3. SRR-4 Circuit Diagram
    Date unknown, but probably around 1961.

  4. PSU Circuit Diagram
    Date unknown, but probably around 1961.

  5. Raytheon. Subminiature tube (valve) type CK6611
    1 April 1957.

  6. Raytheon. Subminiature tube (valve) type CK6612
    1 April 1957.

  7. Raytheon. Subminiature tube (valve) type CK6051
    15 September 1957.
  1. Basic handbook kindly supplied by Pete McCollum [1].

  1. Peter McCollum, The SSR-4 Surveillance Receiver
    Retrieved December 2015.

  2. Military Historical Collection - Gausdal, Agent radio, SSR-4
    Retrieved December 2015.

  3. Wikipedia, Central Intelligence Agency Directorate of Science & Technology
    Retrieved December 2015.

  4. Philip Agee, Inside the Company: CIA Diary
    London 1975. ISBN 0-14-004007-2

  5. Chief CIA base Frankfurt to Chief Munich Liaison Base
    Eaerly report about upcoming meeting. 11 February 1965. 1

  6. CIA Frankfurt, Cable to Langley
    Request for technician with SRT-5 bug for testing. 5 March 1965. 1

  7. CIA Frankfurt, Cable to Langley
    SRR-4 receiver available and confirmation of booked room. 8 March 1965. 1

  8. Chief CIA base Frankfurt (Germany) to Chief KURIOT
    30 March 1965. pp. 4. Includes drawing of the restaurant. 1

  9. Internal CIA memo
    Report about the GEIGE operation. 30 April 1965. 1
  1. Declassified by the CIA in 2006.

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Crypto Museum. Created: Thursday 14 April 2016. Last changed: Monday, 13 March 2017 - 10:15 CET.
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