How to reversibly lock variable capacitor after adjustment? - Page 1
Forum section: electronics / general

author: carl_lab
author_rank: Frequent Contributor
timestamp: July 19, 2017, 09:08:02 pm
content: 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...

author: Muxr
author_rank: Super Contributor
timestamp: July 19, 2017, 09:14:37 pm
content: Nail polish?It should keep the trimmer from moving and when it dries it just flakes away when you adjust it again.

author: carl_lab
author_rank: Frequent Contributor
timestamp: July 19, 2017, 09:33:33 pm
content: Quote from: Muxr on July 19, 2017, 09:14:37 pmMaybe...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

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 20, 2017, 11:50:24 am
content: Not cheap, but if you wanted great adjustability and stability, try a piston cap.

author: carl_lab
author_rank: Frequent Contributor
timestamp: July 20, 2017, 02:12:24 pm
content: Quote from: Conrad Hoffman on July 20, 2017, 11:50:24 amThat'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...

author: Vtile
author_rank: Super Contributor
timestamp: July 20, 2017, 03:47:11 pm
content: What about letter sealant lacquer. That red stuff aanyone have seen on movies. It were used to be made from rosin and filler (chalk iirc) and red pigment ( ie. red iron oxide). Obviously low tech and the moisture impact is unknown, but atleast it is something with known resin.

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 20, 2017, 04:12:41 pm
content: I've also used nail polish successfully.The trick is to allow it to dry for 10 or 20 seconds on the tip of the applicator. Then as you apply it, it has become thick enough that it won't drip inside the rotor.

author: zhtoor
author_rank: Frequent Contributor
timestamp: July 20, 2017, 04:25:57 pm
content: Quote from: Vtile on July 20, 2017, 03:47:11 pmhello all,a 50-50 mixture of common shellac and rosin (colophony) is an excellent sealant (even vacuum sealant), very low cost,very high electrical resistivity, superior humidity sealant.downside being, to be applied in molten state.regards.

author: WhichEnt2
author_rank: Regular Contributor
timestamp: July 23, 2017, 10:31:59 am
content: Why don't just stick square piece of duct tape on the top of capacitor?

author: Ian.M
author_rank: Super Contributor
timestamp: July 23, 2017, 11:23:09 am
content: Whatever you use must be viscous enough that it doesn't penetrate as far as the moving vane (so its presence doesn't alter the capacitance by filling any air gaps with a dialectric), applying it must not ever disturb the rotor position (which makes tapes undesirable) and must also be reasonably easy to remove if future adjustment is required.I can see the logic of the OEM using neutral cure silicone sealant, as its commonly used on electronics assembly lines, and they don't expect it ever to be adjusted again except by a cal lab or their repair department, who can either use silicone remover to break its bond to the rotor and shell or simply replace the affected trimmers.The trimmers you freed up by repeated rotation that are now loose are all FUBARed and should be replaced. One of them has already become intermittent, probably with a bad contact between the mooving vane and its terminal, and the others probably are close to similar failure.Once you've replaced them, nail varnish is the easy option to lock them. You can get natural bristle artists brushes that are much smaller and more precise than the applicator brush in the nail varnish bottle, or simply use a toothpick to transfer some from the applicator to the trimmer. Put a thin skim of nail varnish taken straight from the bottle on the rotor and shell for good adhesion then let it thicken as 'schmitt trigger' recommends before applying the locking dab.