Plug-in Coil forms for the Radio Builder

“Back in the days” home built radio receivers and transmitters often used plug in coils. Plug in coils were used with variable capacitors to create an “L-C” circuit (L being the inductance and C being the capacitance).  The coil-capacitor combination was connected in parallel to tune the cicuit. To narrow the range even further a second variable capacitor would be wired in parallel with the LC circuit.  This  second variable would be of low value and would change the resonant frequency of the circuit a very small amount. It would, in effect be the “Band Spread” of the tuning circuit. One more obvious example was the wonderful old Hallicrafters Recievers. They had the main tuning dial on the left, and a bandspread dial on the right.

Radio amateurs are interested in very small portions of the radio spectrum. These small frequency allocations were best tuned so that the whole CW or Speech portion of the Amateur band would cover the entire swing of the variable tuning capacitor.  By narowing the coverage,  hams could spread out the signals.  While many receivers used a complex switch to change frequencies.  The plug in coil was simpler, less costly, and a favorite amoung our predecessors.

Pastime Projects (www.pastimeprojects.com) introduced new plug in coil forms in October of 2010.  The dimensions and construction of our coils is shown in the attached sketch.

The Pastme Projects coil form is available either glued or unglued. The joint is edge glued between the coil form and the octal base. The glue used in the factory is Loctite Stik n’ Seal Non-Toxic adhesive manufactured by Henkel Corp of Avon Ohio  www.loctiteproducts.com

While the coil form of old may have used 4 pin or 5 pin sockets, these have become scarce.  The use of the octal base makes more sense and give us some choices in the amout of taps and/or coils we can combine on one form.  An obvious disadvantage of the octal socket is more friction in inserting or removing the coil from the base.  We have found that the extra unused pins can be carefully cut off.  We have easily removed every other pin and ended up with a 4 pin socket which is much easier to work with.  It still allows the allignment of the coil into the correct pins in the base because of the keyed center pin on octal bases.  You can choose how many pins you need on your coil for your project. 

 

Why not include plug in coil forms in your next project?  Vry 73 es bcnu Glenn for Pastime Projects.

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2 Responses to “Plug-in Coil forms for the Radio Builder”

  1. Nathan Harrison Says:

    Lovely website, it loads very fast and looks really awesome.

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