EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: Pygmalion on November 09, 2023, 04:25:39 pm
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I have a multimeter that I am very happy with, but lately it has been having problems measuring small resistances. The shortcut measurement sometimes gives 0.2Ω and sometimes tens of Ω.
After inspection of the multimeter, I have come to the conclusion that the problem are the contacts at the rotary switch. The rotary switch contains metal parts that connect copper strips on the circuit board.
My first idea was to clean the metal parts and copper strips with isopropanol and then spray everything with contact spray Kontakt 61.
When cleaning with isopropanol, I realised that the copper strips were already covered with a greasy black substance, and by cleaning them I removed some of it. I suspect that this is some kind of contact lubricant.
Now I do not know how to proceed. Should I find/buy some kind of contact lubricant or perhaps continue with the cleaning and replace this substance with contact spray? What is the best solution to achieve good contacts in this particular case?
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Good Morning Pygmalion, Please post the make and model of your multimeter, so others here on the forum, that have them can advise you.
Also, compare readings on your meter with a known good meter,
and come back and let us know your progress. :popcorn:
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Is your short-circuit measurement done with the usual meter leads or with a very short wire at the meter panel?
0.2 ohms is not out of the question for meter leads shorted together at the probe.
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Is your short-circuit measurement done with the usual meter leads or with a very short wire at the meter panel?
0.2 ohms is not out of the question for meter leads shorted together at the probe.
I did shortcut tests with the usual meter leads. Usually I got 0.2Ω - 0.5Ω, which I think is satisfactory. But lately I have been getting from 0.2Ω up to tens of Ω. And when I touch the rotary switch, the value changes immediately.
So it's definitely a contact problem between the metal parts of the rotary switch and the copper tracks on the circuit board.
I am not asking for advice on how to find the source of the fault, but how best to improve the contact between the metal parts and the copper tracks.
The model name is therefore of little importance here IMHO.
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Clean it and then apply a dilectric grease to the contacts.
Jeff
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Clean, polish with a soft scotchbrite if you see grooves (Barely touch it until shiny, don't remove the gold layer).
If you see deep grooves and/or the copper, it'll be a bit too late, will affect low resistance readings.
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The model name is therefore of little importance here IMHO.
You posted this in the Beginners subforum. If you’re already such an expert why even ask a question?
You were asked because it’s a potentially important piece of information. You don’t need to know or understand why it is important yet. And I don’t understand what you gain by withholding this information.
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It's possible to help people without being a jerk. If you read his question and understood it, you'd realize it has nothing to do with the model of the meter. He is right, you are wrong.
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It's possible to help people without being a jerk. If you read his question and understood it, you'd realize it has nothing to do with the model of the meter. He is right, you are wrong.
No, I’m not wrong.
Since you also don’t seem to understand why it could matter:
- if a service manual is available, it might specify a particular lubricant
- sometimes, a given model of product has known issues that affect it uniquely. For example, the particular lubricant might break down a certain way with age. Or the contacts could be known to fail in a particular way.
- someone else here might have encountered the same fault in the same device. In particular, if the cause of the problem turns out not to be the switch contacts, but something else, the other person might know what it is.
But it also just irks me when people who are asking others for help are secretive about the device they’re working on. Unless it’s a custom piece of proprietary gear made for intelligence agencies (in which case they wouldn’t be asking here), there’s nothing to be gained by withholding that information. Meanwhile, doing so deprives those who want to help of potentially useful information. So an answer like “The model name is therefore of little importance here IMHO.” strikes me as a bit arrogant and entitled.
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With many DMM models the rotary switch is not directly in series with the measured resistor. It is kind of natural to have seprate paths for the voltage reading and test current. So moderate switch problems would not be an issue. There may be a few bad designs though - so the model of the meter can mater.
A few tenths of an ohm is not uncommon with a handhelp DMM, or 2 wire ohm measurements in general.
The problem is more likely with the terminals or probes.
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The model is not a secret. It is Aneng AN870 if this helps.
