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Fluke 77 ohms reading cal off

Discussion in 'Electronic Repair' started by Bob Urz, Jun 16, 2007.

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  1. Bob Urz

    Bob Urz Guest

    One of my handheld flukes (77) now is reading about 2 ohms when its
    leads are shorted on ohms scale. I tried another set of leads off of
    another fluke, the same. I used test clips and bypassed the internal
    protection fuses, the same. There was one pot by the display, did not
    change the Zero ohms cal. Anybody got a clue on how to adjust this or
    what parts to look at? Seems to work OK otherwise. Its the original,
    not a version II or III.

    Bob
     
  2. Guest

    Don't know about the Fluke 77 specifically as I have 2 8060s. The
    sockets on the 8060 are directly on the PCB and those joints sometimes
    crack and give bad ohms readings. I just remove the old solder and
    apply new solder which is good for years of daily use.

    GG
     
  3. Meat Plow

    Meat Plow Guest

    I have a 77 about 22 years old? Isn't there a protect device on
    the minus lead inside? Too late to take it apart tonight :)
    Take it apart and try shorting across minus and ohms right on the board
    Look around if you don't see anything I'll pull mine apart when I wake up
    tomorrow.
     
  4. DaveM

    DaveM Guest


    Do the DC voltage ranges show a good stable zero reading with the leads shorted
    together? If not, you might have a damaged A/D converter, or autozero
    capacitor. If the DC zero is good, you can get a pretty good idea whether the
    problem lies in the front end or the A/D circuitry by applying an accurately
    known DC voltage to the lowest voltage range. If the meter reads correctly,
    then the problem is likely in the ohms circuitry or in the protection circuitry.
    If the DC voltage reading is off, then you might have a damaged A/D. If the A/D
    is damaged, you might be better off replacing the meter rather than spending a
    lot of time hunting parts that might be more expensive than a new meter.

    How much different is the basic model 77 from the series II? I have a copy of
    the service manual for the 77 series II that I can email to you if you think it
    might help.

    Cheers!!!
    --
    Dave M
    MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
    address)

    Life is like a roll of toilet paper; the closer to the end, the faster it goes.
     
  5. Bob Urz

    Bob Urz Guest

    DaveM wrote:

    If you have the schematic, email it to me. I am going to check it
    against my other meters and see what's up.

    Bob
     
  6. carneyke

    carneyke Guest

    You may have something causing the offset (bad solder connection). The
    unit may have been damaged. The pot is for calibrating the 300 mv dc
    range. As you have found it is the only adjustment. You either put in
    an accurate 300mv or 3Volts and adjust the pot. Everything else falls
    in to place (unless it has been damaged). Good Luck.
     
  7. Bob Urz

    Bob Urz Guest

    I took it apart and resoldered the studs for the input leads. I also
    took both fuses out and used emery cloth on the contacts.
    It now reads about .8 to 1 ohm leads shorted. My other 75 reads about .2
    ohms leads shorted. And yes i swapped the leads to see if it made a
    difference. (it did not).

    Voltage readings was about .1 volt difference between the two on DC
    volts. The meter is not useless, i just cannot use it to reliably test
    very low value resistors.

    I wonder what could affect the ohms scale and not the DC volts?

    Bob
     
  8. James Sweet

    James Sweet Guest


    Does it have a software low ohms calibration function? I have a Fluke 79
    that has this. You short the probes together, then press the button in the
    middle of the selector knob and it'll display CAL and moments later 0.00.
     
  9. Meat Plow

    Meat Plow Guest

    Not on my 77 (just checked.) My meter reads 00.1 with the original leads
    BTW.
     
  10. DaveM

    DaveM Guest



    For a digital meter, that 0.1V difference could be significant (unless you're
    measuring an unstable source). What voltage range did you check on the meter?
    If the lowest range, then I'll bet that's your problem.

    --
    Dave M
    MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
    address)

    Life is like a roll of toilet paper; the closer to the end, the faster it goes.
     
  11. Guest

    Well Bob i don't know if your problem is fixed or not...but with those
    meter the rotary switch is sometimes the problem to try cleaning it
    and if that does not seem to work i would try to replace.

    I would also check if that 2 ohms is consistent through out. try to
    find a standard 100 ohm resistor and check if the reading is off by 2
    ohms again.

    tech.

    www.myflukemeter.com
     
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