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Fluke 79 acting up

Discussion in 'Electronic Repair' started by James Sweet, Feb 3, 2008.

  1. James Sweet

    James Sweet Guest

    This thing has performed flawlessly for many years, but when I turned it on
    today something was not right. The ohms range flickers random readings, with
    no probes plugged in I get around 230K on the display, or 2.22M, depending
    on the range, which would imply that the problem is ahead of the scaling
    circuit. When I switch to the low ohms range, the reading starts around 240
    ohms and then drops to -0.00 over a period of 10 seconds or so, frequency
    reading works, but AC volts reads 70mV when plugged into a 120V outlet.
    Something strange is going on! I realized that I don't currently have a
    backup meter, I suppose I could use this as an excuse to buy a true RMS
    Fluke, I don't think I can bring myself to buy a cheapie, except perhaps as
    a backup. Anyone run into this?
     
  2. James Sweet

    James Sweet Guest

    Sorry, it's a 77, not a 79.
     
  3. Jamie

    Jamie Guest

    did you check the fuse in it?


    --
    "I'd rather have a bottle in front of me than a frontal lobotomy"

    "Daily Thought:

    SOME PEOPLE ARE LIKE SLINKIES. NOT REALLY GOOD FOR ANYTHING BUT
    THEY BRING A SMILE TO YOUR FACE WHEN PUSHED DOWN THE STAIRS.
    http://webpages.charter.net/jamie_5"
     
  4. N Cook

    N Cook Guest


    I invested in a moulded rubber surround for my 77, after repairing one
    that had been jarred and the main SMD chip had loosened. Someone I know
    managed to put his knee on his, while working on the floor, and also
    dislodge the main chip. Both repaired after attending to the main chip.
     
  5. This might seem stupid, but have you checked the battery? Is the "lo bat"
    annunciator displayed?
     
  6. James Sweet

    James Sweet Guest


    I have the rubber surround, and it hasn't been jarred, it was sitting in the
    closet where I always keep it.

    I ordered a Fluke 187, I've wanted true RMS for a while anyway. When that
    gets here I'll use it to try to fix the 77 and use that as a backup.
     
  7. James Sweet

    James Sweet Guest


    It's not displayed, but then I suppose it's worth checking the battery
    anyway, I haven't, will do that tonight.
     
  8. Jakthehammer

    Jakthehammer Guest


    You should check its battery, that's where your problem is. Electronic
    meters utilize A/D converter, the converter uses Voltage reference, when
    battery is weak it give faulty voltage reference, as a result you get
    faulty reading. Do not rely on voltage reading, sufficient voltages do not
    mean sufficient Battery without enough current in battery.
     
  9. Jakthehammer

    Jakthehammer Guest


    Sorry if I confuse you, let me try again:

    You should check its battery, that's where your problem is. Electronic
    meters utilize A/D converter. The A/D converter uses bat as Voltage
    reference, when battery is weak it gives faulty reference, as a result you
    get faulty reading. Change the battery with fully charge.
     
  10. Actually, a semiconductor device similar to a Zener diode is used as the
    voltage reference. But the battery has to be at or above the device's
    threshold to provide a stable reference.
     
  11. James Sweet

    James Sweet Guest

    I put a brand new battery in it, the symptoms are unchanged.
     
  12. Meat Plow

    Meat Plow Guest

    Do you have a 77 or was the 79 a typo?
     
  13. James Sweet

    James Sweet Guest

    The 79 was a typo, I have a 77, and I ordered an 87, I'm full of typos
    lately. I really liked the 77, aside from its lack of true RMS, it did
    everything I ever asked it to, I hope the 87 lives up to what I've come to
    expect.
     
  14. Meat Plow

    Meat Plow Guest

    I have an old 77. Serial number 4333xxxxxx. Still works like a charm.
    Zeros out on any setting. Wouldn't mind having an 87 and keeping the 77
    for a backup. Where did you find the best price?
     
  15. Meat Plow

    Meat Plow Guest

    I've found my 77 will work even with a very dead battery. Dead meaning by
    the old taste test. Probably 6 volts or so.
     
  16. clifto

    clifto Guest

    Is the fuse intact?
     
  17. James Sweet

    James Sweet Guest

    I assume so, I didn't do anything that would blow it, but other times the
    fuse has blown it has not exhibited any unusual behavior. I know what I'm
    doing, I tried all the simple stuff, something has failed.
     
  18. James Sweet

    James Sweet Guest

    Ebay, they tend to go for about $160 used and in good shape, closer to $300
    new. There's several different models of 87, make sure you know which one
    you're getting.
     
  19. Jakthehammer

    Jakthehammer Guest


    Correctly, the zener (or an IC reference circuit) are part of the A/D
    circuit. You cannot design a complete A/D without Voltage Referencing.

    Jack....
     
  20. N Cook

    N Cook Guest


    If the fuse is blown you get a consistent reading of about 1 per cent of the
    true reading.
     
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