And I repeat, again, I do not complain that the resistance on connecting measuring cables is 0.2Ω. I complain that sometimes the resistance is tens of Ω. And since it is changed by pushing rotary switch, I can only conclude that the rotary switch contact is in series with the measured resistor.
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For rotary switch care and cleaning - it always descends into a religious or holy war about what to do.
There is cleaning, lubricating and oxidation to deal with, 3 aspects. The different switch constructions matter greatly on how to do all that.
I found the ANENG multimeter rotary switches make poor connection, they are some slag filled copper on ENIG finish on the PCB. Ohms readings way up there.
Factory looked like a mineral oil film was there on the PCB.
I tried - polishing the copper, silicone dielectric grease and a few other chemicals, eventually the fix is applying PPE like (In North America) CAIG DeoxIT F5 (https://caig.com/fader-f-series/) on the contacts/PCB.
The rotary switch works perfect for over 2 years now.
Clean it with IPA and q-tips. PAPER is a great abrasive to scrub off oxide without destroying and plating or surface finish. Anything coarser (sandpaper, scouring pad etc.) is destructive.
Fluke lube was mineral oil and I'll check but a light petroleum grease on the ratchet/shaft like Lubriplate.
Silicones are not preferred because they are like CFC's and can cause a contamination problem, I'd read parts near it can get affected.
WD40 is mostly kerosene and terrible, the solvent which attacks many things, evaporates and then you don't have decent lubrication. Some people love the stuff. ICK.
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I found the ANENG multimeter rotary switches make poor connection, they are some slag filled copper on ENIG finish on the PCB. Ohms readings way up there.
Factory looked like a mineral oil film was there on the PCB.
I tried - polishing the copper, silicone dielectric grease and a few other chemicals, eventually the fix is applying PPE like (In North America) CAIG DeoxIT F5 (https://caig.com/fader-f-series/) on the contacts/PCB.
Clean it with IPA and q-tips. PAPER is a great abrasive to scrub off oxide without destroying and plating or surface finish. Anything coarser (sandpaper, scouring pad etc.) is destructive.
Fluke lube was mineral oil and I'll check but a light petroleum grease on the ratchet/shaft like Lubriplate.
I can confirm that my Aneng also had some sort of black mineral oil, probably the same as your Aneng and Fluke. My idea was to clean with IPA and cotton swabs and then use Kontakt 61 lubricant (https://uk.farnell.com/kontakt-chemie/kontakt-61-400ml/lubricant-contact-61-400ml/dp/837982 (https://uk.farnell.com/kontakt-chemie/kontakt-61-400ml/lubricant-contact-61-400ml/dp/837982)), which is probably something similar to CAIG. I think I came up with the same idea.
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I have seen this type of problem in several of my meters. I have had good luck using a product called DeoxIT D5. I have sprayed it on and exercised the switches. I did not wipe it off. Seems to work. I haven't needed to repeat the treatment on any of the meters where I used it.
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KONTACT CHEMIE (https://www.kontaktchemie.com/koc/KOCproductdetail.csp?division=&product=KONTAKT%2061&ilang=en&plang=en) has no mention of polyphenylene ether products. There were a few patents on PPE as well and it did cause a shakeup.
KONTACT make a lot of claims but no details. 61 appears to be a spray cleaner- solvent plus mineral oil lubricant. 60 not sure how it works.
So I would look at something else, like MG CHEMICALS 801C-P Contact Cleaner, Pen Applicator with Polyphenylene Ether (https://uk.farnell.com/mg-chemicals/801c-p/contact-cleaner-with-ppe-pen-7/dp/2945166) or 801B. (https://mgchemicals.com/products/electronics-maintenance/contact-cleaners/electrical-contact-cleaner-spray/)
I tried probably 5 different fixes and the cheeseball "copper" rotor quickly oxidizes and makes poor connections. Brymen rotary switch uses the same configuration, a copy of a 1989 Actron patent I mentioned here (https://www.eevblog.com/forum/repair/dmm-contact-wipers/msg4343059/#msg4343059) lol but better metal with better plastic tolerances.
I yearn for the Fluke phosphor-coated bronze, multi-leaf rotors - the best. But we all cheap now.
