Hallicrafters SX-28A ‘Super Skyrider’ Receiver Introduction Around January 8, 1946, the Hallicrafters radio company manufactured Model SX-28A, Serial Number HA-30857 - just one in a very long production run of this remarkable communications receiver, the original SX-28 being announced in July, 1940, with production commencing in August, 1940. It has been estimated that around 16,850 SX- 28s and 10,300 SX-28As were manufactured over the 6 years of production (Henry Rogers on the Western Historic Radio Museum website – link below). Some 65 years later, HA-30857 found itself in Ladysmith on Vancouver Island, BC, and not in the best of sorts, having been unused for several years and with many of its components having suffered the ravages of time, probably scores of well-intentioned re-alignments (many likely inexpertly performed) and less-than-ideal storage conditions. Repair had been attempted by Bill West-Sells, a fellow CVRS1 member who resides on Vancouver Island and who has repaired/restored domestic receivers for decades, but who self-admittedly has little experience fixing communications receivers. Bill reported to me that the set had been pulling in a couple of the strongest local stations on the Broadcast Band but was virtually ‘deaf’ on all the Shortwave bands, even after him changing-out several waxed paper capacitors and subbing some tubes. Bill therefore recommended the set’s long-time owner contact someone with more experience in repairing communications receivers of this vintage for help. After a few emails and a phone call, the set was eventually delivered to the author in Coquitlam, BC for electronic repairs and a 1 Canadian Vintage Radio Society (http://www.canadianvintageradio.com/) Hallicrafters SX-28A Gerry O’Hara cosmetic ‘touch-up’ so the owner could once again use the set for shortwave listening and, possibly also on the amateur bands, for which the set is equipped with very useful bandspread for 10m, 20m, 40m and 80m. Homework I had never worked on a Hallicrafters set before, let alone the famous SX-28/28A, though I had coveted one of these classic receivers for many a year. Now, I must admit up-front this model is really the only Hallicrafters I had ever liked the look of – certainly many of their post war sets appear flimsy and not particularly interesting to me. So, starting from scratch I decided to gather some background information, schematics and manuals from web- based sources and to see what the SPARC Museum had in their library. My primary reference materials included the following websites and materials downloaded from links provided on them (schematics, manuals, notes, adverts, etc): http://www.radioblvd.com/SX28Notes.html (great notes on the SX28/28A, lots of background, serial number chronology, etc) http://antiqueradio.org/halli12.htm - well-illustrated SX-28 restoration article http://www.qsl.net/wa2whv/hallicrafters.html - includes restoration logs http://antiqueradio.org/SX-28Gearbox.htm - seminal article on refurbishing an SX-28 gearbox http://pcbunn.cacr.caltech.edu/jjb/Hallicrafters/SX28/SX-28.htm - has links to SX28/28A schematics, manuals (including the Military version) and Bill Feldman’s alignment procedure www.tuberadio.com/sx28/sx28.html - good description and photos of dismantling the coilbox on an SX-28A – but is it really necessary? (see later in this article) (paste the above ‘tuberadio.com’ link to your browser and check out the ‘restoration log’ link – some interesting information here) The military version of the SX-28A manual (the military designation for the SX-28A is AN/GRR2) contains much more servicing information than the ‘civvie’ one, including tables of resistance measurements and a more complete voltage table (be careful using this though as the voltages provided for V1 through V4 are with the tubes removed from their sockets). Not all of 2 Hallicrafters SX-28A Gerry O’Hara the resistance measurements make sense, for example that from the cathode of V9 to ground with the automatic noise limiter (ANL) control fully advanced should (according to the manual) read 450ohms, whereas it actually reads 33 ohms, which is correct if the schematic is studied. Also note that the schematics do not show component values and these have to be referenced from the component list, which rather annoyingly lists many components as being the same as others, so you have to then look up the cross- referenced part – what a good idea that was… (not!). It’s well worth spending a bit of time writing component values directly onto the schematic before working on the radio. There is a lot of useful material available to the would-be restorer and owner of these fine receivers – start with the links above and then