Summary

By shear chance, I obtained three almost identical Motorolla "Goldenview" televisions with 7-inch picture tubes from 1948. One had its knobs, the other two did not. Replica knobs are available for purchase, but they are resin-casted. That would be fine, except that the knobs include gold-colored, painted, concentric circles. On the original knobs, and hence on the casted replicas, the circles are raised ridges, and not grooves. That makes them almost impossible to paint by hand! I tried casting a replica of one of my original knobs, and the hand-painting did not turn out well at all.

So instead, I rendered the knobs in 3D, and printed them on my Halot 1 resin printer. This gave me the freedom to include circular grooves instead of ridges, which are much easier to paint, and look just like the originals. The only hard part was to find a way to resin print very dark brown plastic. Since resin printing depends on UV light being able to penetrate into the resin, there are very few opaque resins, and very few dark colored dyes that are intended for resin printing. I finally purchased some "mahogany" resin from SunLu, and then darkened it still further with some chocolate brown pigment paste, as well as some black, and even some "wine" red pigment paste, all from Ceya. Pigment pastes are intended for resin casting, and are known to interfere with UV resin printing. Indeed, my first attempt using an opaque white resin failed. But when I started with the medium brown "mahogany" resin, I did not need as much of the pigment paste, and I was able to compensate by extending the UV layer exposure time to 7 seconds, and the initial layers to 10 layers at 60 seconds each. That did the trick!

The very high printing resolution of the Halot 1 (0.01mm) produces very smooth surfaces, and accurately forms the "knurls" in the shaft hole. A standard filament printer could never match that.

After printing and curing the knobs, I sprayed them with clear acrylic, allowed them to dry for 24 hours, and then carefully applied the gold paint, scraping off the excess paint outside of the grooves with my fingernail. I'm very pleased with how they turned out.

I've included photos of both TVs with the knobs installed. As you can see, the two TV sets require slightly different knobs. I've included all of the different variants here.