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RCA AR-88
General coverage communications receiver

AR-88 was a valve-based shortwave general coverage communications receiver, developed and built in the early 1940s by the Radio Corporation of America (RCA). Although the receiver was initially intended as the successor to the AR-77 amateur receiver, the outbreak of WWII made it evolve into a professional high-end military-grade receiver for which cost was no object [1].

The AR-88 is a 14-valve (tube) receiver, which covers a frequency range of 535 kHz to 32 MHz. Unlike the National HRO receiver, which had pluggable coil packs for each frequency band, the AR-88 uses a six-position band selector. A special version of the receiver, the AR-88LF, was suitable for LF and MF, covering 70 to 550 kHz (continuously) and 1.5 to 30 MHz (continuously).

The image on the right shows a typical AR-88. It measures 49 x 28 x 49 cm and weights over 40 kg. Judging from its low serial number (100227) it was produced in the early years of the war [1].
  
RCA AR-88, kindly donated by Museum Jan Corver [4].

As most of the first production runs of the AR-88 was supplied to Great Britain, Russia, France and China – as part of the 1941 lend-lease act [3] – it is likely that this one was used in the UK, possibly inside a Y-Station, for intercepting German radio communications. It has a black wrinkle paint finish, which was typical for the early models. It was often supplied as an 'open frame' for mounting in a 19" rack, but was also available as a closed cabinet with a hinged lid at the top, as shown here. Later versions came in a variety of colours and finishings, such as Naval grey. It is estimated that a total of approx. 25,000 AR-88s (all variants) were built during the war [1].

After the war, the AR-88 became a popular receiver for radio amateurs, who used them well into the 1960s. Some receivers are still being used by amateurs and radio enthusiasts today. A typical feature on some of the surviving AR-88 radios, is that most of them have an alternative S-meter behind the rightmost window on the front panel. The reason for this is that during WWII, most receivers were shipped without the original S-meter, due to world-wide shortages of such meters. The receivers that were used for diversity reception didn't need an S-meter, and the AR-88s used for interception in the UK were often equipped with an alternative meter by the British. 1

  1. The device shown here has an S-meter that was supplied by the UK Air Ministry (AM), which is an indication that it was probably used in the UK [4].

RCA AR-88 receiver
Front view of the AR-88
AR-88 with opened top lid
Filters inside the AR-88
Frequency scale and S-meter
Frequency band scales
Frequency fine tuning
S-meter
A
×
A
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RCA AR-88 receiver
A
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Front view of the AR-88
A
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AR-88 with opened top lid
A
4 / 8
Filters inside the AR-88
A
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Frequency scale and S-meter
A
6 / 8
Frequency band scales
A
7 / 8
Frequency fine tuning
A
8 / 8
S-meter

Features
The digram provides an overview of the controls at the front panel of the AR-88. The device shown here is housed in a black metal enclosure, which was optionally available. When mounted in a 19" rack, this cabinet was usually omitted. The device could be powered by a wide range of mains voltages, including 110 and 220V AC, making it suitable for worldwide deployment. It is switched ON by setting the MODE selector at the bottom left to the MOD or CW position. When use as part of a radio station, the MODE selector can also be used to control the transmitter.

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The band selector is used to select one of six available frequency bands, whereas the selectivity selector is used to select the desired bandwidth between 1 (wide) and 5 (narrow). 1 The function of the Noise Limiter (NL) and the Automatic Volume Control (AVC) 2 can be controlled with the 4-position rotary selector at the bottom right.

  1. Here known as broad and sharp respectively.
  2. Also known as Automatic Gain Control (AGC).

Models
  • AR-88
  • AR-88D
  • AR-88F
  • AR-88LF
  • CR-88
  • CR-88A
  • CR-88B
  • CR-91
  • CR-91A
  • SC-88
Wartime use
During WWII, the British intelligence service, GC&CS (now: GCHQ), ran a massive operation for intercepting and decoding German radio messages in morse code, under the codename ULTRA. The majority of the intercepted messages — picked up by a network of so-called Y-Stations — were encrypted with the Enigma cipher machine, which was regarded unbreakable at the time.

The Y-Stations were spread all over the UK, but were also present in other parts of the world, including North Africa and Australia. They were operated around the clock by the so-called Y-Service, which consisted mainly of HAM radio operators and specially trained house wifes, using intercept receivers like HRO-5 and AR-88.

Once the messages were intercepted, they were sent to the codebreaking center at Bletchley Park by despatch rider or via teleprinter lines (telex). There, a team of over 12,000 people, broke the German codes at a large scale on a daily basis.
  
