Excellent post -and informative. Thank you.
Actually, the 100's AF problem wasn't a gear stripping. But rather that the drive gear on the AF motor cracks at the weakest point. Even before fully cracking and losing the AF, the gear will start to slip as it is simply tension held onto the motor spindle. That loss of tension is what causes inaccurate AF and eventually leads to complete loss. The problem was that the gears are made of a cheap nylon polymer and tiny. Here's a broken gear:
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The gear removed, and on the tip of a toothpick to stretch the crack. A UV flourescent adhesion agent was applied, but shows the ragged tear very clearly. The toothpick gives an idea of the size...
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Superglue and other cyanoacrylate adhesives won't stick and just ruin (melt) the plastic, so a UV cured adhesive is placed. I actually overfilled the whole gear to give it more strength as well as coating the inside of the shaft and other surfaces. Here it is curing:
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Once cured, I trimmed, shaved, and polished the adhesive, then shoved the gear back onto the shaft. Clean off the old grease on the other gears and lubed them with some Nye synthetic.
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Lens is working great and nails the AF dead on every time. Well, that was after adjusting the focusing screen, which is a whole 'nother story.