Cleaning decade resistor box contacts (alternative to DeoxIt?) - Page 1
Forum section: electronics / repair

author: NaxFM
author_rank: Regular Contributor
timestamp: January 21, 2022, 11:28:11 am
content: I have a precision decade resistor box which gives me very inconsistent reading, truly a big problem given that the lower decade has 0.01ohm steps.The contacts are black from oxide and even if I already tried two times I can't clean them with the procedure in the datasheet. It says to clean the contact with ether and microfiber cloth and then applying three drops of ricin oil.I got some improvements, but this procedure does not remove the oxide at all, and the lower decade is still useless with the readings that jump all over the place.Is there another safe way to clean this type of contacts?I've read many times of the wonders of Deoxit but here in Italy it's impossible to find and importing it would cost more than what i paid for the resistor box.Do you have any experience with WD-40 contact cleaner? How does it compare to Deoxit? I also need to clean the binding posts, which as you can see are all black and corroded.Of course for my measurements i did not use the corroded binding posts, but i connected the test leads directly to the resistors below

author: bdivi
author_rank: Regular Contributor
timestamp: January 21, 2022, 12:23:34 pm
content: It should be non-conductive and non-corrosive and leaves no residue. I do not believe WD40 is good here, especially with precision equipment such as your case.I have been using with success Motip Dupli Contact Cleaner which should be widely available in Italy.Regards,

author: Gyro
author_rank: Super Contributor
timestamp: January 21, 2022, 12:48:01 pm
content: Try a smear of Vaseline (seriously).

author: bob91343
author_rank: Super Contributor
timestamp: January 21, 2022, 05:40:08 pm
content: I think it's a mistake to use chemicals. I would, instead, polish the surfaces until the oxide is gone.Many a decade box has been ruined by various chemicals. Alcohols, reducers, lubricants. Especially for low resistance boxes. High quality switches are not enclosed and cleaning is possible.

author: Stray Electron
author_rank: Super Contributor
timestamp: January 21, 2022, 06:29:38 pm
content: For circuit board contacts (fingers), I have used denatured alcohol. I'm told that they add a small amount of kerosene to "denature" the alcohol to stop people from drinking it. The alcohol soon evaporates but it seems to leave a very small amount of oily film (kerosene?) on the contacts and I notice that the boards slide in and out of the connectors much easier and seem to make more reliable connections.I think that WD-40 is also mostly kerosene but with some drying agents added. So it might work just as well.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: January 21, 2022, 06:38:41 pm
content: I regularly use one of these, mostly to polish PCB traces to graft wires or whatever onto.https://www.amazon.co.uk/Modelcraft-PBU1019-1-Glass-Pencil/dp/B002Z8IX36It's pretty soft, I can't see it damaging even the the most "woke" of metals, but cleans oxidation off really well without scratching.It's excellent at restoring unobtanium switch contacts.Only drawback is you tend to get microscopic glass fibre splinters in your fingers.Very annoying.

author: 1audio
author_rank: Frequent Contributor
timestamp: January 21, 2022, 06:45:02 pm
content: Are the contact surfaces plated? Its difficult to tell in the picture. If not the classic pencil eraser could remove any oxide and provide a fresh surface. It looks like the wipers are copper and the contacts are brass. Possibly a wipe with white vinegar/ acetic acid to remove oxide on the surface and then an thorough cleaning. when the oxides are strong the usual cleaners can't get through them. 10 milliOhm per step will be difficult to maintain accuracy since the switch would need microOhm contact resistance. That's really hard. Maybe silver on silver? That would be the classic Daven switch.

author: Gyro
author_rank: Super Contributor
timestamp: January 21, 2022, 06:46:25 pm
content: You really don't want to be creating any particles or additional microscopic scratches on the contacts. The leaf contacts have sufficient pressure to maintain cleaning action. You just want something to keep the oxidation under control (so you don't have the exercise the switches too often to mechanically self-clean them), Vaseline works well for that.

