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Hallicrafters SX-28
Intercept receiver (Super Skyrider)

The SX-28 was an AM/CW communication receiver, developed and built by Hallicrafters Inc. in Chicago (USA) in 1940, a few years before the US got involved in WWII. The receiver was intended for use by the Government, the Army and by Radio Amateurs (Hams). It is one of the most popular receivers every built by Hallicrafters. The receiver is also known as Super Skyrider.
 
The SX-28 was known for its excellent audio performance, but also for its stylish art-deco exterior. Although the receiver needs some expertise when put to use, it is generally loved by engineers, even today. During WWII, it became a popular intercept receiver alongside others like the National HRO and RCA's AR-88.

The image on the right shows an SX-28A that was probably produced mid-1944. It was owned for several years by a Radio Amateur who complete reworked and restored it, before it ended up in the Crypto Museum collection.
  
Hallicrafters SX-28A

The SX-28 is a superheterodyne receiver, that covers all frequencies between 550 kHz and 43 MHz, divided over six ranges. The circuit is built around 15 valves (tubes) and features a double pre-selection font-end in the highest four frequency ranges. A single pre-selection front-end is used on the lowest two ranges. Other design features of the SX-28 are the Amplified Automatic Volume Control (AVC), the Lamb Noise Silencer, Calibrated Bandspread and Push-Pull Audio.

The SX-28 was developed in 1940 and was intended to be the 'ultimate receiver' of the moment, using the input from over 600 report, filed by Radio Amateurs and Government Engineers [1]. It performed better than any previous Hallicrafters receiver and was on par with its competitors.

The receiver was first announced in July 1940, but production didn't start until August of that year. The design of the SX-28 was improved several times during the war and around February 1944 it was replaced by the SX-28A, a model that stayed in production for some time after the war, until June 1946. During the war, a special ruggedized version, known as the AN/GGR-2 was released to the Army, and an airborne version, known as the R-45/ARR-7, to the Air Force.

In the UK, many SX-28 receivers were used in the so-called Y-Stations for the interception of coded enemy radio signals, mainly from Germany and Italy. Such messages were subsequently passed on to Bletchley Park for decoding [6]. When production of the SX-28 ended in 1946, an estimated 27,500 of these receivers had been manufactered [5]. Many of these have survived, especially in the US, and are still operational today. The price of an SX-28A in 1944 was US$ 223.
 
Hallicrafters SX-28A Tuning knob

 
Block Diagram
The block diagram below shows the basic operation of the SX-28. The antenna input at the left passes one or two pre-amplifiers (depending on the band) before it reaches the mixer. The main oscillator produces a signal that is 455 kHz higher than the desired frequency, resulting in a 455 kHz Intermediate Frequency (IF) that is amplified in two stages before it reaches the detector.


For the reception of CW (morse) signals, a Beat Frequency Oscillator (BFO) is present. The signal from the BFO can be mixed with the detected signal, before it is passed on to a two-stage audio amplifier. The 2nd AF stage is a Push-Pull Amplifier, built around two 6V6 valves. It produces enough signal to drive a speaker directly. At the bottom right is the Power Supply Unit (PSU). It converts the AC mains (110-125V) to suitable LT and HT voltages. Alternatively, the radio can be fed by a DC source, by directly applying 6V DC and 270V DC to a special connector at the rear.
 
Y-Stations
During WWII, the British intelligence service, GC&CS (now: GCHQ), ran a massive operation of intercepting and decoding German radio messages in morse code, mainly encrypted using the well-known Enigma cipher machine. The messages were intercepted by the so-called Y-Stations, that were spread all over the country, but were also present in other parts of the world.
 
A good example of a Y-Station is the one at Beaumanor Hall [9], a large estate in the small village of Woodhouse (Leicestershire, UK) that was used for military intelligence during WWII. At Beaumanor Park, a series of radio intercept huts, disguised as stables and cricket pavilions, was set up, using a variety of Intercept Receivers including the SX-28 [6]. Most of these were 19" rackmount versions, modified for 240V AC [1].

Other receivers that were used by the Y-Stations include the RCA AR-88, the National HRO, the British R-107, the R-109 and the DST-110 [5].
  
Beaumanor Hall. [unknown source]

Once the messages were intercepted, they were sent to the codebreaking centre at Bletchley Park by despatch rider or via teleprinter lines (telex). There, a team of over 12,000 people, consisting of codebreakers, engineers and WRNS 1 broke the German codes at a large scale on a daily basis.
 
  1. WRNS = Women's Royal Naval Service, sometimes written as WRENS. → Wikipedia

Models
  • SX-28 (August 1940 - February 1944)
  • SX-28A (February 1944 - June 1946)
  • AN/GRR-2 (Ruggedized version)
  • R-45/ARR-7 (Airborne version)
Frequency ranges
  1. 550 kHz - 1.6 MHz
  2. 1.6 - 3.0 MHz
  3. 3.0 - 5.8 MHz
  4. 5.8 - 11 MHz
  5. 11 - 21 MHz
  6. 21 - 43 MHz
Technical specifications
  • Mains voltage: 110-125V AC
  • AC Power consumption: 138W at 117V/60Hz
  • DC Power consumption: 108W at 6V/18A
  • AF Power output: 8W undistorted
  • Sensitivity: 2mV (band1-5), 4mV (band 6) at 0.05W AF output
  • AF output impedance: 5000 or 500 Ω
  • Intermediate Frequency (IF): 455 kHz
Valves
Index Valve Description
V1 6AB7 1st RF Amplifier
V2 6SK7 2nd RF Amplifier
V3 6SA7 Mixer
V4 6SA7 HF Oscillator
V5 6L7 1st IF Amplifier Noise Limiter (ANL)
V6 6SK7 2nd IF Amplifier
V7 6B8 Detector and S-meter Amplifier
V8 6B8 AVC Amplifier
V9 6AB7 Noise Amplifier (ANL)
V10 6H6 Noise Rectifier (ANL)
V11 6J5 Beat Oscillator (BFO)
V12 6SC7 1st Audio Amplifier
V13 6V6 Push-Pull Output Amplifier
V14 6V6 Push-Pull Output Amplifier
V15 5Z3 Rectifier

 
Competitors
The following receiver can be regarded as contemporary competitors of the SX-28:
 
References
  1. Henry Rogers, The Hallicrafters Inc. SX-28, a pre-war masterpiece
    Website: Radio Boulevard. Western Historic Radio Museum.
    1997-2012. Retrieved January 2013.

  2. Philip I. Nelson, Hallicrafters Model SX-28 Communications Receiver (1941)
    Detailed information about restoration of an SX-28.
    1995-2012. Retrieved January 2013.

  3. Philip I. Nelson, Rebuilding the Hallicrafters SX-28 Gearbox
    Detailed information about restoration of the SX-28 tuning mechanism.
    1995-2012. Retrieved January 2013.

  4. Wikipedia, Hallicrafters
    Retrieved January 2013.

  5. Wikipedia, Hallicrafters SX-28
    Retrieved January 2013.

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

  7. The Hallicrafters Co., SX-28 Operating and Service Instructions
    Operating, Alignment and Servicing Instructions. 1941.

  8. War Department, Radio Receiver AN/GRR-2
    Military version of the Hallicrafters Model SX-28-A. 22 November 1944.

  9. Wikipedia, Beaumanor Hall
    Retrieved January 2013.

Further information

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Crypto Museum. Created: Thursday 03 January 2013. Last changed: Saturday, 24 December 2016 - 20:26 CET.
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