you will find that Google really is your friend… Overview of the SX-28/28A Receiver The Hallicrafters SX-28 and SX-28A receivers are 15-tube single-conversion superheterodyne designs (nominal 455kHz IF), covering 550kHz through 42MHz in six bands: 550kHz – 1.6MHz 1.6MHz – 3.0MHz 3.0MHz – 5.8MHz 5.8MHz – 11MHz 11MHz – 21 MHz 21MHz – 42MHz In addition, an electronic bandspread facility is provided in a separate scale for the following frequency ranges, covering the 80m, 40m, 20m and 10m amateur bands: 3.5MHz – 4.0MHz 7.0MHz – 7.3MHz 14MHz – 14.45MHz 28MHz – 30MHz The set is designed to receive AM and CW signals, but with care, SSB signals can be received quite well. Controls include main and bandspread tuning, a six-position selectivity control (three positions with the single crystal filter in-circuit), crystal phasing, ANL, BFO, AGC on/off, antenna tune, RF gain, AF gain, tone, a ‘bass boost’ switch and a standby switch. 3 Hallicrafters SX-28A Gerry O’Hara Referring to the block diagram above, the circuit comprises two pentode RF stages (6AB7 and 6SK7 as pre-selectors), the first stage only used on frequencies above 3MHz. A 6SA7 ‘pentagrid’ (heptode) carries out mixer duty, and a second 6SA7 acts as the local oscillator. Two stages of IF amplification are provided, a 6L7 heptode followed by a 6SK7, feeding a 6B8 dual- diode pentode as detector/AGC/S-meter amplifier. Audio from the detector is fed to a 6SC7 dual triode doing duty as a pre-amplifier and phase-splitter, to two 6V6 output tubes in push-pull, giving 8W of audio. A second 6B8 is used as an AGC amplifier/detector for the RF and mixer stages, a second 6AB7 as the ANL noise amplifier and a 6H6 dual diode as the ANL rectifiers (see below). A 6J5 triode is used in the BFO circuit, which is loosely-coupled to the detector circuit with a ‘gimmick’ capacitor. The power supply is conventional, using a 5Z3 rectifier. The circuit includes some interesting innovations: there are two AGC circuits – one amplified, feeding the RF and mixer stages, and the other conventional, controlling the first IF stage. There are also two ANL circuits: the first provides for shunting of the detector diode load with a capacitor to momentarily bypass audio signals on noise peaks, the second amplifies the noise signal at the IF frequency and places an instantaneous negative potential on the 3rd grid of the first IF amplifier (a 6L7 heptode) at a level selected by the operator thus canceling out noise 4 Hallicrafters SX-28A Gerry O’Hara pulses – this is a version of the Lamb2 noise silencer (‘blanker’). The use of permeability-tuned RF transformers using iron-dust slugs was a fairly new innovation at the time, as was the variable-bandwidth IF and single crystal gate circuitry. Even the S-meter circuit is a little novel having its own amplifier stage, and the audio output impressively has a pair of 6V6 tubes in push-pull for great sounding audio, complete with a switchable bass boost circuit. The receiver is very solidly constructed on a stout steel chassis, weighing-in at 78lbs. The thick steel front panel is separate to the main chassis, with some controls being mounted directly to this. The RF circuits are contained in a series of screened compartments beneath the chassis, reducing heat build-up here, however, a temperature-compensating capacitor is included, this being a negative-temperature coefficient ceramic trimmer located in the tuning gang compartment above the chassis where the four RF tubes are located. The gearbox in the SX-28 uses gears to drive both the main and bandspread gangs, however, the SX-28A uses a dial cord to drive the bandspread gang. Cords are also used to operate band indicators on each of the two tuning dials on both models when the bandswitch is operated. Some other differences between the SX-28 and SX-28A include: - Construction of the RF circuits: in the SX-28A, ‘Hi-Q Micro Set’ coils with phenolic formers were mounted on Paxolin plates, whereas in the SX-28, the coil units were mounted onto the sub-chassis; - The coil box cover is screwed in place in the SX-28 and is held on with retaining clips in the SX-28A, the cover in this model also having a louvered top; - The SX-28 has open ‘spoke’-style tuning knobs and the SX-28A was fitted with closed-in ‘webbed’- style tuning knobs. The SX-28 was introduced in August, 1940 and stayed in production through 1944, with production then swapping to the SX-28A from late-1944/February 1945 through to July 1946, after an estimated total of 27,150 SX-28/SX28A’s were manufactured. The receiver was widely used by the military in WWII and in the post-war period quite rightly became a very sought-after and popular receiver within the amateur community.