Simple Mod Restores CW On Hallicrafters S-38E's
To make this story short, I'll just say that my first three S-38E's worked fine, except none of them worked when you flipped the AM/CW switch to CW. This was clearly a problem related more to design than to a part or adjustment being defective. When Hallicrafters switched from using octal tubes to 7 pin minature tubes, they kept the same basic schematic design only Murphys Law stepped in and changed everything, possibly because of internal capacitance of the tubes. Let's digress for just a moment. When maufacturers needed a very small value of capacitance, they often resorted to making a "gimmick" capacitor. Simply, you can make a gimmick by just twisting two insulated pieces of hookup wire together. You get roughly 2 to 3 pf of capacitance per inch of twisted wire. Hallicrafters used the idea of gimmicks for their CW on the B through E models. As pictured below left, notice the big brown wire winding through the bottom of the first IF can tie points (arrow) then curving around to the tie down and ending. Where that wire passes the IF can, a very tiny capacitance exists that is 1/2 of the feedback for this "BFO." Notice in the right picture I have taken the brown wire out of its tie down and moved it closer to the IF amp tube. I removed the copper wire going from the 1st IF to pin 1 of the IF amp tube and replaced it with a wire about 2 inches long that I wrapped 1 1/2 turns around the brown wire and reconnected it to the same points on the IF can and IF tube socket. I did the same thing to the other gimmick capacitor as shown in the next set of pictures below.
Below is the second half of this modification. In the picture on the left, notice the green wire sort of wrapped half way around the brown wire (arrow). This is the same in all S-38's from the A model to the E model. This picture of the E model, shows the 12AV6 detector tube socket on the left in the 1st picture below, wired as it came from the factory. I disconnected the end at the IF can and wrapped it around the brown wire as seen in the right photo, and reconnected it to the IF can. This very, very, tiny increase in capacitance was enough to provide the needed feedback to make Hallicrafters original idea work just right. I then modified my second S-38E in the same way and I now have two originally non-working CW circuits, working happily away.
Hallicrafters dropped this method for a simulated BFO in the later production units of the S-38E, switching to what they called a "ratio pot" installed on the chassis back, to adjust for the proper operation. That might also be a good mod to make but it would require drilling a hole in the chassis to add the ratio pot. Before you drill, try this nothing-to-buy method. It works!