variable capacitor improvement? (how are they made) - Page 1
Forum section: electronics / rf-microwave

author: coppercone2
author_rank: Super Contributor
timestamp: April 09, 2021, 12:02:17 am
content: So I got a old variable capacitor, the one thing I can think of is to clean everything in an ultrasonic.But what I am also thinking is if I do that, can alodine improve things? I tried to research it but I only come across like "the chassis was" or "all parts were" but I don't see any specific on using it in the rotary contact area. Could this improve capacitor ESR? There is alot of junctions there.So the idea would be to completely take it apart, deep clean it, then give it a thin-medium coating of alodine and reassemble it after a light greasing in an attempt to stabilize ESR.I pretty much have to take it apart I think if I want to follow best practices for 'chassis grounding' type scenarios, so giving it a alodine coat would only be a medium effort. I have two identical ones, so I can do one and see how the measurements compare to the other one.Does anyone see any specific problems, i.e. can this some how ruin my capacitor?I am sort of thinking, because of how it seems the plates are deform fit in, that not much will actually get into the contact area of the plates, it would mainly have an effect on the very end of the capacitor that touches the bushing, maybe.Am I correct in assuming that the way its made is that there is a ribbed shaft made, the plate inserted then the device put into some kind of hydraulic tool to deform the area around the plate to get a crimp contact junction on each fin of the rotor? I don't see how they can be held any way but that.And, does the stator typically come apart or is it also pressed? I don't see the 'crimp marks' on the stationary butterfly plates. The capacitor is a cardwell brand.SO it looks like I can completely take apart the stationary butterfly.. or is it some how welded together in there? Kinda scared to start unfastening it, but I feel like I need to in order to clean out the bushing at least...it seems a little dodgy as I rotate it while its connected to the ESR meter, and the copper shorting jumper thing is somewhat corroded.

author: coppercone2
author_rank: Super Contributor
timestamp: April 09, 2021, 02:39:53 pm
content: that answered nothing

author: tkamiya
author_rank: Super Contributor
timestamp: April 09, 2021, 04:55:55 pm
content: What kind of variable capacitor is it? (picture?)If it is an air gapped variable capacitor, there is really nothing to improve as dialectic is air. Yes, both rotors and staters are typically pressed or crimped. I never heard of anyone disassembling this and successfully putting it back together. If it's grungy, I'd soak it in detergent+water or if that doesn't do it, ultrasonic cleaner. Dry it completely and lube the bearing supporting its shaft.Old trick was to remove every other both rotor and stater and double the voltage handling capacity. I'd literally have to rip them off with no ways to undo it.If it is a "trimmer" type with something other than air for dialectic, I'd just toss it and replace it. I don't know of any way to clean it.

author: coppercone2
author_rank: Super Contributor
timestamp: April 09, 2021, 06:50:11 pm
content: It is a big air core inductorhttps://upload.wikimedia.org/wikipedia/commons/6/6c/Variable_Capacitor.jpgI think if I put alodine on the very end of the aluminum near where the bushing is, it would decrease resistance?That nasty POS needs to be cleaned. I know it works because I was doing it on alot of variable devices that use bushings lately and it decreased the resistance to the specification. This device is much cheaper though (not a genrad standard), but I know if I clean the copper at least, it will decrease ESR.. but if I alodine the aluminum, maybe even more? On my variable inductor I got 150% decrease in ESR reading after I cleaned up all the contacts, right down to dead nuts factory spec from 1960 when measured with the de5000I also thought about putting kool-amp on the copper bit, to silver plate it.Lol, I found termites in the wood (fun repair here!) and when I cleaned the electromechanical parts in the ultrasonic my big tub of simple green colored green water turned a heavy opaque dirt brown. I recommend all old devices be ultrasonic cleaned if possible...I would say about 7 days work on resorting each genrad at a reasonable daily work load, including the exterior wood refinishing

author: cdev
author_rank: Super Contributor
timestamp: April 09, 2021, 09:45:35 pm
content: Ive had good luck using Genie lubricant on a roller inductor, Cleaning and very sparing application of both deoxit and then the Genie lithium grease successfully lowered the contact resistance to very low levels.The Genie stuff (which is sold for use on garage doors) comes in modestly sized tubes that let you use just a drop or two at a time. It is very affordable. I also use it on my screw bearing x-y table..Quote from: tkamiya on April 09, 2021, 04:55:55 pm

