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42-340 trimmer (compensator) values - Printable Version

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42-340 trimmer (compensator) values - Patrick - 01-16-2026

I need to replace the mica on all of these (7, 7a, 23, 23a, and 23b) but I need to know the capacitance range so I can adjust the thickness of the mica, and test each one before I re-assemble it. Does anyone have the value ranges? Thanks

Pat

[Image: https://i.imgur.com/3ioHZnR.jpeg]

RE: 42-340 trimmer (compensator) values - morzh - 01-16-2026

I would just try 3-5 mils thick and see if it works.

RE: 42-340 trimmer (compensator) values - Patrick - 01-16-2026

Actually, I did, with no success. The original was around 2 mils, but unusable, and the values are unstable with the new ones. It's really a pain to remove and replace the trimmers, so I was hoping to get a good measurement.

RE: 42-340 trimmer (compensator) values - morzh - 01-16-2026

Are these the two thin copper plates with screw/nut and mica sheet in between?

You can try this: get some newer type trimmers, open up the cap as much as possible (gently bend the top leaf out), solder the trimmer and see if it works. I would start from 3-20 or 3-30pF.

Or you could buy a 5-mil mica and try to slice it with a scalpel.

I am surprised the mica you have does not work.

RE: 42-340 trimmer (compensator) values - Patrick - 01-17-2026

Not the copper. They are like the other ones you described. I've got sheets of mica 15, 20, and 30 mils that I've sliced before down to 1 mil. I'll try different thickness to acheive 3-20 or 3-30 pf. I'm going to wire the 5 trimmers off the chassis so I can experiment.

RE: 42-340 trimmer (compensator) values - morzh - 01-17-2026

The others I described would not have any mica in them.

Typical mica-sheet type has two plates, one rigid, one movable, either on a flexible cantilever or just an arched piece of conductor. The nut would either get the flexible closer to the rigid by bending that cantilever or flatten the arch with the same effect.

Nica is present together with air gap. With the same gap, the thicker the mica, the higher the capacitance will be. The question is, would the peak spot you are trying to achieve still falls within the cap's range.

An industrial-made trimmer eliminates this crude capacitor's inaccuracy, as well as the dependance on the air quality (humidity etc).

RE: 42-340 trimmer (compensator) values - Patrick - 01-17-2026

Suggestions for industrial trimmers?

RE: 42-340 trimmer (compensator) values - morzh - 01-17-2026

I used Google.

https://www.rf-microwave.com/en/stettner/300-330-405/variable-capacitor-trimmer-4-20-pf/cvc-06/?srsltid=AfmBOoo63YsTtIrQSe9g0us0qDt8-dPOWETrNLwTtomApBFKuMwWoeyy


Amazon has some cheap looking plastic ones. They do not give the lower value, just the highest.

3-40pF more or less. I would start with that.

like this one

https://www.newark.com/vishay/bfc280908002/capacitor-trimmer-4pf-38pf-300v/dp/98K7134

RE: 42-340 trimmer (compensator) values - Patrick - 01-28-2026

There was another problem. A cloth-covered wire that looked good, was broken on the inside. Once I replaced it, I could adjust the RF. The mica I used was 5 mil, and it worked on all trimmers. Thanks, Morzh.

RE: 42-340 trimmer (compensator) values - morzh - 01-28-2026

Good!