Lincoln Kit

     The Lincoln superhet was introduced in December 1926. The components shown above consist of the Lincoln “Fixt Inductance” (oscillator coupler), and IF transformers numbered “1,” “2,” “3,” and “4.” I do not have the original boxes for two of the transformers. The Lincoln appears to operate on the standard superhet circuit of the 1920s, but it looks like they were trying to do something different with the characteristics of their IF transformers.
     Fortunately, my set of transformers were in good working condition when I acquired them. Collector Norman Braithwaite wasn’t so lucky. His transformer marked “1” had an open winding on one of the coils and he had to painstakingly disassemble the transformer to repair it. This type of transformer is probably one of the more difficult transformers to repair. Much of the information I have on the Lincoln kit was obtained from Norman Braithwaite.

     Above: Another view of the “Fixt Inductance.”

     Above: view of the Lincoln “1” transformer disassembled.

     At left: an inside view of the Lincoln “1” transformer. The primary and secondary windings are scramble wound in bifilar fashion.

     Photos above and left courtesy of Norman Braithwaite.

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     The table below lists the electrical parameters that I measured on my set of Lincoln transformers. It appears that the “1” and “4” transformers are essentially wide-band untuned transformers, and the “2” and “3” transformers are narrow-band tuned transformers. The Pressley superheterodyne from late 1924 and the Scott superheterodyne using the 400 series IF transformers used a similar technique of staggering the wide-band and the narrow-band IF transformers.
     The Lincoln set of transformers are very unusual because the peak frequencies of the narrow-band “2” and “3” transformers do not match the peak frequencies of the wide-band “1” and “4” transformers. The set of four transformers actually peak at three different frequencies! The unusual bandpass response is not due to faulty transformers because my transformers have essentially the same resistance values as Norman Braithwaite’s transformers. There’s not very much that can go wrong with these transformers if their windings have the correct resistance. The lower bandpass point of the “4” transformer is low enough that the transformer should work OK with the “2” and “3” transformers. However, the lower bandpass point of the “1” transformer drops off before it reaches the upper bandpass points of the “2” and “3” transformers. How good can this be? Lincoln may have been trying to accomplish some type of impedance matching with the input and output of their IF amplifier, or they could have been trying to widen the overall response of the IF amplifier. Who knows?
     Lincoln is not the only company that had IF transformers that peaked at different frequencies. The Sangamo IF transformers used in the Pressley superheterodyne peaked at two different frequencies; the narrow-band AT-60 transformers peak at 65 KC while the wide-band IF-60 transformers peak at 44 KC.

     Norman Braithwaite also provided the following resistance measurements from his set of transformers.

     Sales literature for the Lincoln superheterodyne. This is a later version of the kit that includes the IF transformer labeled “X.” The circuit has a few slight modifications from the earlier standard 1920s circuit. Photo courtesy of Norman Braithwaite.