Bletchley Park, Britain's wartime codebreaking center.

As many receivers were required for the war effort, the UK ordered large quantities of AR-88 receivers from 1941 onwards. These were provided by the US under the so-called lend-lease act of October 1941 [3]. The receivers were supplied as stand-alone as well as rackmount units.

The image on the right shows an intercept room at Beaumanor Hall, one of the most prominent Y-Stations in the UK. In the room, an long array of intercept desks was present, each with its own operator. The image on the right shows a single desk with the necessary paperwork and a pair of headphones. At the right is a 19" rack with two RCA AR-88 receivers. The device at the front left is a so-called undulator, a recording device that could write morse code as a series of square waves onto a strip of pre-gummed paper.

The intercept rooms were located in the main building (Beaumanor Hall) as well as in a series of wooden huts, disguised as stables and cricket pavilions. Other receivers were used here as well. The signal from the various antennas were distributed to all receivers on the estate [6].

Per lend-lease agreement, the receivers had to be returned to the US or destroyed, when they were no longer needed. For that reason, many AR-88s were destroyed at the end of the war.
  
Radio intercept room at Beaumanor Hall, showing several AR-88 receivers. The National Archives HW41-119 [7].

This is probably the reason why so few AR-88s are found today. The ones that did survive were often bought back by the UK (for a fraction of the actual price) after they had been returned 'pro-forma'. The manufacturer – RCA – didn't want to see any of them back again on the US market.

The AR-88 also played an important part in intercepting German Wireless Traffic (WT) that originated from the Lorenz SZ-40/42 cipher machine. The SZ-40/42 was codenamed TUNNY by the allies, and was in fact a heavy mechanical coding machine with 12 cipher wheels, that was used for the encryption of teletype traffic (telex).

The telex signals were first encrypted using the SZ-40, and then sent via short wave radio, using Frequency Shift Keying (FSK). At the other end the traffic was decrypted using another SZ-40.
  
Re-enactment at TNMOC during the Tunny Gallery opening day [8]

In the UK, the German signals were intercepted by the various Y-Stations, and sent to Bletchley Park (BP) for decoding. At BP, a team of methematicians led by Bill Tutte, managed to create a working electronic equivalent of the TUNNY machine, without ever having seen a real Lorenz SZ-40. Unlike the mechanical SZ-40, the TUNNY machine used electronic valves (tubes). The image above shows Sarah Marlin and Debby Minney in ATS uniform, re-enacting the intercept of TUNNY traffic at the Tunny Gallery opening day at The National Museum of Computing in May 2011 [8].

In Great Britain, the AR-88 was not only used for interception of enemy radio signals, but also for communication with ships and airplanes. The photograph on the right was taken in the secret Underground Headquarters (UGHQ) under Fort Southwick, hidden deep down in the Portsdown Tunnels [5]. On D-Day, this was the Allied communications center for Operation Overlord.

An AR-88 is visible on the left, being operated by a WRENS (Womens Royal Navy Service). The clothing of the other operators suggest that this was the Naval wireless transmission room (W/T).
  
AR-88 in use in the Naval Wireless Transmission room of the Underground Headquarters during WWII. Reproduced here by kind permission from Bob Hunt [5].

Portsdown is located to the north of Portsmouth (UK). It consists of approx. 5 miles of tunnel constructions, hidden in a 120 metres high chalk hill that is locally known as The Hill. Most of it was created during WWII. More information about the underground headquarters (UGHQ), the Portsdown Tunnels and Fort Southwick, is available from Bob Hunt's excellent website [5].


Similar receivers
Block diagram
Below is the simplified block diagram of the AR-88. At the left is a 2-stage RF pre-amplifier built around two 6SG7 valves, the output of which is mixed in a 6SA7 with the signal from a VFO built around a 6J5. The VFO is adjusted in tandem with the tuned circuits of the RF pre-amplifiers.


The output of the mixer is followed by a selectable bandwith filter and several IF amplifier stages (3 x 6SG7), each of which has selectable band-pass filters to increase the overall selectivity of the receiver. The signal is then fed to an AM detector (½ 6H6) followed by a limiter (6H6), an audio pre-amplifier (6SJ7), and finally a power amplifier stage (6K6GT). For the reception of CW signals (morse code), the signal from a BFO (6J5) can be injected at the input of the third IF amplifier.

An Automatic Gain Control (AGC) – built around ½ 6H6 – takes its input from the detector and various IF stages, to ensure that the two RF amplifier stages (RF1, RF2) are not overloaded when receiving strong signals. Two further valves – a 5Y3GT double rectifier and a VR-150 stabiliser – are used in the mains power supply unit (PSU) and are not shown in the above block diagram.

