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Fixing a DMM

Discussion in 'Electronic Basics' started by Ivan Vegvary, Sep 16, 2012.

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  1. Ivan Vegvary

    Ivan Vegvary Guest

    Hey, I know this is crazy, but I want to fix a "cheap" DMM. It is a Kelvin 50LE.

    This, and similar meters can be purchased for 3 to 5 dollars. Said meter works except the Ohm section does not function. Fuse is fine. Is there an easily identifiable component that has probably been fried?

    I have several analog and digital meters to help with this repair. All advice (other than "throw it away") would be greatly appreciated. I could throw away $20 before I could throw away a tool, any tool.

    BTW, wife often catches me watching tool porn on the computer (Snap-On, Harbor Freight, etc.).

    Ivan Vegvary (has tool fetish)
  2. Phil Allison

    Phil Allison Guest

    "Ivan Vegvary"

    ** Look for a low value resistor on the PCB that may be open - or a diode
    that might be short.

    .... Phil
  3. fungus

    fungus Guest

    All of the Ohm section or only particular
  4. Bob Masta

    Bob Masta Guest

    Even if you can't fix it, you don't need to toss it. Since
    the basic voltmeter circuit is still OK you can always use
    it as a dedicated panel meter in some future project.

    Best regards,

    Bob Masta

    DAQARTA v7.00
    Data AcQuisition And Real-Time Analysis
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  5. Tom Biasi

    Tom Biasi Guest

    Are you willing to spend $20.00 to fix it?

  6. Mark Zenier

    Mark Zenier Guest

    The magic number is 7126, that's the basic part number for (my best
    guess for) the chip in them. A low power IC for battery powered DVMs.
    It's originally from Intersil, but has had a large (half a dozen)
    different second source manufacturers, many of them in Asia. There's also
    the original 7106, but one time I checked, more companies in Asia made
    the 7126. (Also 7116, and 7136. Too lazy to dig to the bottom of the
    pile to find the databook, I might really mean the 7136, but I think
    that's the one with autohold or something like that).

    There are a number of application notes for these chips that give
    schematics for the various functions. And the manual for a similar
    meter, from one of the big names like the old Fluke 802x series
    (8024, 8026?) or a Beckman of the same era (1980s), might give
    you a very close schematic.

    The chip is ratiometric, it compares the voltage across the measured
    input with the voltage across the reference input.

    For voltage measurements, they have a voltage divider for the measured
    input and an internal reference for the reference. Really cheap meters
    use the on chip reference, more expensive ones use an external chip
    (often in a transistor package). The reference is either 100 millivolts
    or 1 volt. The chip will display up to +/- 1.999 times that.

    For current, they have resistor shunts to turn the current into a voltage.

    For ohms, they feed a current through both a reference resistor, across
    the reference input, in series with whatever is at the test probes.

    If all the ohms ranges are out, first check the range switch, then what
    feeds all the reference resistors. If just one (or two) ranges is out,
    check the reference resistor for that range.

    Mark Zenier
    Googleproofaddress(account:mzenier provider:eskimo domain:com)
  7. tuinkabouter

    tuinkabouter Guest

    Did you replace the battery already?
  8. Or the fuse, though I don't know if those cheap ones have them.

    I once had a weird problem with my DMM, and it worked but not right.
    Either the battery was too low, or one of the fuses had gained resistance.
    SOmething odd. I ended up opening it up, thinking the switch wasn't
    making proper contact, and then saw nothing, suddenly realizing it was
    something like the fuse or battery (I can't remember which).

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