Monthly Archives: July 2015

Servicing a Fluke 12 Multimeter

One of my most-used tools on the workbench is my Fluke ’12’ multimeter. I’ve had it almost 20 years, and it’s my favourite meter because it was clearly designed by someone who had to fix things. It’s rugged, especially in its bright yellow holster, and has so many thoughtful features: it has big buttons and a switch, instead of a rotary knob, so it’s easy to use with one hand. It autoranges quickly and reliably. In continuity and resistance modes, it automatically switches to measuring voltage if it detects one, so you don’t need to worry about changing ranges when debugging things. It doesn’t have fiddly extra features. It doesn’t even have a current range, because it would make no sense: measuring current usually involves breaking a circuit, which you can’t easily do when working on a circuit board. It switches itself off when unused for half an hour or so, saving the battery.

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It’s been completely reliable apart from needing a new set of test leads last year (for the first time). However, recently the big, chunky buttons had become reluctant to respond, and needed firmer and firmer presses until they didn’t work at all. That meant I was stuck measuring either DC voltage or continuity. Time to pull it apart and see what’s wrong.

Removing the test leads and holster, then taking out the four screws, reveals the view you get when changing the battery. I’m sure Dave at EEVBlog, master of the multimeter teardown, would approve. Lots of chunky components and very solid construction.

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The PCB is held in by the plastic clips at the top and sides. Easing them back lets it and the plastic frame underneath it come out:

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The black plastic internal frame (under the PCB in this photo) is marked as being made of polycarbonate, so it’s very strong. None of that high-impact polystyrene rubbish. It’s nice that the membrane for the buttons bears against the frame, not straight against the PCB. This is nice industrial design.

The button membrane is connected by a zebra strip to the PCB. The PCB itself looks nice and clean, and the zebra strip is OK, but the contacts on the membrane look tarnished.

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I wanted to test the membrane using the continuity check function of my trusty Fluke 12…oh, hang on, it’s in pieces. Break out the equally trusty Avo 8.

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The membrane itself works fine. I cleaned up the contacts using DeoxIT D5 on a piece of paper. I also cleaned all the plastic parts, including the holster, in the office sink with washing-up liquid. Here’s the result, showing resistance mode to prove that the buttons work.

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Looking good, working as well as it did when new, and ready for the next 20 years.

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