Electronics > Metrology

How to reversibly lock variable capacitor after adjustment?

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carl_lab:
I have to adjust variable capacitors in a frequency compensated input devider of a 7 1/2 digit DMM.
 
The capacitors look like these Murata TZB4R (but red and blue, not green type):



They all seem to be sealed by silicone-rubber-like sealant or adhesive, probably to prevent disadjustment by vibration.

Problem 1:
I turn the rotor for adjustment to correct position, but after removing the adjustment tool, the rotor is turned back slightly by the silicone rubber. I tried a little bit to "overshoot"  the adjusting angle, but without success. Then I tried to remove the silicone rubber sealant mechanically (turning the rotor, scratching it off the rotor by tweezers, screwdriver, needles etc.).

Problem 2:
The rotors now rotate very easy, but at least one of them is extremely touch sensitive (DMM display is jumping around). The visible part of the rotor (the part with cut out cross for screwdriver) is wiggling around loose. So I decided to replace the worst variable cap by a fixed cap.
The other ones seem to be adjustable in some degree, so I need something to prevent unintended disadjustment by vibration.
I don't want to use silicone rubber, because it's a PITA in the case you need to re-adjust the trimmers and have to remove the crap before.

What do you think, can I use some molten wax from a candle?
I know wax was used to fix adjustable threaded cores in inductors or filters of older TV/radio/RF circuits...

Muxr:
Nail polish?

It should keep the trimmer from moving and when it dries it just flakes away when you adjust it again.

carl_lab:

--- Quote from: Muxr on July 19, 2017, 09:14:37 pm ---Nail polish?

--- End quote ---

Maybe...

I thought about using screw locking lacquer, but I fear, it could lock the small rotor forever...  ;)



I tried using a little amount of hot-glue, but it didn't work very well.

That's the construction of the caps (it's the reflow soldering type without cover film):



http://www.mouser.com/ds/2/281/t13e-522726.pdf

Conrad Hoffman:
Not cheap, but if you wanted great adjustability and stability, try a piston cap.

carl_lab:

--- Quote from: Conrad Hoffman on July 20, 2017, 11:50:24 am ---Not cheap, but if you wanted great adjustability and stability, try a piston cap.

--- End quote ---

That's a great idea, but there is not too much space for the replacement parts (original size is about 4x4x3mm).
I need 2-6pF and 4-20pF.

I don't know exactly where I've seen them before, but I think that it was probably in my defective electronics pile...   ;)

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