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Author Topic: Bakelite Dial Restoration  (Read 7856 times)
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« on: March 13, 2011, 08:18:20 PM »

Radio equipment from the 1920s and early 30s often uses moulded bakelite dials, with paint-filled depressions forming the markings.  More often than not, an original piece of equipment or a used dial will show deterioration of the painted markings.  The paint may be dirty and faded, or else pieces of paint have become detached and flaked from the dial, or both.

A typical dial may be a simple all-in-one bakelite disc with a set-screw and bushing or a screw-on clamping mechanism to attach it to a shaft, or it may be part of a planetary vernier dial mechanism.  All-in-one dials are often seen on 1920s era TRF broadcast receivers. Planetary vernier dials are often seen on late 20s and 1930s ham equipment.  Probably the most popular planetary vernier was manufactured by National, and is known as the type A Velvet Vernier. The most often seen type A dials are 4" in diameter, made of black bakelite. A brown all-in-one bakelite dial is often seen on early Atwater-Kent broadcast sets.

The 4" black bakelite National  dials come in three distinct versions.  Two have the Velvet Vernier logo on the bottom part of the dial, and the other doesn't.  I believe the older original version is the one with the logo.  Another difference between early and later versions is that the originals have one set screw on the knob.  Later, post WW2 ones, have two set screws 90 apart on the shaft.

Of the ones with the logo, there are two varieties: one has very neat, distinct calibration markings/numbers, while the other has rather sloppy ones.  At first I thought the ones I had with the sloppy markings had been damaged when Hammy Hambone tried to remove the original white or golden paint with a sharp metal object like a needle (something you commonly see done to a lot of the hamfest finds), but after removing the original flaky paint and repainting the markings on many of these, I concluded that they were that way originally, sloppily cast at the factory.  There are also some differences in the way the hub where the dial plate screws to the vernier assembly was cast. All the type A dials I have seen without the Velvet Vernier logo have had the better, more distinct markings.

Regardless of the type of dial in need of repair, here are some hints for a successful restoration without further damaging the dial.

First of all, NEVER clean out the old paint with a sharp metallic object.  Sharpen a wooden or plastic toothpick, and it helps to use paint stripper.  From my experience, paint stripper does not damage the bakelite (but it might not be a bad idea to test your stripper on a tiny spot on the back side of the dial first, just in case; manufacturers reformulate chemical products all the time with no warning). Secondly, NEVER NEVER EVER clean these dials with a cleanser  like Fantastik or 409.  It will wash away the glossy skin of the bakelite and expose the filler, which is finely-ground sawdust.  Once the filler is exposed, the sheen is lost for ever.  Nothing will ever restore it back to original. It is impossible to polish sawdust.

The older original National type A planetary mechanisms look to be brass plated.  Later versions are  mechanically identical, except they are nickel plated.  Some of the nickel plated mechanisms take fine threaded screws while the older ones use coarse threaded screws, #4 IIRC.

To restore the markings on the National or any similar bakelite dial, after stripping the painted markings down to bare bakelite, use a white (or gold) lacquer stick, available from Antique Electronic Supply in Tempe, AZ.  Rub the stick over the cleaned-out markings, then use a soft cloth to carefully wipe away the paint from the surface of the dial, leaving it embedded in the grooved impressions that form the markings.  It may take several attempts to get this right, completely removing all the paint from the surface while leaving the impressions thoroughly filled in.  Lacking a lacquer stick, I have successfully used white oil-based artist's paint.  Something that helps is to dampen (but not saturate) the cleaning rag with mineral spirits.

When mounting some National type A dials, I have found it necessary to place a set of thin flat #4 washers between the dial plate and the vernier mechanism to space the dial far enough away from the panel to keep it from dragging.

National also made a smaller version of the type A dial, with a smaller bakelite disc about 3 inches in diameter, but otherwise identical to the larger dial. Shortly after WWII, the type A dial was replaced by the type AM (type A Metal) dial with a nickel plated metal skirt. The  newer type AM dial is about the same size as the smaller bakelite type A.

Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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