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SX - 28 A

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überarbeitet am 19.10.2010

Even the earliest TRF receivers sold under Hallicrafters brand carried the "Skyrider" designation; from 1934, the receiver circuitry has been changed to a superheterodyne set, and the model designation became "Super Skyrider", the earliest beeing the S-4 to S-6. In 1936, their successor was the S-9, this model has also been sold with a crystal filter to improve selectivity, it got the SX-9 designation. In 1937, the S(X)-16 and it's companion, the S(X)-17 with an additional RF stage and a Noise Limiter, were the top of the range sets; these receivers came with a unique dial in form of an engracved metal disc mounted on the tuning capacitor's shaft.
In the following few years, there has been a rapid progress in developing high end receivers until Hallicrafters presented the SX-28 als an ultimate pre war dream receiver. It came with all kind of "goodies" to improve reception and can be considered as one of the ancestors of all "World Band Radios" in the next 25 years.
As American state and military authorities ordered the set in large amounts, the SX-28 (and SX-28A respectively) have been the first mass produced Hallicrafters communications receivers.

SX - 28
1940/1
Single conversion, 540 kHz - 43 MHz, IF 455 kHz, 15 tubes
SX - 28 A technically improved for military / governmental use
R - 12 matching console speaker
PM - 23 matching table cabinet speaker

 

Single conversion, ZF 455 kHz

Analog dial, Bandspread dial

AM, CW with BFO

Sensitivity
AM <14 MHz 2 uV, >14 MHz 1 uV

Selectivity 6 dB
LC filters 14 / x / 5 kHz, Quarzfilter 3 / x / 0.030 kHz

RF gain, Antennea tuner, Bandspread dial
Crystal Phasing- / Notchfilter, Noise limiter

The dimensions of the rugged metal cabinet of the SX-28 are 521 x 254 x 375 mm, it's weight is 34 kg, the front panel has engraved letterings for brand and model designations and a black crinkle finish. A hinged lid gives You access to the tubes and the innards of the receiver.
The receiver's output is 600 Ohms, so You need a 600 Ohms speaker, as the matching PM-23 table speaker or the huge R-12 console speaker, the matching transformers have been integrated in the speaker cabinet.
Decorative rims and the rounded edges of the cabinet contribute to the fame of the SX-28 Super Skyrider as beeing one of the examples of using "Art Deco" in industrial design - later, there have not been many military receivers in such a pleasant design.

In the middle of the upper region of the front panel, You find three dial windows: the signal strength meter at the left, the main tuning dial, a shadow indicates the bandrange on the backlit orange - brown dial and the bandspread dial with its 0 - 100 marks and the the frequency indications for the spread amateur bands.
The main tuning knob at the left hand underneath the S - meter and the bandspread tuning knob are both of the characteristic Hallicrafters "steering wheel" type, these bakelite knobs tend to break (what happended to one knob of my set, let me know, in case You have a spare one...). The mechanics of the tuning arrangement are all metal gears, so there is virtually no backlash; there is a mechanical brake to prevent detuning of the main tuning. The accuracy of the analog non linear dial is not sufficient on the higher bands, but there is a 0 - 100 logging scale on the main and bandspread tuning knobs; You might find it useful, to write down the figures for tuning in a known station later. By comparing spoken frequency announcements and the settings of the logging scale, You can reach an accuracy of around 5 kHz.
At the left hand of the receiver, You find the mains switch, it is combined with the tone control (like found on later Hallicrafters sets) and not with volume control. Underneath the BFO pitch control, You find a BASS BOOST function acting similar like the one found on Sony's Ghetto Blasters in the eighties. In the left lower corner, You find the AF Gain control and the headphones jack.
In the lower part of the frontplate between the main and bandspread tuning controls, You find the RF Gain control, the band range selector (550 kHz - 1,6 MHz, 1,6 - 3 MHz, 3 - 5,8 MHz, 5,8 - 11 MHz, 11 - 21 MHz und 21 - 43 MHz) and an antenna trimmer for tuning an external antenna.
At the right side of the front panel, You find (from top) the Noise Limiter and the Crystal Filter control used to shift the filter's passband and to use the Filter as a Notch Filter to eliminate unwanted interfering signals. The rotary control below the RECEIVE - SEND (i.e. Standby) switch lets You select one of the three coil filters (14 - 5 kHz) or three crystal filters (3 - < 0,1 kHz). Next to this, You find the AVC and BFO switch.

In the inside of the set, You can recognize the mains transformer at the left and the "cage" shielding the main tuning capacitor and the RF stages.

From the high impedance antenna input, the RF signal will reach two amplifier stages (6AB7 and 6SK7), the first RF amplifier stage will be bypassed in band ranges 1 and 2 with usually high signal levels. The RF gain will act on the kathodeconnector of this tube and on two IF amplifier stages. The oscillator signal from V4 (6SA7) will be mixed in the mixer stage V3 (6SA7) to end up in a 455 kHz intermediate frequency. The coil and crystal IF filters have to be passed between two IF amplifier stages (6L7 and 6SK7). The diode of V7 (6B8) acts as AM demodulator, here, the signal voltage will act on the AVC (automatic gain control) and drives the signal strength meter. The AF driver 6SC7 will amplify the audio frequency signal until it reaches the push-pull final stage with 2 x 6V6GT. As the audio output connectors have high impedance, You have to connect to a speaker with integrated 600 / 8 ohms transformer.

In practical use, I can more easily use the SX-28 for DXing as the earlier owner has connected a frequency counter. Otherwise finding a station on a known frequency might turn out quite difficult, as long as cannot identify the interval signal ar hear a spoken identification. Use the main tuning dial for coarse tuning and adjust the bandspread control for optimum readability. There are marks to which the main tuning dial can be set at, so that the bandspread dial will directly read the frequencies of the amateur bands.
The SX-28 will accept random wire antennas up to 20 or 30 m without overloading, sensitivity is good in comparison with more modern receivers. A major problem is the poor selectivity of the LC filters acting on the 455 kHz intermediate frequency. Sometimes You can eliminate an interfering adjacent channel signal with the crystal filter set to broad position. You can easily eliminate interference during CW reception, when You set the receiver to Crystal Filter Narrow and carefully adjust the Crystal Phasing control. Reception of single sideband transmissions is possible using the BFO, but the receiver has no product detector with selectable USB/LSB reception. According to John Bryant, there existed a special version of the SX-28 using two intermediate frequencies and narrow band filters for selective SSB reception, this was built for the FCC.
Due to the single conversion design, the set is subjected to the risk of spurious signals, use an external preselector to prevent this.
The audio performance of this set on AM is superb, specially with the BASS BOOST option activated.

These minor drawbacks are compensated by the joy of using a sixty year old set with a superb Art Deco design. As the SX-28 / SX-28A has been made in large numbers and has been sent to Russia according to the "Lend Lease Act", not many examples of this set have made their way to Switzerland.

© Martin Boesch

further reading:
d / e: Hallicrafters SX-28 at www.radiomuseum.org
e: the Hallicrafters SX-28 - the classic Shortwave Receiver, John Bryant, fine tuning proceedings 1992/93
e: Dachis, Chuck: Radios by Hallicrafters, Schiffer Publishing Ltd, Atglen, PA 19310, USA
e: Osterman, Fred: Receivers Past & Present, Universal Radio Research, Reynoldsburg, OH 43068, USA, ISBN 1-882123-07-7