Connect with us

Unstable ohmeter

Discussion in 'Electronic Repair' started by Peter Easthope, Jul 2, 2013.

Scroll to continue with content
  1. I have a Sanwa BX-505 analogue multimeter where the needle wavers and can not be zeroed on the x1, x10 and x1k scales. Zeroing is possible on the x10k scale. The ohmeter is really an electromagnetic voltmeter connected in acircuit with dry cell, resistor, zeroing potentiometer, leads and a subject resistance.

    Scans of the relevant pages from the manual are here.
    http://carnot.yi.org/SanwaBX505Manual.jpg

    What is the cause of this behavior?

    Thanks, ... Peter E.
     
  2. Phil Allison

    Phil Allison Guest

    "Peter Easthope"

    I have a Sanwa BX-505 analogue multimeter where the needle wavers and can
    not be zeroed on the x1, x10 and x1k scales. Zeroing is possible on the
    x10k scale. The ohmeter is really an electromagnetic voltmeter connected in
    a circuit with dry cell, resistor, zeroing potentiometer, leads and a
    subject resistance.


    ** I take it that analogue multimeters are new to you.

    The ohms ranges depend on AA and a 9v batteries - they must have sufficient
    voltage or the needle will not zero.

    Try some new AAs and make sure all the contacts are clean and bright.

    A little WD40 sprayed on a rag is good for this.

    BTW:

    I used to own your meter's bigger brother:

    http://www.radiomuseum.org/r/sanwaelec_multimeter_n_501n50.html

    It featured a 2uA, taut band movement.


    ..... Phil
     


  3. What he said.

    I would only add that pushing, poking, tapping around etc while you are
    looking around inside might help reveal the one or more bad connections that
    might be there.

    Also, since these problems tend to be pretty intermittent, you might think
    it is fixed, only to recur later, so don't give up after one or two failed
    tries, you'll fix it soon enough if you keep trying.

    I have a Simpson 635 and a 270 series 2 that have had similar problems, and
    a Fluke 8840A which also had issues with the low range ohms.

    When somewhat more current is required, as in a Rx1 or Rx10 low ohms
    function, poor connections become a much larger percentage of the total
    circuit resistance.

    Speaking of which, for troubleshooting, one trick would be to remove the
    battery, short across those terminals, and measure the total resistance
    through your lead wires and through the meter itself using another
    multimeter. Meanwhile, the aforementioned pushing, poking, etc might show up
    the culprit on the other meter.

    Have fun!

    Mark Z.

    Mark Z.
     
  4. Sometimes it's just a matter of pulling the leads out of the jacks, and
    putting them back in.
    I didn't think analog multimeters had fuses.

    But, one time one DMM was reading odd, and I ended up opening it up,
    expecting the contacts on the switch to need cleaning. NO. It turned out
    eh fuse was bad, it didn't open but had a higher than normal resistance.
    I can't remember what was "high", but it wasn't a large amount, just way
    more than it should have been.

    Michael
     
  5. Phil Allison

    Phil Allison Guest

    "Michael Black"
    ** Some did, to protect the current ranges.
    ** A 0.5amp fuse is often used in the ohms and 200mA ranges of many DMMs.

    Some 0.5amp fuses have 2 to 5 ohms of resistance while others have only 0.3
    ohms.

    Depends on the fuse wire being copper or tin alloy.


    ..... Phil
     
  6. A freshly charged AA rechargeable didn't help.
    Testing the probes and leads was easy. Remove the black lead.
    While holding the red probe on the black socket move the lead
    in various ways. No correlation between needle movement and
    lead movement evident. Likewise for the black lead.
    The wiper-leaf-spring is held on the back of the switch with
    one nut. That was removed and all the contacts were cleaned
    thoroughly with a cotton tipped applicator. The dirt was only
    some blackness left on the applicator. With the switch
    reassembled the ohmmeter was dead stable again. Appears the
    problem was just the barely visible trace of dirt on the contacts.
    There is a spare fuse in plastic clips beside the working fuse.
    The two fuses have about the same resistance. Swapping fuses
    made no improvement.

    Thanks to everyone for the tips, ... Peter E.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-