Circuit diagram
The original circuit diagram [B] – extracted from the original instruction manual [A] – is available for download below. However, due to the complexity of the selectable band-pass filters – which affect nearly every stage of the receiver – it is cluttered with lines and rotary switches, and might therefore be difficult to understand. For this reason, the Dutch Radio Monitoring Service (RCD) made its own circuit diagram in 1958, in which the bandwidth selectors are omitted [C].

The simplified circuit diagram also shows an additional EF80 valve, which is used as a cathode follower. It buffers the output of the mixer and provides an output for a panoramic display (panadapter) that can be connected externally. This was probably a modification by the RCD.

 Original circuit diagram
 Simplified circuit diagram


Specifications
  • Frequency
    535 kHz - 23 MHz
  • Bands
    6 (see below)
  • IF
    455 kHz
  • Impedance
    200Ω 1
  • Antenna
    7.5-15 m (25-50 ft.)
  • Sensitivity
    < 1µV at 0.5W output
  • Selectivity
    5 positions (see below)
  • Output
    2.5W into 2.25Ω or 600Ω 2
  • LT
    6V, 4A
  • HT
    250-300V, 90mA
  • Mains
    100-165V or 190-260V AC 50/60Hz
  • Valves
    14 (see below)
  • Dimensions
    489 x 489 x 279 mm
  • Weight
    40 kg
  • NSN
    5820-17-601-6969
  1. Except on lowed frequency band (535 kHz - 1.6 MHz).
  2. Receivers with S/N below 00300 have 20Ω output instead of 600Ω.

Valves
  • 6SG7 (5x)
    RF amplifier, IF amplifier
  • 6SA7
    Mixer
  • 6J5 (2x)
    LO, BFO
  • 6H6 (2x)
    2nd detector, Noise limiter
  • 6SJ7
    AF Amplifier
  • 6K6GT
    AF Power Amplifier
  • 5Y3GT
    Rectifier
  • VR-150
    Voltage Regulator
Frequency bands
  1. 535 kHz - 1.6 MHz
  2. 1.6 - 4.5 MHz
  3. 4.5 - 12 MHz
  4. 12 - 16.5 MHz
  5. 16.5 - 22.5 MHz
  6. 22.6 - 32 MHz
Selectivity settings   at -6dB
  1. ~ 13 KHz/s
  2. ~ 7 kHz
  3. 3 kHz
  4. 1500 Hz
  5. 400 Hz
Documentation
  1. General Purpose Communications Receiver Model AR-88D, Instructions
    RCA, Camden (NJ, USA). IB-25927-3. Amended 26 November 1943.

  2. AR-88 Circuit Diagram
    RCA, Camden (NJ, USA). IB-25927-3. Amended 26 November 1943.
    This circuit diagram should be inserted as Page 21 of the manual [A].

  3. RCD Communication Receiver AR 88, simplified circuit diagram
    RCD (NL), 2 July 1958.

  4. Reception Set AR-88, Description (provisional)
    Electrical and Mechanical Engineering Regulations. E-752. Issue 1, May 1945.

  5. AR-88, Promotional poster
    RCA, RL 13701-S. Date unknown.
References
  1. Henry Rogers, RCA's Amazing AR-88 Receivers
    Website: Radio Boulevard. Western Historic Radio Museum.
    1997-2012. Retrieved December 2012.

  2. Wikipedia, RCA (Radio Corporation of America)
    Retrieved December 2012.

  3. Wikipedia, WWII Lend-Lease Act
    Retrieved December 2012.

  4. Cor Moerman, AR-88 Receiver - THANKS !
    RCA AR-88 receiver featured on this page kindly donated by Museum Jan Corver.
    Budel (Netherlands), December 2012.

  5. Bob Hunt, Portsdown Tunnels
    Website, Retrieved December 2012.

  6. Kevin Coleman, Personal correspondence
    Volunteer at Bletchley Park (Station X) and Beaumanor (Y-Station).
    December 2008 - January 2009.

  7. The National Archives, Image of radio intercept room at Beaumanor Hall
    National Archives (UK) reference: HW41-119.

  8. John Robertson, Photograph of Y-Station re-enactment
    The National Museum of Computing (TNMOC), TUNNY Gallery opening day, 26 May 2011.
Further information
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Crypto Museum. Created: Wednesday 05 December 2012. Last changed: Tuesday, 30 March 2021 - 09:46 CET.
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