author: Nusa
author_rank: Super Contributor
timestamp: April 09, 2021, 10:19:06 pm
content: Quote from: coppercone2 on April 09, 2021, 06:50:11 pmThat picture is of a capacitor. While a capacitor and inductor both store energy, they do it in different ways and behave differently. Here's a picture of a roller inductor, if that's what you actually have:https://en.wikipedia.org/wiki/Inductor#/media/File:Rollspule.jpg

author: bob91343
author_rank: Super Contributor
timestamp: April 09, 2021, 11:23:17 pm
content: The common shaft driven variable air capacitor has, of course, to have two terminals for each section. One terminal is obvious, the solder connection on the stator. The connection to the rotor, usually ground, is not so obvious. Of course it's not via the ball bearing, often filled with grease. It is by means of a spring clip that goes over a slot in the shaft, usually opposite from the bearing, that also bears against the frame.Expensive, high quality units may have several of these clips to lower the resistance. If you want to reduce the ESR, perhaps these spring clips could be removed and cleaned. Or cleaned in place while rotating the shaft.

author: cdev
author_rank: Super Contributor
timestamp: April 09, 2021, 11:32:03 pm
content: Ive seen a lot of interesting ways to DIY a high voltage variable capacitor. Of course the bearing's electrical resistance and possible loss at that point is a key area for improvement.For the same reason that a so called buterfly capacitor is viewed as desirable for loop antennas a trombone capacitor is similar.They can be made out of nested tubing with an appropriate insulator.Glass also makes an excellent insulator for a variable cap. And although I have never tried this I understand that distilled water can be used and works very well.I have used a foil-covered soft plastic ribbon folded on itself to make a loop.Quote from: camila on April 09, 2021, 01:39:56 pm

author: coppercone2
author_rank: Super Contributor
timestamp: April 10, 2021, 12:38:35 am
content: clearly I meant to write capacitor, everywhere else it says capacitor, unless you mean the part where I said I restored a variable inductor that uses a similar method to the variable capacitor with a weird coupling clip, or maybe its the near identical clip on the other side, I am not sure, but the idea is the same with both clips, they are just shaped differently, because multimeter resistance seems to show that they are both connected to the outer frame, i.e. both sides of the butterfly capacitor shaft have some kind of tensioned spring clip that is connecting the shaft to the outer frame from each end.the picture I posted is of the clip on a cheap variable capacitor (CARDWELL) and its asking if alodine is appropriate for the region the super dirty tarnished clamp is in, and if silver plating the copper would help as well.I am basically saying I have NO IDEA what a silver plating like kool-amp and alodine work in a region with a spring clip that has such a 'sharp' geometry, perhaps it does nothing, I thought its utility might be restricted to something more along the lines of a flat joint, like a panel that is riveted into place, rather then a joint that is more akin to something wedged into the earth (dug in). On one side the shaft is pressed against the body with a U shaped bit of copper, where the alodine/silver looks somewhat useful, but on the other side its wedged up against the X shaped bit of copper, which looks like it really digs into everything, so I don't know.and I am pretty sure camila is a troll account, I would say 90% of the posts attached to it are nonsensical/weird

author: coppercone2
author_rank: Super Contributor
timestamp: April 10, 2021, 02:00:39 am
content: I decided to take a picture because there is way too much confusion in this threadI want to know if there are plating solutions for the 2 areas I circled. I wanted to put a thin to medium layer of alodine on the aluminum in the parts circled and the parts around the other side of the clips, and preferably the whole capacitor.I also want to know about silver plating the clips with kool-amp.The procedure would be to disassemble as much as possible, clean as much as possible with detergent in the ultrasonic along with brushes/scrubbing with non abrasives, then to apply alumiprep33 (in low concentration to avoid too much etching) and then alodine. maybe with scotch ultra-fine to abrade it a little (they do not sell this one in the hardware store, it is much more fine then a dish scotch brite). I don't have the best alodine for electrical though, if anyone knows a source for the electrical kind (not the common one), the performance would be some what better. With alodine the thicker you go the higher the resistance is but it gains durability. I don't know about it on joints though,.