author: NaxFM
author_rank: Regular Contributor
timestamp: January 21, 2022, 08:44:05 pm
content: Quote from: 1audio on January 21, 2022, 06:45:02 pmThe color of the contacts is exactly that in the picture, I'm 100% sure they are not silver.I don't know anything else, except that they souldn't be black.Vinegar to remove the oxid doesn't seem a bad idea, if it doesn't work it's easily washable from the contact surface, so i guess I'll give it a try...Quote from: Gyro on January 21, 2022, 06:46:25 pmBut vaseline doesn't remove the oxide, right? It justs makes everything smoother, but no really cleaning and deoxiding action, which is what i'm looking forQuote from: Audiorepair on January 21, 2022, 06:38:41 pmWell, i may try it, at this point i'm open to every possible solution...Quote from: bob91343 on January 21, 2022, 05:40:08 pmProbably what i'll try before anything else, thanks!Quote from: Stray Electron on January 21, 2022, 06:29:38 pmIt doesn't remove the oxide tho, so the wiper still contacts the oxide and not the clean surface

author: Gyro
author_rank: Super Contributor
timestamp: January 21, 2022, 09:05:08 pm
content: Quote from: NaxFM on January 21, 2022, 08:44:05 pmYou don't have oxide on the areas of the contacts where the leaf contacts run, they are bright. The contact pressure and wiping action are enough to mechanically remove it. You just want to avoid it building up again between operations. That's where excluding the oxygen using a thin inert protective film (Vaseline) helps. Getting hung up on maintaining cosmetically shiny contacts over their whole surface is pointless - with brass/copper contacts, the non wiping areas will always tarnish again.P.S. In metrology labs, it is/was standard practice to exercise rotary switches from end to end several times to mechanically clean the contacts before use.

author: NaxFM
author_rank: Regular Contributor
timestamp: January 21, 2022, 09:10:09 pm
content: The contact resistance is unreliable, expecially since it must be much less than 0.01 ohm for the last decade to be somewhat useful. |If you look at the picture there are contact oxidized even in the path of the wiper, and that is after cleaning them two times with isopropil alcohol and applying the ricin oil like it says in the datasheet

author: Gyro
author_rank: Super Contributor
timestamp: January 21, 2022, 09:15:28 pm
content: If you've got IPA on hand for cleaning, then I suggest you try it - it only needs to be smeared on the contact surfaces themselves (not the insulation), much more controllable than a spray, so it's easily removed. You may be surprised.

author: bob91343
author_rank: Super Contributor
timestamp: January 22, 2022, 04:26:36 am
content: I have used fiberglass brushes with success. Too heavy a hand may remove any plating but I don't think that applies to most high end decade boxes. Sometimes a business card can remove extraneous particles and perhaps some oxidation.I would NEVER use WD-40 on anything I cared about. In particular, electrical stuff.

author: abdulbadii
author_rank: Frequent Contributor
timestamp: January 22, 2022, 06:53:26 am
content: is strong acid on Italy market sold freely?in some places is illegal unless with certificate or alike

