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AT-3
Automatic transmitter

AT-3 was a semi-automatic hybrid spy radio transmitter, developed between 1956 and 1959 by Hughes 1 in (California, USA) for the US Central Intelligence Agency (CIA), and introduced in 1960 for use in clandestine operations. It is the central piece of the AS-3 spy radio station, 2 and has a built-in burst keyer — equivalent to the later KE-8 — that accepts a CA-3 tape cartridge. It sends pre-recorded (coded) messages in morse code as a burst, at a speed of 300 words per minute.

The transmitter measures approx. 26 x 22 x 6 cm 3 and weights 3.5 kg. It has a frequency range from 3 to 30 MHz and produces an output power of no less than 25W in CW. It is the first CIA transmitter with a built-in burst keyer, that allows pre-recorded (encrypted) messages on a magnetic tape to be played back at high speed, in order to reduce the risk of interception and discovery by means of Radio Direction Finding.

If necessary, the AT-3 can also be used for bare manual transmissions, by using the built-in morse key, or connecting a larger external one.
  
AT-3 transmitter with CA-3 tape cartridge and internal morse key

Being a development from the late 1950s, the transmitter is a so-called hybrid, which means that it is partly built with transistors and partly with valves (tubes). In fact, there are just two valves — one for the exciter and one for the power amplifier (PA). All other circuits are transistor-based.

Development of the set started in 1956, and took approx. four years. The CIA introduced it in 1960. Two years later, in March 1962, the AT-3 was evaluated by the US Army, but was found inadequate. Furthermore, the Army did not recognise the benefit of using a burst encoder [4].

The unit featured here was probably manufactured in 1961, as the valves have the date codes of weeks 39 and 42 of 1960 respectively. A self-adhesive label below the AT-3 name tag, shows that a modification was carried out on 1 December 1962. The serial number of this unit is 917, which is imprinted at the bottom, and written inside the case shell. It is known that 10 proto­types were made in late 1960, followed by a production run of 250 units in late 1960 or early 1961 [5].

  1. Full name: Hughes Aircraft Company. After 1985 referred to as Hughes Electronics.
  2. In CIA parlance: an agent radio set.
  3. Batteries, crystal, receiver, printer and battery pack not included. Internal morse key retracted.

At-3 transmitter with C)-3 coder and CA-3 tape cartridge
AT-3 transmitter
AT-3 transmitter with CA-3 tape cartridge and internal morse key
AT-3 transmitter with CA-3 tape cartridge and internal morse key
AT-3 - top view
AT-3 transmitter with RR/E-11 receiver attached
Retractable internal morse key
AT-3 transmitter - rear view
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At-3 transmitter with C)-3 coder and CA-3 tape cartridge
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AT-3 transmitter
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AT-3 transmitter with CA-3 tape cartridge and internal morse key
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AT-3 transmitter with CA-3 tape cartridge and internal morse key
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AT-3 - top view
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AT-3 transmitter with RR/E-11 receiver attached
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Retractable internal morse key
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AT-3 transmitter - rear view

Features
The diagram below gives an overview of the controls and connections of the AT-3 transmitter. There are connections on all four sides: a 2-pin socket for an external morse key at the front, a 2-pin antenna socket at the left and a socket for connection of a 12V DC source at the rear.

Furthermore, there is a 9-pin female socket at the right front – for connection of an RR/E-11 or RR/D/11 receiver, and another one at the left front for connection of an optional HELL printer. For emergency purposes, an internal morse key is available behind a hinged lid at the front side.

Click to see more

All controls are at the top surface. At the front right are the MODE selector, the voltage indicator and a socket for the crystal. Towards the centre is the valve-based exciter, which should be tuned to the desired frequency. At the left is the power amplifier (PA) stage, which is also valve-based. The PA range selector and the PA tuning scale should be adjusted for maximum power output.

At the right is the burst keyer, which consists of a motor-driven magnetic tape reader. It has a bay – hidden below a hinged lid – that accepts a CA-3 tape cartridge. The image above shows the AT-3 transmitter with the CA-3 tape cartridge installed. Once the transmitter has been tuned to the desired (crystal) frequency, the MODE selector is set to TRANSMIT and the MSG-button is pressed to transmit the pre-recorded message at high speed in morse code (burst transmission).

