Mica Trimmer Capacitors

I picked up two sets of mica trimmer capacitors from eBay:

Mica trimmer capacitors

Mica trimmer capacitors

The big cap on the left goes from a bit under 1000 pF to just over 2500 pF, maybe a 50% range. I got four of the things, they can be disassembled, and I could reduce the total capacitance by maybe half; the tuning range would drop by even more, so it may not be worth it.

The smaller trimmer has different sections: 300-400 pF and 500-700 pF, with about 25% range. As nearly as I can tell, the 700 pF section has one more pair of plates than the 400 pF section.

Given that compression caps work by mashing a stack of mica sheets, I think more pressure makes them more stable and running near the high end of their range will be a Good Thing.

Soldering these New Old Stock relics may be challenging, as the stacked flat metal leads aren’t in pristine condition: properly wetting all the leaves will require plenty of flux.

Resonating the loop antenna requires an external capacitor around 1000 pF. Paralleling a 120 pF fixed cap with both sections of the dual-section cap should do the trick: 900 pF minimum, 1200 pF maximum. Putting the fixed cap on a jumper would reduce the total capacitance, which seems easy & sensible.


  1. #1 by Hexley Ball on 2016-11-21 - 11:08

    “…they can be disassembled, and I could reduce the total capacitance by maybe half…”

    Speaking of mica compression caps, did you ever see this project where Manfred Mornhinweg rolled his own compression caps (along with his own high-current RF fixed mica caps): http://ludens.cl/Electron/fmtx/fmtx.html

    As Manfred says, “I made my own mica compression trimmers, using brass and copper sheet, brass base plate, brass compression washer, and mica sheets originally intended for TO-247 capsule mounting. All connections in the trimmers are soldered, not just riveted like in many factory made trimmers.”

    Those sound like something that could just as well have come out of your Basement Machine Shop :-)

    • #2 by Ed on 2016-11-21 - 20:21

      That guy is crazy, but in a good way… [grin]

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