About a week ago I reported the mechanical issue I had on my 121 with the switch and input terminals here ;
https://www.eevblog.com/forum/testgear/eevblog-121gw-multimeter-issues/msg1403241/#msg1403241So after returning from holiday I thought I would have a look inside at what may be happening with my meter.
What I can see happening with mine to cause my issue was the small flexing of the PCB by the input plugs movement
was causing referred movement at the switch area and since my switch was rather 'sloppy' in its location hole this had caused
either contact or contact location issues on their relevant PCB tracks and thus the mode failures.
Further investigation of the switch rotor showed quite a lot of lateral movement in the PCB locating hole.
So next step was what to do to reduce this lateral 'slop'!.
After removing the PCB from the switch knob I could test fit various rod diameters, (rear ends of twist drills in 0.1mm inc.), to lightly expand
the switch plate plastic fingers that go through the main PCB to remove the slop but still have easy rotational movement.
The best size for my meter was 6.1mm OD. To help alignment further I made the ID be a good sliding fit to the hex shaft of the knob at 5.6mm.
The resulting sleeve I machined up can be seen in the photo below in its final position (I left it longer so its easily removed.). The material used was PEEK which is
reasonably stiff and has good insulation properties, (I also had it anyway
). Did not want to use PTFE which would have been too soft at the cross section used.
As an additional exercise I also cut up some 0.3mm PTFE sheet as spacers under the knob circlip, see photo below. Two pieces provided a good take up
of the space between the housing and the circlip. This reduced the knob free play up and down but allowed some compliance in the well in the top housing and the circlip too
due to the soft flexible nature of the PTFE.
So the end result of this exercise was a considerably less 'wobbly' feel to the knob and more importantly all the issues I had reported have now gone
completely and any movement of the input plugs have no effect on the switch nor does wiggling the knob itself set off any failures of the mode its in.