Fluke 175: Repair

I recently scored two Fluke 175 multimeters that were sold on ebay as a set waiting for repair. According to the seller, the first multimeter was fully functional except for the beeper. The second meter had a strange error rendering it unusable, but came with a working beeper. The sellers suggestion then was to transfer the known-good beeper to the first meter.

A word of caution: Multimeters can be used to measure sources that can cause damage, serious injury or death. If you are not authorized or qualified to work on such devices, don’t do it. I don’t take any responsibility for actions you take or the results of these actions. You do everything at your own risk.

With this problem description I was confident that I would be able to at least repair the first meter fairly quickly – it might be as simple as repositioning the piezoelectric disk which might very well be still functional. And sure enough, two minutes into the “repair” the meter was fixed by aligning the beeper correctly.

For the second meter I hoped for something more exciting, like a failed component. And indeed, the error seemed to be at least a bit more complicated: The meter did only power up in certain switch positions (e. g. ohms/continuity), with the display showing invalid readings in any of the “working” ranges. Also, in continuity mode the beeper was on permanently, explaining the seller’s remarks regarding the functional beeper on this unit. At this point I expected an issue with the rotary switch. Opening the meter revealed two things:

  • The unit uses a physical rotary switch. I never saw a teardown/picture of the 170 series meter with this switch style. Considering the ICs’ date codes (’01) it makes some sense though. The first meter was a much more recent version with the usual PCB switch contact setup (a gold plated pcb and upgraded circuitry)
  • one of the metal contacts of the switch came loose – all of the plastic pins must have snapped off due to aging plastics and/or mechanical stress. No surprise that the meter didn’t work properly

A quick web search showed that the switch isn’t readily available anymore. I found only one seller and a hefty price tag of about $100. So the only way forward was a switch repair.

The information presented can not be used as an instruction to repair multimeters. In particular, this repair does NOT guarantee safe operation of the meter. Under no circumstances should this meter be used to measure line voltages/currents. This repair might not be permanent.

a) Top-right Contact held in place by intact pins, b) Bottom-left contact with pins snapped off from aging/mechanical stress and glued back in place

This involved gluing the contact back in place using a strong two-component adhesive (UHU plus endfest 300). I carefully applied small amounts of the adhesive to the transparent rotating part of the switch using a small screw driver. Then I brought the metal contact into position and applied more glue at either end of the contact as well as to the holes, mimicking the plastic pins that snapped off.

With this approach I hope to increase the shear strength as much as possible without the risk of gluing the rotating part of the switch to the stationary switch assembly. The mentioned adhesive cures best at elevated temperatures, increasing its strength while reducing the curing time. The easiest (and hopefully sufficient) option for me was to just place the PCB on top of the fan outlet of my computer, heating the board (and switch) to roughly 35°C.

In theory it would have been fine to wait some three to six hours before using the switch, but I decided to wait until the next day. Despite being very careful with the application of the adhesive I had to use some moderate force to break the rotating part free – some adhesive ended up somewhere I didn’t want it to.

Next I functional-tested the meter. To my delight the meter works properly again – I couldn’t find any other problem. I’m curious to see whether/when the repaired switch breaks again.

A Fluke 179 with its two Fluke 175 siblings.

Finally I compared the readings of both Fluke 175 with my fairly new Fluke 179 and all three seem to agree almost perfectly, so no need for any further calibration/adjustment efforts. I call that a success. By the way: Cleaning the meters properly took me longer than the actual repairs.





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