author: cdev
author_rank: Super Contributor
timestamp: April 10, 2021, 07:20:02 pm
content: Cardwell is a good brand, I think. One of the best there is. Yes, the bushings at the end are the critical spot and a point where loss occurs.I would investigate this Genie stuff (lithium grease based, officially its made for garage doors. Its been recommended by high end variable inductor manufacturers.. or so I have heard.. (Palstar? antenna tuners)I bought three tubes of it which should last a lifetime. I use it very sparingly.Anything is better than graphite mixed with Vaseline. (what MFJ recommends, or even worse, WD-40! yuck..You will probably have the best luck if you clean it with your ultrasonic cleaner first. That will loikely help the bushings.Cardwell's bushings may the same as the ones on an EF Johnson variable cap I have (I think Cardwell bought EF Johnson's capacitor business - or makes the same capacitors now, (Hammarlund too, ) and they are good and I suspect can all be cleaned some way or another. I have a bunch of similar caps to yours. I have a bunch of different sizes and values. (seehttps://cardwellcondenser.com/products/) Hope that helps.I dont know anything about "alodyzing" (maybe you mean anodyzing? >>?>>anodyzing them. Ive always as you said understood anodyzing to turn the coat into an insulator. I would leave the star shaped curved metal piece the way it is, unless its damaged in some way, just clean it well with IPA and you should be good. That really goes for almost the entire capacitors. maybe lubricate them a bit. DeOxit is probably the best for that. They are built very well.

author: coppercone2
author_rank: Super Contributor
timestamp: April 10, 2021, 07:30:02 pm
content: no, its not anodizing, its the opposite of anodizing. its a flexible gummy coat of chromate. Its called chromate conversion coating, its meant to protect stuff without anodizing, to increase conductivity, to make paint work better, to allow stuff to work unpainted (its thin, for lightness in air) and its meant to coat stuff that needs to flex where anodizing reduces the strain life of it (when stuff shakes in an airplane if you anodize the wrong things it can like shake less and break sooner).Its a little nasty to work with but so long you use gloves you are OK, the only problem is some people like spraying it, which can mist, but you can also 'baste' the part with a sponge or just dip it.There is a few different kinds, 1200 is not the best one for conductivity, but its the only one you can get in small quantities easily, otherwise I think someone would need to drain some off a drum for you, not sure.. was hoping to find a vendor for the other ones. Its conductivity and durability is related to thickness.The only downside it has is that it has a maximum temperature, if you reach it, it starts to flake off. And supposedly its self healing, so it re-flows into the places it was scratched from, in some cases, from microscratches.When I come across some fubar aluminum part that needed alot of work, i.e. in stuff I buy, I give it a alodine coating sometimes, instead of self etching primer (I noticed,.. this stuff does not etch all that well)

author: cdev
author_rank: Super Contributor
timestamp: April 10, 2021, 07:35:56 pm
content: Is that a toxic environmental substance?I vaguely remember reading about a site nearby that is heavily contaminated with chromate. Don't remember the implications of that but I do remember its nasty to have around, as far as cost of cleanup.Quote from: coppercone2 on April 10, 2021, 07:30:02 pm

author: coppercone2
author_rank: Super Contributor
timestamp: April 10, 2021, 09:22:02 pm
content: its chromic acid based, so you should not flush it down the drainwhat they like doing on planes is basically soaking them in the stuff from a spray and letting it drip into the sewer. if you have a good sewer I guess its ok, but if its like, not managed properly, you get chromic acid going into the ground. If you do little parts in a bowl or spray over a bowl with out fine mist its not going to contaminate much. The contamination comes with painting a giant oversized object that cannot be contained. Basically I think so long they monitor the sewer its ok, they can treat it with chemicals to precipitate it out and filter out.if you want to try, sodium metabisulfate used for treating it is on ebay for cheapand I tend to baste my parts, like its a oven meat, so there is no splash or splatter or mist or dunking

author: tkamiya
author_rank: Super Contributor
timestamp: April 25, 2021, 04:11:04 am
content: I've never heard of ESR as being an issue for those air variable capacitors. Those beryllium copper contacts at both end of the shaft is a source of intermittent connection. I've seen the shaft having a welded contact which connected to braid copper, which was soldered to the ground terminal. I'm thinking other than cleaning, replating that springy copper with silver, etc, that's pretty much the practical limit.I think you are in totally experimental territory....Yes, stators do come apart. Once they are apart, they are pain to align correctly.

