Repair: Digital Mutlimeter

Discussion in 'General Electronics Chat' started by Georacer, Dec 17, 2012.

  1. Georacer

    Thread Starter Moderator

    Nov 25, 2009
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    I had bought this digital multimeter some 4 years ago: <snip> (Link unmounted due to security warnings. This is an alternative Russian page: http://www.mastech.ru/catalog/mult/ms8240b.html)
    It is a Mastech MS8240B.
    In this site there is the exact image of the one I have: http://www.genieshop.gr/index.php/v...o-polimetro-plires-me-piniometro-MS8240B.html

    It is Chinese but it is very rugged and the plastic feels not cheap at all (which wasn't).

    Up until now it worked fine but the last two weeks it shows some worrying signs.
    The current meter displays Over Limit while nothing is connected. On the 10A scale I can actually see the display rise gradually to 10A and then go OL.
    On the volt measuring mode, the DC measurements are also rising, but capped, depending on the scale selection. On the AC mode, the measurements are erratic and deviate from zero slightly.

    I don't remember any recent case of abuse that might cause it, except maybe measuring AC mains current for a small transformer in a smaller scale than the adequate one.
    Both 10A and 250mA fuses are intact, though.

    I 'm not saying that when I test batteries or other voltages the numbers are completely wrong, on the contrary, but I don't feel I can trust it for unknown voltages.

    Has this happened to anyone? Any guesses about what could go wrong?

    Thanks in advance.
     
    Last edited: Jan 5, 2013
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Is to test batteries.

    Did u replace the batteries for new ones.?

    If problem persists, make sure the batteries are good indeed. I have encountered drained new batteries.
     
  3. Ian Rogers

    Member

    Dec 12, 2012
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    Similar problem here!!

    Weird things happening.. Mine had a multi switch in the center.... On inspection the tracks were slightly worn and the copper was all in the grease.... After a clean and new white grease, its back to normal...
     
  4. absf

    Senior Member

    Dec 29, 2010
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    I have one with auto-range but at half of your price. My problem is the 2x AAA battery holder giving intermittent faults. After changing the battery holder the problem was solved.:)

    One thing - never use China-made battery. They always leak before you discover them.

    The rotary switch also sometimes giving trouble.... Just give it a few fast turns clockwise then anticlockwise would clear the problem.:)

    Allen
     
  5. Georacer

    Thread Starter Moderator

    Nov 25, 2009
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    Hm... The rotary switch might have something to do about it. With a firm push and twist it seems to reduce the problem, not erase it though.

    I might open it and clean it, but I 'm hesitant, since it's a 1000VDC rated instrument and I don't want to compromise that rating by mistake.
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    My first thought would be to clean the contacts on the rotary switch with isopropyl alcohol.
     
  7. PackratKing

    Well-Known Member

    Jul 13, 2008
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    I have done similar resurrection on some old analog meters, that have had severe tarnish of the function selector, or battery leakage. My favorite is an old "Micronta" 260 "range doubler" - collectors' item been offered $250 for it , even with a satellite battery holder attached to the back.

    Best bet is to strip it down for access [ not always easy - very time consuming ] and polish all traces on the function switch, douche out all rotarty pot controls w/ PCB cleaner and apply a very light coaty of dielectric grease.
    That has restored accuracy to all but one of the meters, where battery leakage had gotten into the wiring, at which juncture they are not worth the effort, unless it was a really good meter to begin with
     
    Last edited: Jan 8, 2013
  8. Georacer

    Thread Starter Moderator

    Nov 25, 2009
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    I know it was a Greek link. I posted it only for its picture, as I said. It was the only one I found with the correct picture after a quick search.

    You guys convinced me. I 'll try to open it but only if find the right grease first, though.
     
  9. Georacer

    Thread Starter Moderator

    Nov 25, 2009
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    I finally got to open my multimeter.

    There's an image and detail of what I saw in the first two pictures.

    I used alcohol on a paper tissue to clean the dirt and the result is in the two last pictures.

    I 'm afraid that a mistake in the design/manufacturing process resulted in a burnt wire instead of a burnt fuse.
    The multimeter still works, but with a faulty/erratic behaviour in some modes.
     
  10. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    It looks like the rotary switch was not perfectly concentric with the underlying PCB tracks. This is not surprising considering how many ranges and functions the designer tried to fit into that rotary switch.

    Even if the switch was perfectly aligned during manufacturing, any lateral stress on the switch may cause mis-alignment. All it takes in one time during a voltage measurement and, poof, the PCB track is burned.

    It would be nice if someone else could post a picture of a similar (functioning) PCB to show whether nor not that burned section was formerly a copper trace or just a burned section of bare PCB so you can try to fix it.

    If you elect to scrap it, I strongly recommend an auto-ranging model that takes care of most of those functions in software and allows a much simpler rotary switch. E.g. Klein M2000 is a well made unit for about $100 new.
     
  11. ScottWang

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    Aug 23, 2012
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  12. GopherT

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  13. Georacer

    Thread Starter Moderator

    Nov 25, 2009
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    Firefox reports a security threat too. What was in that page?
    EDIT: I realized that you were referring to the link I had made in the first post.

    That would be ideal, but I think it's unlikely.
     
    Last edited: Jan 5, 2013
  14. ScottWang

    Moderator

    Aug 23, 2012
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    That warning shows that the site could be steal the infos from the visitor, or doing something bad to the visitor.

    Probably when the visitor visit that site and it will auto download the resident program into the computer of the visitor.
     
  15. Metalmann

    Active Member

    Dec 8, 2012
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    I got the virus warning also, but it said that site had nothing bad happen in the last 90 days???:confused:
     
  16. Georacer

    Thread Starter Moderator

    Nov 25, 2009
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    Let me see what I had posted there. I 'm sure the warning didn't pop up when I first posted it.
     
  17. Georacer

    Thread Starter Moderator

    Nov 25, 2009
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    I removed the initial link. I can't find any official page for the product, so I replaced it with a Russian page I found. You can find it in the first post. If anyone else locates an official one, please let me know.

    At any rate, my multimeter is a Mastech MS8240B.
     
  18. Metalmann

    Active Member

    Dec 8, 2012
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    Those pics remind me of electromagnetic clutch facings on old screw machines in the 60s.
    The parts the wiper rides on looks like it could be made a little thicker.

    After we paid a guy to fly here from Germany, he showed me how easy it was to make those sliding surfaces like new again.

    We used 1000 grit, wet/dry sanding cloth, on a flat surface plate; to remove all burnt traces.

    The company put this guy up in hotels for two weeks, when all it took was a few strokes on a surface plate.;)
     
  19. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I used to clean copper traces with ScotchBrite or ultra-fine grit sand paper until my very non-technical brother reminded me that he used to use a eraser from the end of a pencil.

    He remembered the pencil eraser was the recommended way to clean the brushes and track on his old Tyco Slot-Car race track. It is very efficient and leaves the copper very bright and smooth.
     
  20. Georacer

    Thread Starter Moderator

    Nov 25, 2009
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    The problem isn't dirt on a trace. It's the possibility that where the charred board lies now, there may be an intact metal trace under the silkscreen.

    I 'm wondering if soldering a thin wire on top would inhibit the operation of the selector.
     
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