MODE selector (function switch)
Oscillator section (with crystal installed)
Voltage and current indicator
Antenna matcher
Output indicator
Connector for external morse key and lid behind which the internal morse key is stowed.
Pulling-out the internal morse key
Operating the internal morse key
Hinged lid over tape cartridge bay
Opening the tape cartridge bay
CA-3 tape cartridge installed
Tape cartridge installed (lid removed)
Removing the tape cartridge (press button on cartridge)
Antenna terminals
Socket for connection of (HELL) printer
Socket for connection of receiver
Power receptacle
Oscillator section (without crystal)
Oscillator section (with crystal installed)
Crystal installed
Instructions
B
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B
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MODE selector (function switch)
B
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Oscillator section (with crystal installed)
B
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Voltage and current indicator
B
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Antenna matcher
B
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Output indicator
B
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Connector for external morse key and lid behind which the internal morse key is stowed.
B
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Pulling-out the internal morse key
B
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Operating the internal morse key
B
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Hinged lid over tape cartridge bay
B
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Opening the tape cartridge bay
B
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CA-3 tape cartridge installed
B
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Tape cartridge installed (lid removed)
B
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Removing the tape cartridge (press button on cartridge)
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Antenna terminals
B
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Socket for connection of (HELL) printer
B
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Socket for connection of receiver
B
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Power receptacle
B
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Oscillator section (without crystal)
B
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Oscillator section (with crystal installed)
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Crystal installed
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Instructions

MODE selector
At the right half of the control panel is the main function switch, or MODE selector. It has three settings: OFF, TUNE and TRANSMIT. Note that in the OFF position, the filaments of the valves are already powered, so that the valves are pre-heated when the MODE selector is set to TUNE or TRANSMIT. In this state the device consumes approx. 0.98A. For this reason, the battery pack should be disconnected when the device is not in use. In the TUNE-state the current is 2.87A.

Operation
  1. Connect antenna and ground
  2. Connect battery pack or other 12V DC source
  3. Insert crystal in holder
  4. Set MODE selector to TUNE
  5. Set exciter tuning knob to desired frequency
  6. Set PA tuning knob to desired frequency
  7. Set antenna load selector to position 1
  8. Depress internal morse key
  9. Adjust exciter tuning knob for maximum reading on rightmost meter
  10. Adjust PA tuning knob for maximum reading on leftmost meter
  11. Try next setting of antenna load selector
  12. Repeat 9-10 until maximum is found
  13. Release internal morse key
  14. Set MODE selector to TRANSMIT
  15. Insert pre-recorded tape in cartridge bay
  16. Press MSG
  17. Once the message is transmitted, set MODE selector to OFF
  18. Disconnect battery or other 12V DC source


Some of the parts of the AS-3 radio station

Parts
AT-3 transmitter
Suitable crystals
Burst encoder CO-3
Magnetic tape cartridge CA-3
Single-band receiver
Double-band receiver
Battery pack BP-3
AC Power Supply Uniy AP-3
12V adapter (terminal block)
Transmitter   AT-3
The AT-3 transmitter is the central piece of the AS-3 spy radio set. When all accessories are removed, it looks like the image on the right. In this state, the dimensions are 26 x 22 x 6 cm; small enough to fit it inside a regular Samsonite briefcase of the era.

At the right side, it can be expanded with a single- or double-band receiver. At the left side it can be expanded with a printer. A battery pack or AC/PSU should be connected at the rear.

  
AT-3 transmitter

Crystals
The transmission frequency of the AT-3 is determined by a quartz crystal that must be inserted into the crystal socket at the centre of the control panel. The crystal frequency – in the range 3 to 30 MHz – corresponds directly to the transmission frequency.

The sockets accepts HC-6U crystals with a pin distance of 12.5 mm and a pin diameter of 2.5 mm. Examples of such crystals are shown in the image on the right.

