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The FBIS is covered in the January 1945 issue. That article includes a picture of a wall with twenty rack-mounted SX-28 receivers. After the War, the SX-28/28A continued to be used by the FCC to monitor domestic radio transmitters. The September 1946 issue of Radio News, page 92, shows a picture of one installed in an automobile equipped for monitoring by the FCC.
The SX-28A is a very similar receiver. It is also claimed that many receivers labeled as SX-28 are really SX-28A. According to some, any SX-28 with the factory fuse holder installed is actually the SX-28A. This does not necessarily follow. An article by Radio News in the February 1943 issue clearly shows the fuse holder in a picture of the back panel as well as the fuse in the published schematic portion. Had the SX-28 actually been the SX-28A, I find it hard to believe that the Radio News editors, authors or letters to the editor would not have identified it as such. The fuse was simply an added feature in later versions of the SX-28. The timing of the addition of the fuse and possibly some other refinements may be correlated to the Radio News advertisements from November 1941 to July 1942 of the "new 1942 model of the SX-28". The price of the "new" 1942 model went to $179.50.
One cosmetic distinctive between the SX-28 and the SX-28A, according to the circuits and pictorials of the two models as published in the 12th and 16th volumes of Rider's Perpetual Troubleshooters Manual and the Halllicrafter manuals for both, is the standby socket on the rear panel. The SX-28 has one (a small rectangular AC outlet) and the SX-28A does not. That stand-by socket is mounted vertically on some SX-28 chassis and horizontally on others. The function performed by the socket can easily be duplicated using the external power socket, justifing its removal in the "A" version. The other cosmetic distinctive is thought to be the tuning and bandspread knobs. The SX-28 knobs have openings between the spokes. The SX-28A knobs have the areas between the spokes filled in. Whether this is a clean distinctive is somewhat doubtful.
The major electronic difference is the placement, style, and servicability of the coils and trimmers around the bandswitch. This will be obvious to anyone attempting alignment. One cannot use the pictorial location of the coils for one model when aligning the other. Note that some "A" version manuals merely repeat the original SX-28 schematic (which shows the standby socket even though the receiver may not have one), further adding to the confusion. A later version of the SX-28A is also distinctive, using a 6AB7 instead of a 6SK7 as the second RF amp. Details of that modification are in the 16th Rider volume. Both the SX-28 and the other SX-28A version also use the 6AB7 for first RF and Lamb noise silencer amplifier. However the earliest SX-28 uses the 6SK7 for all of these functions. The 6AB7 is not just a plug-in replacement as the screen voltage requires a change in circuit. It would be interesting to track some of the variations within the SX-28/28A family relative to serial numbers.
The PM-23 speaker is a match for the SX-28 as well as the Hallicrafter SX-23, 24, and 25. Many PM-23 speakers were made without the stylized bright chrome "h" on the lower left side of the grille. I assume the elimination of the "h" was either a cost-saving measure or a war-related saving of chromium.