[Hallicrafters] [Boatanchors] Variable Capacitors needed

[Carl]

Try a thin coat of Krylon Clear, you wont need much to build up 
dielectric strength.

Carl
KM1H


----- Original Message ----- 
From: "Robert Nickels" <ranickel at comcast.net>
To: "Jack Schrader" <w1jhs at myfairpoint.net>
Cc: "boatanchor network" <boatanchors at mailman.qth.net>; 
<W9RAN at oneradio.net>; <hallicrafters at mailman.qth.net>
Sent: Tuesday, February 17, 2009 2:09 PM
Subject: Re: [Boatanchors] [Hallicrafters] Variable Capacitors needed


I should have included a link to these photos of the capacitors in
question.

http://smg.photobucket.com/albums/v652/ranickel/Grungy%20Capacitor/

These were taken before and after I initially cleaned them by soaking in
the lime-rust remover "CLR".  That removed much of the grunge, but there
was still arcing between the plates (370 volts) so I tried mechanical
means,  running a folded strip of very fine grit sandpaper between each
set of meshed plates.   This cured the arcing - for a while - but now
it's back.    The appearance is slightly better than shown in the
"after" pics here but since the plates look clean and there's no visible
material in the airgap,  and since the arcing is occurring at various
points depending on the angle of rotation,  I suspect there are surface
irregularities that are causing the charge to build up and overcome the
dielectric strength of the air, which is 3 × 10^6  V/m, or ~76 volts per
1/1000th of an inch.   That means the specified plate spacing of .025"
should withstand 1900 volts, whereas mine are breaking down at 1/4 that.

I think the idea of applying a thin conformal coating makes sense, but
I'll glady accept further suggestions!

Thanks and 73,

Bob W9RAN
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