  
Suitable crystals

Burst encoder   CO-3
The AS-3 radio set was probably the second US spy radio set to use high-speed morse burst transmissions to reduce the risk of detection and discovery by radio direction finding (RDF). The first one was the RS-1, which it replaced.

The CO-3 (shown on the right) was a simple device that allowed the bare morse elements – dots and dashes – to be recorded onto a CA-3 tape cartridge. Once recorded, the tape was played back on the built-in keyer of the AT-3 transmitter at a speed of 300 words per minute.

 More information

  
CO-3B Dot-dash Coder

Tape cartridge   CA-3
Messages – generally encrypted or otherwise coded – were pre-recorded with the CO-3 coder shown above, onto a CA-3 tape cartridge as shown in the image on the right. The cartridge contains regular magnetic audio tape of the era, and can hold up to 150 five-character words.

Recording a message was time-consuming, as each dot, dash and space had to be entered separately. But once it was recorded, it could be sent in a matter of seconds, which significantly reduced the on-air time and hence the risk of discovery by means of radio direction finding.

  
CA-3B Tape Cartridge

Receiver   RR/E-11
RR/E-11 was among the first solid-state spy receivers developed by the CIA Radio Lab. It was intended for use as part of the AS-3 radio set, and could be plugged into the right side of the AT-3 transmitter, in which case power (12V) and antenna signal were supplied by the transmitter.

The receiver, which was released well before the AS-3 radio station was ready, could also be used stand-alone, in which case it could be powered by a 6V or 12V DC source, whilst antenna and headphones were connected directly to the unit.

 More information

  
RR/E-11 receiver

Two-band receiver   RR/D-11
RR/D-11 was a plug-in receiver for the AS-3 radio station, that was similar to the RR/E-11 shown above, albeit with two frequency bands instead of one. The receiver is approx. 2.5 cm (one inch) deeper than the RR/E-11, but can be plugged into the same socket of the transmitter.

In many respects, the RR/D-11 is similar to the German FE-8 (BN-58) receiver of the same era.

At present, no image of this item is available.
  
Please help us to find this item

Battery pack   BP-3
For portable and mobile operation, the AT-3 can be powered by a purpose-built battery pack, designated BP-3, which is plugged into the 9-pin male receptacle at the rear centre. The BP-3 delivers a voltage of 12V DC at 10A.

At present, no image of this item is available.
  
Please help us to find this item

AC power supply unit   AP-3
According to the original documentation, an external power supply unit (PSU) was available for powering the transmitter directly from the mains (110 - 240V AC).

At present, no image of this item is available.
  
Please help us to find this item

12V adapter
It was also possible to power the AT-3 directly from any other suitable 12V DV power source, such as the battery of a vehicle, by using the adapter shown in the image on the right. It plugs straight into the receptacle at the centre of the rear panel, and has two screw terminals for connection to the external 12V DC source.

It has a 15A fuse and an internal germanium power diode that blows the fuse when the 12V DC source is connected the wrong way around.

The side panels are currently missing from this item.
  
12V adapter

AT-3 transmitter
CO-3 coder and CA-3 cartridge (closed)
CO-3 coder and CA-3 cartridge (open)
CA-3 cartridge installed in CO-3 coder
Operating the coder (adding a 'dot' to the tape)
Operating the coder (adding a 'dash' to the tape)
12V adapter
Suitable crystals
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AT-3 transmitter
C
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CO-3 coder and CA-3 cartridge (closed)
C
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CO-3 coder and CA-3 cartridge (open)
C
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CA-3 cartridge installed in CO-3 coder
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Operating the coder (adding a 'dot' to the tape)
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Operating the coder (adding a 'dash' to the tape)
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12V adapter
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Suitable crystals



AT-3 interior

Interior
The AT-3 consists of a metal base plate onto which all parts and circuits are mounted, covered by a somewhat rounded metal case shell. The case shell is held in place by 17 screws, divided over the four sides, close to the bottom edge. After removing these screws, the case shell comes off.

