The AM Forum

I just finished rebuilding a multi-section ceramic rotary switch for the antenna tuner project.  I disassembled several switches that had identical 10-position ceramic rotary wafers and combined the hardware to form a 5-gang rotary switch by combining wafers onto one frame assembly and modifying the end bearing of another switch so that the shafts of the two switches can be ganged together.

The ceramic insulation used for the wafers as well as the insulated rotary shaft are made with unglazed porcelain.  After reassembling the modified switch, I noticed metal deposits all over the ceramic.  It seems that even lightly brushing nickel plated brass across unglazed ceramic leaves a metallic deposit, kind of like a pencil leaves a graphite deposit.  Part of the disassembly and re-assembly involved slipping brass sleeves over the ceramic rods and fastening the parts together with metal screws and bolts, so multiple metallic deposits ended up on the insulation between contacts that will be switching high voltage rf.

I was concerned about possible breakdown due to the metallic leakage path between HV contacts. If an arc occurred, it would likely leave carbon tracks impregnated in the ceramic, something I have found to be totally impossible to remove.  I have seen carbon tracks render tube sockets, HV standoffs and  feed-throughs, and ceramic switches useless.  I tried several different solvents and cleaners, as well as plain old soap and water, but nothing seemed to touch the contamination.

Then I tried "erasing" the metallic marks using the rubber on a #2 wooden pencil, and found that this removes them completely, along with the finger smudges left behind from handling the ceramic during assembly.  I carefully cleaned the entire switch assembly using this method and now the ceramic pieces look sparkling clean like they had just come from the  factory.

Send the XYL shopping and run it through the dish washer