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As the OP was very reluctant to give the model, until later on... I guess is the time to look for a better multimeter. No need to invest more in fixing the unit than it was to purchase it |O.
This is the contact in this cheapo instrument.
(https://lygte-info.dk/pic/Aneng/AN870/DSC_7496a.jpg)
This is the other side of this ONE CHIP multimeter:
(https://lygte-info.dk/pic/Aneng/AN870/DSC_7500a.jpg)
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As the OP was very reluctant to give the model, until later on... I guess is the time to look for a better multimeter. No need to invest more in fixing the unit than it was to purchase it |O.
This is the contact in this cheapo instrument.
This is the other side of this ONE CHIP multimeter:
This judgemental comment is proof that I was rightly "reluctant" to provide a multimeter model. :-//
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Buddy, make it judgemental or not, by the time you realize spending money on cheap stuff costs you more money in the long run, you will have spent at least double of the cost of a decent multimeter.
You are not the first, not the last, to fall in the trap of expensive "cheapo instruments".
If you are on a budget, start looking at a UNI-T. I guarantee the cheapest/decent will last at least 10 years before you run into contacts problems.
NB: I said nothing about 3V multimeters, or unsafe ones. Dave (you know who) said enough.
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by the time you realize spending money on cheap stuff costs you more money in the long run
If you are on a budget, start looking at a UNI-T
:palm:
I don't see the issue with him cleaning up his contacts and putting a dab of something on it? How is that a huge investment? I putz around with junk all the time, not trying to justify my time at any hourly rate.
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As the OP was very reluctant to give the model, until later on... I guess is the time to look for a better multimeter. No need to invest more in fixing the unit than it was to purchase it |O.
This is the contact in this cheapo instrument.
This is the other side of this ONE CHIP multimeter:
This judgemental comment is proof that I was rightly "reluctant" to provide a multimeter model. :-//
Internal construction of handheld DMMs varies quite a bit from model to model: this is why people trying to give a useful answer want to know the model.
Why the reluctance?
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I don't see the issue with him cleaning up his contacts and putting a dab of something on it? How is that a huge investment? I putz around with junk all the time, not trying to justify my time at any hourly rate.
Me neither, but when you do not have the cleaning agent, and you have to buy it, I guess it is far better to buy a decent one that does not need any maintenance. I bet the unit is 1-2 years old, and not even used on a daily basis.
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I have a multimeter that I am very happy with, but lately it has been having problems measuring small resistances. The shortcut measurement sometimes gives 0.2Ω and sometimes tens of Ω.
After inspection of the multimeter, I have come to the conclusion that the problem are the contacts at the rotary switch. The rotary switch contains metal parts that connect copper strips on the circuit board.
My first idea was to clean the metal parts and copper strips with isopropanol and then spray everything with contact spray Kontakt 61.
When cleaning with isopropanol, I realised that the copper strips were already covered with a greasy black substance, and by cleaning them I removed some of it. I suspect that this is some kind of contact lubricant.
Now I do not know how to proceed. Should I find/buy some kind of contact lubricant or perhaps continue with the cleaning and replace this substance with contact spray? What is the best solution to achieve good contacts in this particular case?
The black is just copper oxide. This is from moisture in the air oxidizing the copper surfaces and being scuffed off mechanically over a period of time. Probably from the contacts rather than the board. I can't tell but it looks like gold traces on the board. Isopropyl alcohol works great for cleaning that stuff off.
You can just use any Silicone based dielectric grease from an auto parts or hardware store. Mineral oil base seem to turn cloudy after time but work as well. DeOxit F5 Fader works to but doesn't seem to last as long as the grease.Main purpose of the grease is not just for lubrication but also a barrier to moisture.
Hope this helps and I'm glad to see you trying to keep stuff out of the land fills.
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As the OP was very reluctant to give the model, until later on... I guess is the time to look for a better multimeter. No need to invest more in fixing the unit than it was to purchase it |O.
This is the contact in this cheapo instrument.