The interior, shown in the image on the right, consists of several tuning mechanisms, circuits and sub-circuits, merged in a construction that is typically American for the era. Considering its age, this particular unit is very well preserved.

The transmitter is a typical hybrid, in the sense that transistors are used in the oscillator and keyer circuits, whilst valves are used for the exciter (pre-amplifier) and the power amplifier (PA). The 6883 PA valve is accessible at the left side, and is retained by a spring-loaded clip. It has a removable anode connection at the top.
  
AT-3 interior seen from  the font right

The 6883 is a compact yet high power valve that was used in audio amplifiers as well as in the RF output stage of transmitters. It is capable of delivering no less than 25 Watts to the antenna. Removing it might require the gentle use of a screwdriver, as it is firmly seated in its octal socket.

The exciter valve is a bit more difficult to reach, as it is deeply embedded in the device. Luckily, it can be reached through a removable lid in the bottom panel. The lid is held in place by a single screw, and once it is removed, the small 6688 valve becomes (partly) visible as shown in the image on the right. Also visible, below the valve, is the exciter's unevenly spaced tuning coil.

In order to withstand severe transport shocks, the designers have chosen to use the Premium Quality (PQ) 1 variant of the valve in this case. It is extra strong and has gold-plated contacts.
  
Exciter valve

Despite the fact that the 6688 valve is accessible through the lid, removing it can be tricky, as there is no space to hold it. This can be solved by using a push-tool that is inserted through the hole at the centre of the valve's socket, which can be reached from the front of the device.

The remaining circuits are very difficult to access and may require the transmitter to be partly dis­assembled. Most of the sub-circuits are bolted to the bottom panel and can be loosened by removing the relevent screws from the bottom. In some cases it may be necessary to remove one or more of the tuning knobs, for which two small hex wrenches are stowed inside the device.

The largest sub-circuit is the internal power supply unit, which is located at the rear centre. It holds the 9-pin male receptacle to which the battery or external PSU should be connected.
  
Power module (interior)

It converts the 12V DC source supply into -70V, +215V and +450V for the valve-based circuits. The filaments of the valves and the transistor-based circuits are powered directly by 12V. Note that the -70V voltage is also used for the (transistor-based) keyer. The image above shows the power converter with its upper case shell removed. At the right is the black toroidal transformer.

  1. Equivalent to the Philips designation Special Quality (SQ).

AT-3 interior seen from  the font right
AT-3 interior seen from  the font left
AT-3 interior seen from the left rear
AT-3 interior seen from the right rear
Interior - top view
Anode circuit of the 6886 PA valve
PA valve retained by spring-loaded metal clip
PA valve (6883)
Bottom side, showing the lid that gibes access to the oscillator valve
Exciter valve
Exciter valve
Oscillator valve PQ 6688/E
PQ mark on the oscillator valve
Oscillator tuning mechanism, with the 6688 valve socket at the bottom left.
Oscillator tuning mechanism and 6688 valve wiriing (bottom right)
Antenna coil selector and antenna relais
Antenna socket and antenna relais (wiring)
Miniature hex-wrenches stowed inside the device. Needed for removing the knobs.
Tape reader (keyer)
Electronic circuit of the tape reader (keyer)
Internal power module
Power module (interior)
Large transistor inside the power module
High-voltage fuses
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D
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AT-3 interior seen from  the font right
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AT-3 interior seen from  the font left
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AT-3 interior seen from the left rear
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AT-3 interior seen from the right rear
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Interior - top view
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Anode circuit of the 6886 PA valve
D
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PA valve retained by spring-loaded metal clip
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PA valve (6883)
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Bottom side, showing the lid that gibes access to the oscillator valve
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Exciter valve
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Exciter valve
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Oscillator valve PQ 6688/E
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PQ mark on the oscillator valve
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Oscillator tuning mechanism, with the 6688 valve socket at the bottom left.
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Oscillator tuning mechanism and 6688 valve wiriing (bottom right)
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Antenna coil selector and antenna relais
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Antenna socket and antenna relais (wiring)
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Miniature hex-wrenches stowed inside the device. Needed for removing the knobs.
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Tape reader (keyer)
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Electronic circuit of the tape reader (keyer)
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Internal power module
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Power module (interior)
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Large transistor inside the power module
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High-voltage fuses