author: trobbins
author_rank: Frequent Contributor
timestamp: January 22, 2022, 09:51:53 pm
content: My vintage decade box has only 1 ohm lowest steps and has three leaf contacts in lieu of your six, but I can appreciate your pain as my box appeared to not have been used for decades (pun intended) and each of the brass step contact faces had built up a visible oxide layer such that normal rotation exercising made no improvement to contact resistance integrity. As my box needed a few repairs, I was more inclined to use a simple but brutal approach with a light use of 1000 grit abrasive paper to visibly remove the darker oxide layer on the step contact surfaces - that action restored the consistency of measurements for about 95% of times I set a position on the lowest ohm decade. Your knob and multiple-leaf and common ring arrangement seems more robust and appropriate than what I have where the 3 leaf contacts pass through the body of the knob to get to the common ring, and I can notice minor milliohm resistance changes by forcibly rocking or pressing on the knob. My box also shows grooves made by the 3 leaf contacts. You may need to resort to a fine abrasive in order to clean oxide from the groove walls - perhaps there is a finer practical abrasive technique than 1000 grit paper - such as a polishing head on a Dremel with some paste (sort of like intermediate polishing of a mirror glass blank).How are you measuring the 'resistance' of the box as you change settings? I used a HP3497A with 1mA CC and kelvin connection and 1 milliohm resolution, but you likely have to go another digit resolution.I just have to be careful to use multiple rotations/measurements to confirm that any single measurement does not incur some contact variability, especially when relying on the 0.1% tolerance of the steps. I have just used a typical electrical contact cleaner/lubricant to suppress on-going oxidation, but can appreciate the use of vaseline as that is commonly used to exclude atmosphere/moisture on high current mated surfaces. There is a high class box operating manual that does indeed identify deoxit as well as WD40, so I wouldn't immediately recuse WD40.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: January 22, 2022, 10:11:43 pm
content: Quote from: trobbins on January 22, 2022, 09:51:53 pmIs not the solution just simply cleaning the paths used by the wiper?Cleaning the uppermost surfaces is not going to do very much by scraping them with sandpaper.You need to get down to the path the wiper uses and clean that instead.This is where the glass fibre brush becomes very useful indeed.

author: trobbins
author_rank: Frequent Contributor
timestamp: January 22, 2022, 11:38:32 pm
content: The abrasive paper effectively cleaned into the wear grooves made by the wiper leaves. Yes it also cleaned oxide from the un-wiped surface area of each step terminal - a consequence of using an abrasive paper technique. Each groove is sort of a wide furrow due to the straight profile of each leaf's contact width and the rotation radius of the leaf.Given that surface irregularities affect the flatness of both the step terminal and the leaf edge, there may have been some residual contact resistance accruing over time from just that aspect and nothing to do with surface oxidation, but doing a high quality flat regrind to all surfaces was never going to be practical.

author: bob91343
author_rank: Super Contributor
timestamp: January 22, 2022, 11:53:09 pm
content: I would also consider jeweler's rouge. I have some of it, a sort of reddish brown, that works well. It's extremely fine and I have polished eyeglass lenses with it.

author: 1audio
author_rank: Frequent Contributor
timestamp: January 23, 2022, 12:07:11 am
content: Looking at the areas marked there may be a contact force/flatness issues. On some contacts there is a clean 6 groove wear, on others it looks like the contacts are not flat so parts of the contact wipers are not making contact. Maybe the contacts are not consistently flat enough? Machining them flat is probably not an option but maybe the wiper assembly can be adjusted to have a little more contact force on the bad contacts?

author: nctnico
author_rank: Super Contributor
timestamp: January 23, 2022, 12:20:36 am
content: To me it looks like there is already some kind of oil /liquid in there. I would probably take it apart, give everything a good wash with some kind of alcohol, put it back together and use vaseline to lubricate rotating parts and contacts.

author: jdragoset
author_rank: Frequent Contributor
timestamp: January 23, 2022, 04:32:01 am
content: Dremel tool with SS wire brush

author: geggi1
author_rank: Frequent Contributor
timestamp: January 23, 2022, 10:13:36 am
content: Dremmel tool with a polishing head. Use some chrome polishing paste like Autosol.After polishing clean everything to remove any residues of chemicals and oxsides.Get some quality contact spray to prevent future oxidation, but dont spray it on use a brush or q-tip to apply because you dont want the spray all over your decade box.

author: NaxFM
author_rank: Regular Contributor
timestamp: January 23, 2022, 11:22:47 am
content: Thank you everyone! You've given me plenty of idea to clean every sort of contact.For now i ordered one of those fiberglass pencils and I'll try with that first just because it's easier. Of course I'll brush very lightly, don't want to scratch the surface finish.

author: jpanhalt
author_rank: Super Contributor
timestamp: January 23, 2022, 11:29:07 am
content: Be sure to practice with the fiberglass pencil before using it on the contacts. I use such a pencil to remove solder resist. They are sold to remove scratches on auto paint too. They, or at least the one I have, are far more abrasive than, say, 3M's Scotch-Brite white pads (#7445) .