It was well-known that the HRO's legendary performance was in-part due to its plug-in coil sets.
But, how to effectively eliminate the valid arguments against plug-in coil use in a new product?
Certainly, National was having fabulous success with the HRO receiver, which had been in production since early 1935.
Five marked holes in the front panel allowed viewing a white "flag" that was mounted to the catacomb and indicated which tuning range was selected.
When shipped, the coil catacomb was screwed to one side where a guide pin was located to prevent any damage due to rough handling.
This type of approach eliminated problems of lead length, shielding and stability along with isolation of unused tuned circuits - problems that were commonly found in broadcast receivers using conventional rotary bandswitches. Handling three individual coils for each band change, storage of the unused coils and how to remove B when changing coils all added to the counter-belief that plug-in coils were archaic.
James Millen, National Co.'s General Manager and Chief Engineer, was one of the designers that insisted the best receiver performance was achieved using plug-in coils.
were very popular and accounted for a lengthy production history.
The NC-100 Series "MOVING COIL" Receiver production spanned from 1936 up to 1949. Certainly the design of the "MOVING COIL" method for band change used in the NC-100 receivers impressed the commercial operators of the late thirties with modified versions being ordered by the Bureau of Air Commerce and the CAA for use in airports around the country (even as late as post-WWII.) The NC-100XA version impressed the U. Navy who ordered special versions with low Local Oscillator radiation that became the famous RAO family of WWII receivers.
To keep the costs down by keeping the physical size of the catacomb relatively small only three sets of coils were used per tuning range.