05 Apr 2017, 22:30
The mercury switches operate in a closed-open-closed mode. It's the "open" period that determines whether the watch displays the time or not.
The switch must close between 1/10 and 2/10 of a second after it opens, or there is no display. This 100-millisecond window (time-domain- sensitivity, I think it's called) was intended to minimize the number of "false" display demands created by normal movements, and thus conserve battery life.
The commonest problem the switches develop is immobility of the mercury bead (it begins to stick to the iron electrode, making it impossible for the switch to "open", and so disabling the function). Replacement is the only remedy for this. In between is an entire spectrum of functionality, possibly due to slightly varying sizes of the mercury bead (I am guessing that a lighter bead might be somewhat more difficult to "get moving", due to the difference in getting a smaller mass to "break" away from the stickier, gold-plated brass, electrode, but, that's just a guess). Conversely, a heavier bead might be more easily dislodged, creating a "sensitive" switch. Again, replacement would seem to be the only cure here, as well (the much-smaller switches used in some 800 modules...which appear in the 5503 Dress and some Sport watches, are, of course, subject to the same maladies). It it also conceivable that there could be a defect in the part of the circuit that reads the open-to-close time, and interprets any interruption of the closed condition of the switch as a legitimate time demand (I don't see any possible fix for this).
Replacing an Auto-Command switch in a Calculator impresses me as being very much akin to brain surgery; you have to do a let of damage to get to where you need to go, do the fix, and then repair all the damage you just did, on the way out. You have to split the module wide open, and the switch is on the main circuit board, right next to what must be, judging by its' size, the calculator chip itself. Every wire bond on the board (and there are scores of them) is open and unencapsulated, begging to be dislodged or broken. Given the risks of doing additional or even catastrophic damage, I see this exercise as taking a very big gamble for comparatively little gain.