[goldenarrow]

There is a comment I got from Pentax Service in Russian
There are (were) two issues with 16-50

1. Sudden SDM switching off. The issue has been resolved in lenses that is produced since 2013
2. There is design deficiency (or peculiarity). The SDM consists of two main parts: rotor and stator. They are separated with very small distance (about microns). So any stuff between rotor and stator increases friction and power of the piezoelectric elements is not enough to rotate the rotor.
   The sources of the stuff are: dust, chemical reaction (oxidation), lubricant evaporation. The latter two are originate from prolonged lens disuse. In order to avoid this it is enough to use the lens once per week.

Thank you for mentioning this interesting info. Don't think I have heard that this might be the main reason why the SDM fails so often. Maybe only place in the internet where this is mentioned (at least in English!)....

[paolotrabuio]

It's a software fix. If you read the "definitive link" above, it details how to put an older pentax digital body into "debug mode" which allows you to load the lens internal programming onto your memory card, edit it (to always use the screwdrive, and ignore the SDM regardless of the camera body) and then re-load the changed software back into the lens. The lens is permanently changed regardless of what body it goes onto afterwards.(the software is inside the lens, not the camera).

I did what the link suggestes and now my 16-50 work perfectly with "screwdrive". Thanks a lot for the explanation that made me confident on result.
Have a nice light!
Paolo