Andre,
Looking at the picture of the burned coax I think I see the problem. It may not
be a capacitor current issue but a capacitor voltage issue. It appears that the
ends arced. This will take very roughly 10 kV RMS for the way you have the ends
dressed. To fix this build new hard line capacitors and this time run the inner
insulation 2 cm beyond the shield (at both ends).
While running the rig someone can observe the tuner to see if there is an
corona discharge happening at the ends of the coaxial caps. If there is you
need to reduce the field stress some more. This can be done by extending the
inner conductor 1/2 cm beyond the inner insulation. Dressing the cut ends of
the inner conductors with a file to remove burrs. Then tin them to further
smooth the edges.
If more electric field reduction is needed you can fashion corona balls using
solder. To do this you apply the soldering iron the the wire. Then, while still
feeding solder (it is a ball by now about 1/2 to 1 cm diameter) remove the iron
first and then the solder wire. With some practice you can make some nice round
corona balls. What you are trying to do is to reduce the field stress at any
point to below 35 kV/cm.
Some might tell you to apply RTV. This will work temporarily but due to partial
discharges within the RTV (inside gas bubbles) the RTV will fail after awhile.
It might hold up in intermittent amateur use and then it might not.
Dave WX7G
_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband
|