The problem... the mode switch has two shafts,
coupled between the front detent (1/4 inch shaft)
and the rear contacts (3/16 flat walled shaft). The
coupler is plastic and this one had become loose;
not gripping the contact shaft as it should.
This allowed the rear shaft to slide back with use
until it was clear of the coupler.  I had a couple of
choices here to fix this. I could have taken the
switch apart and used epoxy to glue the shaft into
the coupler. I decided against this approach
because the coupler was so close to the
contacts... any epoxy on the contacts would ruin
the switch. Another reason is that once glued
together, the switch could never be disassembled
again. Also, the switch can be inserted 180
degrees off, and making that correct orientation
during assembly was a risk I did not want to take..
Instead, I chose to make a retainer for the back of the shaft, out of Styrene plastic. I used heavy walled plastic pipe
and a small section of sheet plastic. This was assembled to make a cap, which was worked  to the precise length to
just clear the shaft. This was glued to the plastic shaft support at the rear of the switch. This keeps the shaft where it
should be and the switch is still serviceable should the need arrise...
Problem Switch...
A loose shaft
coupler  allowed the
rear shaft to slide
free from the forward
knob and shaft. You
can see the location
of the shaft coupler.
It sits right in front of
the first contact wafer.
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