If you reduce the air gap even with “insulation” the breakdown voltage goes
DOWN not up. Now sometimes , if the air gap is sufficient , the “insulation”
will modify the field of a sharp point or rough areas on a surface enough to
keep it from breaking down – but it’s the field modification – not the
“insulation” .
Don’t feel bad , others had not thought about that either - years ago – on a
military project – an air variable we had was arcing , and the capacitor vendor
said he had the solution , he made us a special by putting teflon coating on
the rotor and stator plates – it broke down at a lower voltage than before
!!!!! After the fact it was obvious – but we all suckered ourselves in .
But having said all that some Glyptal or about anything that will smooth that
area – cover any sharp points and not decrease the air gap too much may just
work.
My recollection of that cap is it is made by TT or someone for them with
spacers and plates all bolted together – you might be able to get a new
plate/plates – or if you tore it apart you might be able to get the area very
smooth . If it is out near the edge you could just remove that area completely
and smooth off the cut edge .
73 Hank K7HP
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