Originally posted by robjmitchell I'm sure it's not the nylon gears, its the motor on top. The photos show that the screw drive feeds in from right and the tiny micro motor via the left into the nylon gears that then drive the focus mechanism.The solution was to install a more powerful SDM motor (from various tidbits on the web) This would cause more failure of nylon gears if that was the problem.
The SDM failure is characteristic of motor burn out, a broken gear would create a loud noise as the mechanism slips and hits. It would be sudden and complete, not the slow death seen in SDM failure. From the looks of it to replace the SDM motor requires pulling out the motor and gear assembly to reach an screw that holds the motor and its nylon drive gear in place. Kind of hard to tell what their doing when we cant speak the language though
Rob, the SDM is a piezo-electric motor, so "burnt out" gives the wrong impression of the failure. It's not like an electromagnetic motor where coils carry currents, which can overheat and literally burn. Having pulled my 16-50 SDM drive apart more times than I care to recall, I can tell you that it jams at one extreme of movement, and will free up with the application of a modest amount of wrist action via a Philips head screwdriver on the shaft, but in the finish they just keep jamming at shorter intervals (I converted my copy to screw drive when the degree of frustration just got too much). I suspect the culprit is distortion of the piezo elements, but I haven't actually pulled the motor itself apart to test that theory. I really doubt the replacement motor has that much more torque than the original, to the extent that it would strip the plastic gears in the drive train. I'd be more leaning toward misalignment in the repair job, or bearing wear, but I'd be happy to be corrected by someone with design experience in the specific area (mine's been in somewhat larger gear trains!).