author: coppercone2
author_rank: Super Contributor
timestamp: April 25, 2021, 04:47:36 am
content: yes, the genrad is off after cleaningbut at least the bushings shine. I think it has to do with correct spring tension, I am getting a nasty linearity but also my LCR measurement stuff sucks.The underside of the brushes was in such bad condition though.Can't complain, with the termite holes, I think most people would have thrown it into the trash lolIf you have a genrad air variable capacitor, the spring bits do not need to come off, the only problem in mine (no oxidation) but really really old chunky grease. I could not resist to look. It looks like you can clean the brush without effecting alignment on those, if you use very long q- tips on the bushing after you remove the brush. But ultra nasty congealed grease is not good for me. There is a stack of spring steels that glued themself together, it seemed improper.

author: bob91343
author_rank: Super Contributor
timestamp: April 25, 2021, 05:24:48 am
content: In your picture is visible the clip that connects the rotor to the frame, in the right hand circle. The left hand circle does nothing; that is the bearing and is not expected to make any contact.This clip must be clean and springy to maintain quiet contact as the rotor is turned. Any corrosion here is bad.However, disassembling it is risky. You might bend the clip and ruin its ability to provide low impedance contact. What we have is an interface between the clip and the capacitor frame, which is stationary. The rotor rubs against the clip and here is the sliding contact. Both interfaces need to be clean and free of oxidation. If you insist on chemical treatment, use something that leaves no residue.

author: coppercone2
author_rank: Super Contributor
timestamp: April 25, 2021, 05:57:06 am
content: that one is fine. my genrad however.. I plan on getting some gauge blocks to see if i managed to bend a plate or something when I undid the screw tension and ruined alignment. but that bastard was squeeking alsoyou would not happen to know how do calibrate the radial bend plates on those capacitors would you for trimming?

author: vk6zgo
author_rank: Super Contributor
timestamp: April 25, 2021, 11:17:21 am
content: Quote from: tkamiya on April 09, 2021, 04:55:55 pmRemoving plates also reduces (technically halves) the capacitance, too!

author: coppercone2
author_rank: Super Contributor
timestamp: April 25, 2021, 06:47:37 pm
content: so if its really bad your supposed to tear em out?

author: bob91343
author_rank: Super Contributor
timestamp: April 25, 2021, 11:01:38 pm
content: Bending plates for calibration is a tedious task. The number of sub-plates gives you a clue as to how accurately you might be able to trim. What you do is turn the plates fully open so that the only section left meshed would be what you would trim. Then progressively close the capacitor plates and progressively bend the trimmer sections. The final adjustment is the least critical, as its variation is a very small part of the total capacitance.You can bend the plate uniformly or you can twist it slightly to vary its effect. It's hard to explain but once you do it, you kind of get trained how to do it.

author: cdev
author_rank: Super Contributor
timestamp: April 25, 2021, 11:06:25 pm
content: Your capacitor looks fine, I would not do any surgery with it unless there was some obvious problem with it. youre an experienced user.Frankly, its very nice looking cap. Maybe a little less space between the plates than I would prefer for a loop for the reasons I think you explained in another thread.But all in all it looks very clean. Se I last posted I have tried the Genie stuff on my main (receiving) variable cap which I usually use for quick on the fly receiving loops made from insulated wire I have lying around. It seems as if it works well. I apply it with a toothpick. down into the bushing metal thing that applies pressure to the center bearing.. The capacitor has a bit of a reduction gear so it was already pretty smooth feeling.But it didnt hurt anything and the resistance of the capacitor bearing is very very low.I don't think I could hope for lower. And it doesn't make any abrupt changes. I think this Genie stuff is a good lubricant and protectant for electronics applications. In addition to its intended use, for garage doors.

author: coppercone2
author_rank: Super Contributor
timestamp: April 26, 2021, 12:23:49 am
content: that brass brush corrosion does not seem to come off without mild abrasives like brasso, or a LONG time in the ultrasonic.I bought 100% deoxit red solution to experiment with it, I only ever used the 5% spray.