Circuit diagram
The circuit diagram of the AT-3 transmitter is currently unknown, although the diagram of an early prototype can be found in the Preliminary Instruction Manual [A]. You can help us to expand this page by supplying the operator's manual, the circuit diagram(s) and/or any other information related to this item. As always, your help will be much appreciated.  Contact us


Restoration
When we obtained the transmitter featured on this page, it was in reasonable condition, albeit dusty and dirty. Apparently it had been stored in a damp place for several years, as corrosion was found on the screws, the tuning coils and at the places were two different types of metal are in contact with each other. However, treating the corroded parts and applying a drop of oil and grease to the movable parts, appeared to be enough to make the device fully operational again.

So far, the following work has been done on our AT-3:

  • Exterior cleaned
  • All exterior screws derusted
  • Receiver socket derusted
  • Dymo label residue removed from front panel
  • Oscillator coil cleaned
  • PA coil cleaned
  • All moving parts oiled and greased
Specifications
  • Frequency
    3 - 30 MHz
  • Output
    25W
  • Supply
    12V DC / 10A
  • Valves
    6688, 6883
  • Dimensions
    26 × 22 × 6 cm
  • Weight
    3.5 kg
Connections
Power socket
The AT-3 is powered by a 12V DC source, which should be connected to the 9-pin male socket (DE9/M) at the rear. Although this socket has 9-pins, the pins of each row are interconnected, in order to withstand high currents. The socket is suitable for connection of a BA-3 battery pack, an AC power supply unit, or a 12V terminal adapter. The latter contains a fuse holder (with a 15A fuse) and a heavy germanium diode that blows the fuse when the polarity is accidentally reversed.

  1. +12V
  2. +12V
  3. +12V
  4. +12V
  5. +12V
  6. 0V
  7. 0V
  8. 0V
  9. 0V
Receiver interface   DE-9/F
At the right side of the transmitter, close towards the front, is a 9-pin DE-9/F socket for connection to an (optional) receiver. The socket is suitable for connection of the RR/E-11 and the RR/D-11 receiver.

  1. Antenna
  2. Ground
  3. n.c.
  4. n.c.
  5. Jumper to 6 when used with 12V DC
  6. +6V DC input
  7. n.c.
  8. +12V DC input
  9. Audio output
Exciter valve · PQ 6688/E
An Amperex 6688 valve (tube) is used in the exciter of the AT-3 transmitter. It is a low-profile penthode valve with 9 gold-plated contact pins, as indicated by the PQ marking on the exterior. The valve is equivalent to the Tesla E180F and to the 6J9P.

 6688 datasheet


PA valve · 6883
The PA-stage of the AT-3 transmitter is built around an RCA 6883 penthode valve (tube). The valve is identical to the 6146, but has a filament voltage of 12.6V instead of 6.3V. It delivers an output power of 25 Watts. The valve is equivalent to the 8032A, 8552 and QE05/40F.

 6883 datasheet (12V)
 6146 datasheet (same but 6V)



Identification   serial numbers
The model number of the unit is engraved in a small black tag that is located near the left edge of the top surface, just below the ventilation holes of the PA valve. It is held in place by two screws, so that it could be swapped if the Army adopted it as well. The serial number is imprinted at the bottom of the case, near the ventilation holes of the PA valve. It is also written inside the upper case shell. It is estimated that at least 260 units were made. The following numbers are known:

  • 917
    Crypto Museum (Netherlands)
If you own an AT-3 transmitter with a serial number that is not listed here, please let us know, as it may help us to make an educated guess of the actual quantity that was produced.  Contact us

Serial number at the bottom of the case
Serial number, pencil writted inside the top cover
Modification Work Order (WMO) 44, carried out on 1 December 1962
E
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E
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Serial number at the bottom of the case
E
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Serial number, pencil writted inside the top cover
E
3 / 3
Modification Work Order (WMO) 44, carried out on 1 December 1962

