Multimeter Range Switch Contacts: Whoops!

One of my multimeters began reporting bogus values that improved by working the range switch back-and-forth, which suggested the switch contacts need cleaning. Taking the meter apart was easy, right up to the point where I removed the range switch from the PCB by compressing the four locking tabs on the central shaft:

Multimeter range selector switch

Multimeter range selector switch

Just before taking that picture, the switch launched half a dozen spring contacts across the bench, my shirt, and the floor… I recovered four for the picture and later found a fifth smashed on the floor, but the last contact remains AWOL.

The contact in the middle, the oddly shaped one with small tabs on the ends, is a prototype replacement conjured from 6 mil phosphor bronze stock:

Multimeter range switch contacts

Multimeter range switch contacts

The little domes ensure a good sliding surface, but require two bends in the middle of the contact and some way to shape the metal into a dome. After a few experiments, I filed the end of a nail into a rounded chisel that worked pretty well:

DMM switch contact punch

DMM switch contact punch

The original contacts came from 3.5 mil stock and have considerably more flex; 6 mil stock is what I have.

I think I should make half a dozen contact springs to replace the entire set, a task requiring more time than I have right now. For the record, the overall process goes like this:

  • lay out overall shape, slightly longer than needed
  • cut center opening with abrasive wheel
  • cut out contact
  • punch contact domes (from back = dimples)
  • bend to shape
  • trim ends to length (not done in picture)
  • dress raw edges (not done in picture)

Given the number of parts and the fiddly accuracy required to make the slot, this might be a good job for the Sherline, although clamping each little proto-spring down while getting the abrasive wheel in there seems daunting.

Perhaps cutting the slots and punching the dimples would work better before cutting out the contacts, with a sheet clamped on four sides? The center will be floppy, what with all the slots, but grinding slots on the middle contacts first might be helpful. Would adhesive under the sheet to hold down the middle gunk up the abrasive wheel?

So many projects …

Memo to self: Springs! Always expect springs!

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  1. #1 by Raj on 20-September-2012 - 08:25

    What brand & model is it ?

    • #2 by Ed on 20-September-2012 - 08:39

      It’s an ancient house branded meter from Marlin P Jones & Associates: MPJA 9903.

      We all know it’d be easier to just scrap it and buy a newer, cheaper, better meter…

      • #3 by Erik on 20-September-2012 - 10:22

        I wouldn’t scrap it under those circumstances, but I might just figure out which traces to jumper over and turn it into a panel meter. It would be amusing, for instance, to have a voltmeter constantly watching the 240/120 distribution in the house, or current to the bench, or something like that. I saw a neat hack with a pendulum, a potentiometer, and a voltage regulator – 0 to 9 volts indicated 0 to 90 degrees. Or use a D/A output to turn it into a handheld longboard speedometer. Millions of uses, some practical. :-)

        • #4 by Ed on 20-September-2012 - 11:18

          or current to the bench

          Now there’s a thought… [grin]

      • #5 by Raj on 21-September-2012 - 02:40

        I was wondering if one could cannibalise another old meter or even a new 4$ meter!

  2. #6 by Red County Pete on 20-September-2012 - 11:27

    Hmm, guess I should take a look at my old Fluke. DC voltage was fine, but the last measurement of AC gave me 85V or less on a 110 circuit. (Bad diode in a bridge?) Considering the real voltage was about 105 (that bad neutral I had at church), the Fluke had to be swapped out in a hurry. It’s rather old, and the LCD got a bit messed up in a freeze 20 years ago, but it still works on DC. Be nice to have a backup DMM I can trust.

    • #7 by Ed on 20-September-2012 - 12:27

      a look at my old Fluke

      I still miss my old true-RMS Fluke meter. It just up and died one day, refusing all the usual laying-on-of-hands repairs. The new meters just aren’t the same…

      But expect springs when you take the cover off your meter!

      • #8 by Red County Pete on 20-September-2012 - 16:24

        I’ve dismantled an old Lowrey and am doing the same to a Thomas organ. Lots of springs, though most are captive. Most.

        • #9 by Ed on 20-September-2012 - 20:05

          most are captive

          Good luck finding the rest! [grin]

  3. #10 by RonL on 20-September-2012 - 21:49

    If you want a cheap replacement, get one of the Harbor Freight specials, like this one.
    http://www.harborfreight.com/7-function-digital-multimeter-69096.html

    It’s $4.99 and you can knock another buck off with the 20% off coupon on the main HF web page.

    For most things, it works well enough.

    • #11 by Ed on 20-September-2012 - 21:56

      For most things, it works well enough.

      Yeah, I have a couple of those craptastic meters from other sources… some of them actually worked for a while, too!

      The MPJA meter has had a thermocouple input that came in quite handy. Fortunately, it also used a standard thermocouple connector, so I could swap in the Fluke temperature meter Aitch gave me a while ago…

    • #12 by Raj on 21-September-2012 - 02:38

      I bought a yellow one for 2$ locally here in India.

      • #13 by Ed on 21-September-2012 - 08:14

        Yeah, but you’re closer to the source! [grin]

      • #14 by RonL on 21-September-2012 - 08:56

        HF has a meter for every need! :-) For $19.99 ($16 after coupon), you can get one with temperature measurement capability.

        http://www.harborfreight.com/ac-dc-digital-multimeter-37772.html

        It’s hard to be sure from the picture, but it looks like it has a standard thermocouple connector on it.

        Overall, the quality doesn’t compare to a Fluke, but in my experience they tend to be accurate and last long enough that I think they are good value.

        • #15 by Ed on 21-September-2012 - 09:07

          they tend to be accurate and last long enough

          By now, that’s about all one can expect…

          Some day soon, I must ride out Spackenkill Road with a 20% coupon in tow!

  4. #16 by Raj on 21-September-2012 - 07:08

    A blub lit up and I started to hum the Bee Gees tune Idea!.. you probably can do with one contact short! There is one which is used as a power switch. Remove that spring and add a small toggle switch somewhere.

    • #17 by Ed on 21-September-2012 - 08:22

      one which is used as a power switch

      I can’t find a pattern that goes all the way around. The ring from the outside could be a power switch for half the rotation, but they’re definitely switching plenty of other circuit functions with the other contacts!