Live To Air – National NC-101X finally back together tonight newly installed coil rack, with the bronze bushings and fabricated band indicator. Slides like it’s on glass, super smooth and low effort.
Listening to the melodious tones of the old Gray Hair Net, using Amplitude Modulation, on 160 meters shortwave. Exact frequency is 1.945 MHz. Not bad for a 80 year old radio, I think the original owner W1KEK would be pleased to know that his treasured set, that he bought back in 1937, has returned to the air.
Very pleased with how this project turned out. The audio is very sweet, with some nice wide bandwidth for hifi AM audio. Just waiting for the electrolytic filter capacitors to finish the project.
Here’s the solution to the sliding coil deteriorated bushing problem on the National NC-101X shortwave receiver. Unlike that other National products with “plug in” coils, like the HRO, the early NC-100 series use a “catacomb” coil assembly that slides inside the chassis to switch bands. A very ingenious design, and it works very well..for 80 years or so, and then the plastic bushings deteriorate from age and use.
The chances of finding new old stock bushings is slim to none, and even if they were available, they’d be just as deteriorated as the ones inside my receiver.
The solution, order a 1/2” to 9/16” bronze bushing from McMaster Carr, machine it down so the outside diameter is .512 inch, press in place using Loctite to retain, ream the ID so the shaft slides through the bushings freely.
I’d like to thank Marc KA2QFX for his very gracious and kind skills in the machine shop, machining the new bushings to fit properly into the sliding coil form.
After two long months, I can finally see the end of the NC-101 project in sight.
Fabricating a new band indicator became a necessity since spare parts for a 80 year old radio are just not something you can find at your local Radio Shack, if you can even find a Radio Shack.
So the old sheet metal, aluminum actually, band indicator, which provides a white dot in each of the band windows as the coil rack moves, was literally falling apart. Given the flimsy nature of the failed unit, I almost wondered if someone made this one.
So, one of the final steps on this project, now that the bushings are installed in the moving coil housing, was to fix or fabricate a new band indicator.
A trip to my local hardware store yielded me some nice 22ga sheet metal to start the project. Thin enough to work with on the bench with simple hand tools, yet rigid enough to last another 80 years. To get the correct form, I straightened the old one out, and used it as a template for the new replacement, scribing around it.
Carefully I trimmed it out, and then used a file for the final shape and deburring. Several coats of white Krylon did the trick.
Better than it was before, better, stronger and faster.
While this is for the 101X, most it likely applies and hold true for any of the National moving coil products.
What had occurred to this unit, is that the plastic bushings had been brittle, loose and broken over the last 80 years, the coil became unusually binding when I attempted to move it. Concern that I might damage one of the unobtainable coil sets, or the contact fingers, let me to a solution. Since National is no more, I figured I could find a current product, and adapt that to the application. McMaster-Carr is an excellent source of such items, and small hardware, and a quick search of their online catalog found several possible candidates. Since the shipping was more than the $5 parts, I ordered two sets of the plastic and one set of the bronze bushings to try.
One thing I did have to satisfy myself on, was if the coil Assembly itself was a ground potential. Being insulted by the plastic bushings, I wasn’t sure if the coil housing was actually electrically insulated. Having another NC-101 on hand made it pretty straightforward work, I removed the bottom panel and using a ohm meter verified that there was continuity from ground to the coil assembly itself. I was glad to discover this, because it meant I could use bronze bushings without fear of grounding the sliding coil.
Step by Step Removal of the Sliding Coil Assembly
The first step is to remove the bottom cover of the unit, this will expose the moving coil.
Then the cabinet needs to be removed, which necessitates removal of the knobs, and disconnecting the s meter.
The next step is the front rack and gear which moves the coil back-and-forth. You can actually start with this step if you want. There is a nut on the front panel it needs to be loosened and some hardware, including two insulator bushings, that attach a gear to the front panel. You’ll need to remove the top part of the sliding coil housing was necessitates removing six screws, so you can slide the gear up over the housing for removal. With that done, entire sliding coil assembly should lift free of the chassis.
With a cabinet removed on the rear the chassis there are two flat head screws which retain the shaft on which the coil rack slides. Removal of the screws may take some effort, they likely been in there for over a years, and you don’t want to strip them out. The use of an impact screwdriver with the proper size bit, would be recommended.
Once you have the screws loose on the sides, the rod will still be retained has there are two dimples in the chassis which fit into the countersunk ends of the rod. You’ll also note that there is a small braided ground cable, which is retained to the rod via a small machine screw and nut. Be sure to remove these, before attempting to remove the coil.
Once you have the braided cable loose, using a large screwdriver carefully pry out the shaft from the counter sunk ends. It will take some effort, but the shaft will finally pop free.
With the the shaft now for free of the chassis, you can carefully pick the sliding coil rack out from the receiver. I would recommend putting the cover back on the top of the catacomb, just so no stress is put on the fragile coil forms while working with it on the bench.
In regards to the bushings, you’ll likely find that the bushings size is slightly larger any opening in a moving coil assembly. You’ll need to do one of two things, either machine be opening in a coil assembly to a slightly larger diameter to except the bushings, or machine the bushings to a slightly smaller outside diameter, for a tight press fit into the coil assembly. Either choice is acceptable, since the original bushings lasted 80 years, it is unlikely that replacements will be required again, especially if the bronze style bushing as fitted.
Progress continues tonight on the National NC-101X project. First real desoldering and parts replacement work. Remove and replaced dry rotted old line cord and chassis feed through grommet. Installed new a three wire line cord, with grounding chassis, for electrical safety.
Tomorrow evening I start with replacement of the old wax paper capacitors, 21 total. Then onto the replacement electrolytic, reinstallation of the moving coil assembly, paint some Extend rust convertor on the bare chassis spots, and finish off with an alignment.
I’m always intrigued when some former owner leaves his handiwork or mark on radio gear from the past. I recently came across the HRO receiver coil and caught my eye how much effort the previous user had put into making up a logging scale for the set. He must have been an avid listener to have taken the time to create such. I imagine that one could go back through the station list and get a pretty good idea of the year and even location of the previous owner.
Had a few minutes this morning to get Scott W9WFA’s old National NC-101X out of storage and onto the new 1930s station desk. I’ll be moving the other 101X over once the restoration is finished, but it’ll be on the workbench for a bit, so time to fire up something else.
Listening to 40 meter CW on the National NC-101X receiver with the green “magic eye” tuning indicator. Bands are pretty lousy this morning, much static.
Transmitter is a MOPA (Master Oscillator Power Amplifier) built by amateur radio operator AB9ZG, modeled after one of the early Aero designs.
CW “Bug” (keying device for transmitter) is an early Vibroplex “Zephyr” model built in New York City. Believe this belonged to my Elmer Dick W2UJR. “Cans” (headphones) are vintage Brandes “Superior” model, built also in New York City in the late 1920s or early 30s.
Antenna is a traditional balanced feed line 160 meter dipole up about 50’, right on the coast.
Photos on wall are of a teenage ham radio operator from the 1930s who lived in Kennebunk, Maine. The other of some young radio hams from Syracuse, NY in a home built ham shack.