Missing items
The following items are currently missing from our AT-3:

  • BP-3 battery pack
  • AP-3 AC adapter
  • TP-3 printer
  • User manual
  • Technical documentation
  • RR/D-11 receiver
Literature
  1. Development of AS-3 Portable Radio Station, Progress Report No. 1
    1 October 1956. 1

  2. Development of AS-3 Portable Radio Station, Progress Report No. 2
    1 November 1956. 1

  3. Development of the Semi-Automatic Two-Way Radio Station AS-3, Progress Report No. 3
    1 January 1957. 1

  4. Development of the Semi-Automatic Two-Way Radio Station AS-3, Progress Report No. 4
    11 March 1957. 1

  5. Development of the Semi-Automatic Two-Way Radio Station AS-3, Progress Report No. 5
    1 May 1957. 1

  6. Development of the Semi-Automatic Two-Way Radio Station AS-3, Progress Report No. 6
    1 July 1957. 1

  7. Development of the Semi-Automatic Two-Way Radio Station AS-3, Progress Report No. 8
    1 November 1957. 1

  8. Contract RD-122, Task Order 1 - AS-3
    15 November 1956. 1

  9. Contract RD-122, Task Order 1 - AS-3
    18 December 1956. 1

  10. Contract RD-122, Task Order 1 - AS-3
    7 February 1957. 1

  11. Contract RD-122, Task 1 - AS-3 - Trip Report
    18 March 1957. 1

  12. AS-3 Contract RD-122 - Trip Report
    10 May 1957. 1

  13. Trip Report - Discussions on RS-11 and AS-3 Equipment
    10 May 1957. 1

  14. Trip Report - RD-122 - AS-3
    24 June 1957. 1

  15. Trip Report - RD-122 - AS-3
    18 December 1958. 1

  16. Trip Report - AS-3
    17 April 1959. 1

  17. AS-3 Agent Communication System (Project 2108)
    13 December 1956. 1

  18. Semi-Automatic Agent Communications Set, AS-3 (Project 2108)
    30 July 1957. 1

  19. TP-3 Motors
    19 November 1957. 1

  20. Preliminary Evaluation of TP-3, Hellschreiber Transistorized Printer
    19 December 1957. 1

  21. Trip Report - AS-3 Tests (hints to Hellschreiber usage)
    23 April 1959. 1

  22. Development of a Minature DC Motor for the TP-3 printer
    30 December 1959. 1

  23. Technical Notes on AT-3 Transmitter
    26 August 1960. 1

  24. Army Evaluation of AS-3 Equipment
    5 March 1962. 1

  25. Defects in AS-3 Prototype - Attachment 'A'
    Date unknown.

  26. Suggested AS-3 Accessories
    8 May 1959. 1

  27. Conference Report, Radio Station AS-3
    19 July 1957. 1

  28. Conference Report, Radio Station AS-3
    27 May 1957. 1

  29. Conference Report, AS-3
    31 October 1958. 1
Documentation
  1. Preliminary Instruction Manual for Radio Set AS-3
    With circuit diagram of an early prototype (not the final design).
    CIA, 11 February 1959. 1

  2. AP-3A Power Supply Instruction Manual
    Date unknown. 1  Older version
  1. Sanitized copy approved for release by CIA on 14 February 2013 — 2 April 2013.

References
  1. Pete McCollum, The AS-3 HF Radio Set
    Retrieved June 2020.

  2. Louis Meulstee, AT-3
    Wireless for the Warrior - Volume 4 Supplement, Chapter 143.
    October 2017. Retrieved June 2020.

  3. CIA, Request for Bids, AS-3
    30 April 1956. 1

  4. CIA, Army Evaluation of AS-3 Equipment
    5 March 1962. 1

  5. CIA, Technical Notes on AT-3 Transmitter
    26 August 1960. 1
  1. Sanitized copy approved for release by CIA on 14 February 2013 — 2 April 2013.

Further information
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Crypto Museum. Created: Monday 31 August 2020. Last changed: Friday, 04 September 2020 - 15:20 CET.
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