This is the other side of this ONE CHIP multimeter:
This judgemental comment is proof that I was rightly "reluctant" to provide a multimeter model. :-//
Well, I don’t judge you for having an Aneng — I have an Aneng among my stable of meters.
The tone aside, the comment’s point of choosing how much to invest in a low-cost device is a very valid one. If you already have the suggested contact grease around, great! But it may not make economic sense to invest $25 in that grease for a meter that cost little more than that.
But the fact that others reported the same black grease in Aneng models and a detailed description of what did and didn’t work to fix the problem is precisely what I meant in the value of sharing model information.
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by the time you realize spending money on cheap stuff costs you more money in the long run
If you are on a budget, start looking at a UNI-T
:palm:
I don't see the issue with him cleaning up his contacts and putting a dab of something on it? How is that a huge investment? I putz around with junk all the time, not trying to justify my time at any hourly rate.
I think the intent of the comment was that it may not make sense to spend $25 to buy that grease just for this purpose, if you don’t already have it.
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Cleaning switches is part of owning any electronics. That $25 can I've used on a dozen pieces of equipment and it's still not empty.
When the maintenance costs are too high compared to the multimeter's price... cheaper to bin it and buy a new one? Strange timeline this is.
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I'll just try KONTAKT 61 for now and report the results. Can't hurt trying something new.
Also, apart from this contact problem, I am generally pleased with Aneng 870. My education and job have nothing to do with electronics, so I use it exclusively for my hobbies, along with low cost USB oscilloscope and signal generator.
But the fact that others reported the same black grease in Aneng models and a detailed description of what did and didn’t work to fix the problem is precisely what I meant in the value of sharing model information.
Where can I find those descriptions and information?
[BTW, Fluke 101 also has only one chip.]
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When the craze happened a few years ago I bought two AN8008 boxes. A month later I wanted to check a series of 1k 1% resistors and several were out of spec by a few percent. So Initially I thought I bought "mislabeled" resistors from Ali. Then I checked the resistors with a decent DMM and all resistors were within spec. So those aneng meters can go in the garbage bin. I can't use a DMM I can't trust. Even blowing wind over them from between your buttocks it too much effort for these things.
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You get what you pay for, it was a $25 DMM and I would not expect much Fluke which is 20x the price.
After the DeOxit F5 on the rotary switch, the ANENG 8008 I have works great. It does have a heart attack outdoors in cold weather at -30°C. The LCD froze, the rotary switch made a bad connection on any function, the PVC test leads all freeze up super stiff - I did laugh pretty hard. It took about 1 minute to croak after pulling it out of my warm pocket. Total failure lol.
I have a personal issue with the Brymen DMM rotary switch, I assume it's fixed now but Dave did not do a follow up to BM786 Switch Issue. (https://www.eevblog.com/forum/testgear/bm786-switch-issue/) Very much the same as the ANENG in design. Just tighter tolerances and better metal I guess. Too bad the Fluke rotor is space age technology in comparison.
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I probably have over a dozen VOMs, analog and digital ones. My least expensive one is a Radio Shack that cost less than $10, probably less than $8.
SO WHAT? I have used it many times and it actually WORKED! Well over two thirds of the problems I have repaired were due to either open or shorted components and both of them look the same on a $8 meter and a $800 one.
BTW, it still works. But my first DMM, a Simpson that cost over $200, doesn't. And I didn't abuse it, it just quit. Go figure!
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I just want to inform everyone about the developments. The problems with the multimeter became more and more pronounced, so I finally took it apart, cleaned the contacts and applied Kontakt 61. Now it works perfectly again, 0.2Ω short contact every time without any problem. The only question is - how long.
Now I want to buy another budget multimeter as a gift. Do you still recommend me to buy UNI-T instead of Aneng? Is there a particular model with similar features to the AN 870 that you would suggest?
For example, I see that the UNI-T UT139 (C,E) have similar features to the Aneng AN870, but it has the same type of rotary switch contacts, so I do not know how it would perform better.
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Now I have a question. How do I clean the contact of the rotary switch on the Triplett 630 type 5? I can't see how I get to the contact part. Looks like it's sealed.