|[AMRadio] Need 75/80M Final Tank Coil for BC610E|
k4kyv at charter.net
Mon Jan 27 11:57:37 EST 2014
>>>Need one in good condx...not falling apart...not looking for a fixer
one..I have that...This is the large dual coil plug in assembly with the
adjustable link in the center....thanks for any help...Jeff...W0XV >>>
You may have to bite the bullet and repair the one you have, or one in
better shape if you can find it.
Those coils used a plastic substance called cellulose acetate for the
insulative strips. I believe it's close to the same stuff as celluloid, and
shares a common characteristic: it self-destructs over time, turning brown,
shrinking and finally decomposing into a powdery substance. Just like the
classic movie films that have self-destructed while in storage. B&W
continued to use this material to make their air-core coils, while EF
Johnson, Bud and National used polystyrene. The latter is much more stable
and does not deteriorate with age if not exposed to UV. I often see Bud,
National and Johnson coils at hamfests still in perfect condition.
Unfortunately, all the BC-610 coils I have seen were made by B&W, and now
most of those are falling apart. Sometime the shrinkage of the plastic is
enough to break the ceramic bar that holds the banana plugs. The
deterioration also causes a degradation of the dielectric properties of the
plastic. I once had the plastic strips on an HDVL coil literally burst into
flames while I was transmitting.
The most convenient way to fix the coils is to cut out replacement strips
from plexiglass sheet, and use plexiglass glue to secure them in place on
the wire. Plexiglass, an acrylic plastic is not the best insulator in the
world, however. Polystyrene would be superior, but it appears that solid
polystyrene sheet has become unobtanium. I checked several vendors of
plastic material, and the only polystyrene I could find was the foam
variety, like what is used to make plastic coffee cups and packing material,
not solid transparent sheets or rods. Years ago, solid polystyrene was
readily available at electronic suppliers. I recall buying mine from
Berstein-Applebee in Kansas City. You could also buy polystyrene glue,
similar to plexiglass glue for bonding the pieces together.
Polystyrene is not for every application, however. I once made the mistake
of using polystyrene rods for open wire spreaders. It deteriorates very
rapidly when exposed to UV. In about a year, the rods had turned to a brown
powder, similar to what happens to the cellulose acetate. Plexiglass works
much better for spreaders because it is highly resistant to UV damage. So,
best to use plexiglass outdoors, and polystyrene in localities not exposed
to too much daylight, if only one could find the stuff.
If anyone knows of a present-day source of solid polystyrene, please let me
I believe, but not 100% sure, that polycarbonate may be a better dielectric
insulator than plexiglass. It is certainly more rugged and less prone to
cracking and breaking, so that may be an option as well.
If the coils are not too badly deteriorated, I have repaired them using
Two-Ton Clear epoxy, but that does not stop the plastic from deteriorating
further and losing its dielectric properties.
Probably more than 90% of the BC-610 coils you find to-day will have
deteriorated plastic, even if they are unused, still sealed in the original
box. Maybe if they were stored in the optimum humidity and temperature
environment they will be in better shape.
I have seen both the rotatable link inside the coil and the fixed link
coils, although all the BC-610s I have ever had my hands on used the
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