The TH7DX first sold in 1982. The dark aluminum is not a problem on the
exposed surfaces, it is likely oxidation. When you take the antenna apart,
clean the mating surfaces (inside and outside) with a Scotchpad,
steel-wool, or other type of light abrasive until the metal is shiny. Use
Penetrox-A or another type of anti-oxident (for aluminum) on these surfaces
when re-installing. ARRL has a list of available anti-oxidents.
The hardware used on the TH7DX has always been Stainless-steel except for
the 4 long boom-to-mast bolts. There was a conversion kit (product 392s)
available from 1982-1986 which allowed a TH6DXX to be converted to a TH7DX.
This kit also included all new SS hardware. Sometime in the mid-80's, the
SS U-style element compression clamps were replaced with standard all-SS
compression (hose) clamps. Unless the SS bolts have galled or stripped
threads, these do not normally have to be replaced. Use heavy motor oil on
SS threads to prevent galling. The 4 long boom-to-mast bolts were
originally cadmium-plated, but are currently SS.
Inspect the trap-caps. If this is an old antenna, or if it has been in a
very sunny environment these black plastic caps on the trap ends will need
replaced. Also, the outboard caps should have a 0.25 inch drain hole in
the sloping surface for water drainaige. This hole needs to be lined up
with the drain holes in the trap body, and directed towards the ground when
installed. Inspect the screw inside one end of each trap. This
self-tapping screw shorts the outer capacitor shell to the inner tubing on
the inboard end of the trap. Before abt 1990, this screw was plated. If
SS, then tighten. If plated, you may wish to replace. If you see evidence
of bugs or insects inside the traps, use compressed air to blow out their
nests, etc. If you have a grid-dip meter, check the traps for resonance.
The meter will show a resonance of approx. 27.6 MHz on the short 10 meter
traps and 20.6 MHz on the 15 meter traps.
The monoband elements of the TH7DX used a plastic rope in the tips to
prevent or reduce wind vibrations. This is recommended.
The TH7DX comes with the BN-86 balun. This may need to be replaced if
damaged. We also recommend upgrading to the BN-4000 or Palomar beam-balun
or homemade RF choke if very-high power is to be used. Also it is
recommended for club stations or multi-op stations where lots of different
people operate. (The BN-86 may arc if used on the wrong band with high
power). The BN-86 is rated at 1500 watts PEP with VSWR less than 2:1.
In the 1990's we added a "DX" setting to the TH7DX. RDE-2=33 inches (838
mm) and FDE-2=42 inches (1067 mm) this lowers the VSWR curves on all 3
bands but does not affect the gain or patterns. New resonant freq's are
14.140, 21.250, and 28.950 MHz. The bottom edge of each band ends up at
approx. 1.5:1 VSWR.
This is all I can think of right now.
73 Roger WB0DGF (Hy-Gain)
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