Posted by
Kenneth Christiansen on
Oct 23, 2010; 9:07pm
URL: http://elecraft.85.s1.nabble.com/Prevent-Snow-Dirt-Static-damage-to-KPA100-after-upgrade-tp5666395.html
40 years ago we had a windy dust storm in Bismarck ND and I lost the RF
amplifier in my new solid state Heathkit receiver three times that day
before I realized the dirt and high winds were putting a static charge
on my antenna. I found there was a capacitor between the antenna and the
RF amp and the static would arc across the capacitor and burn out the
RF amp. Of course tube type receivers did not have this problem. I found
out a 100K resistor from the antenna to ground would prevent this and
not load down the antenna.
Several years ago my friend I each bought a new IC-746PRO. The first
time we had a blizzard with 50 MPH winds and snow my friend and I both
burned out our new rigs with snow static trying to check into the ND
Weather Net. The K2 survived just fine. The IC-746PROs were both fixed
under warranty but it still cost both of us shipping. I found out the
IC-746PROs did not have any path to discharge the antenna charge from
the snow static.
I recently installed the KPA100 Upgrade and one of the steps was to
remove R10 the 100uh choke between the antenna jack and ground. There is
no other path to ground and I have read on the reflector about people
having their SWR and Power measuring diodes go bad and it was easy to
see why. I plan to use this rig to check into the ND Weather net this
winter so knew I needed to fix the problem.
MFJ on my remote tuner uses a 40 K resistor to protect their circuit.
That is less than the 100 K I have used over the years so it should
protect even better.
I put in the following change to fix my problem. I bought four 10K half
watt resistors in a package from Radio Shack for $1.99. I soldered them
in series and soldered one end to the center pint of the antenna jack J2
and the other end to ground. I calculate that the resistors would
withstand a 16 to 1 VSWR and I do not think the KPA100 would be able to
put out 100 watts to a 16 to 1 VSWR so the resistors should be safe. 40K
should protect the diodes from any problem and under normal conditions
should dissipate 0.2 watts.
There is an easy way to protect the amplifier without making a
modification to the amplifier it self. Buy a PL259 T adapter and a PL259
plug. Solder one end of the 40 K series resistor to the center of the
PL259 and the other end to the shell. The resistors can be covered by
tape to prevent an accidental RF burn. Screw the PL259 /40K assembly to
one side of the T adapter. Put the other two ports of the T adapter in
series with the coax to the antenna or the coax to the 50 ohm side of a
tuner.
The one remaining precaution is to touch both sides of the antenna lead
to ground just before hooking them up to discharge any static
electricity on the antenna. This fix will not protect the equipment from
a lightning strike but it does allow me to use my rig to check into the
ND Weather net during a blizzard.
I hope this can save someone else from having rig trouble after a high
wind snow or dust storm.
73 for now
Ken W0CZ
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