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If there is an obvious carbon track and it won't clean off you can drill out or grind the carbon track off and rebuild with silicone caulk...The HV rect bottom socket cups in some sets will arc through and carbonize and that is my typical solution to that...The silicone caulk has a higher dielectric strength than the average plastic they used back then so it at least in the case of carbonized holes is usually a sound solution...YMMV.
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