The HRO wasn't hassle-free though and Millen realized that for the SWL (Short Wave Listener) and intermediate-level hams, in other words, those who didn't have the experience or couldn't afford the 0 HRO receiver, there had to be a design that would provide the excellent performance of plug-in coils without all of the hassles and expense.
- National's mechanical engineers offered a solution that solved most of the negatives of plug-in coils and retained most of the advantages.
The mechanical action simulated plugging in a three coil set for each band with the ease of turning a knob while keeping all of the unused coils isolated and shielded.
"Switching noise" was eliminated by routing the RF and IF amplifier screen voltage through the foremost pin split-contacts of the LO coil section.
Another portion of the design involved the PW gear drive used on the NC-100 series.
A large band selector knob on the front panel of the receiver would turn a rack and pinion gear mechanism that would move the coil catacomb into place, thus engaging the proper coil set pins into short, fixed position, spring-contacts mounted under the tuning condenser in insulator blocks.
When shipped, the coil catacomb was screwed to one side where a guide pin was located to prevent any damage due to rough handling.
When the receiver was installed, this screw had to be removed to "unlock" the coil catacomb.
Due to the nature of physically moving a metal box underneath the receiver's chassis the catacomb width dimension was limited by the chassis width and five tuning ranges were what could be fit into a metal box half the width of the chassis.
A finely finished round metal rod that ran the full length of the chassis was mounted at the rear of the chassis to act as a rear bearing for the catacomb.