This is a DIY instruction for attempting to repair your expensive Pentax SDM (DA*) lens wich has lost its ability to focus. Please do not attempt this fix if your lens is stil covered by warranty! Only try this if you are confident with your own skills - including handling all the tiny parts and tight fittings of a modern lens.
This guide is done based upon findings by several Pentax users online. Repect goes out to RiceHigh for getting my attention through his blog post and wayli for his original post at the Chinese Xitek forum.
Don't blame me, Pentax or the Internet if you fail this operation or even further damage your lens trying to repair your lens! I take no responsibility for any damage that may happen by following these instructions.
Start by removing two small black screws (marked in red) that hold the SDM contact itself. Then remove the SDM contact-module by carefully pulling it straight up. You can try to help it a bit with a small spudge or a flat screwdriver. Don't use excessive force.
Image shows lens with SDM contacts module removed.
Remove the five metallic screws around the lens mount itself (marked in blue in above photo). Remove the mount itself by pulling it straight up. Take your time - don't rush as you risk spreading small parts all over the place!
Using a pair of tweezers - remove all seven golden spring loaded contacts from the lens. Note that one of them has no head, it's just a golden spring.
Using tweezers, unmount the spring that connects the aperture coupling with the lens body. Remove the coupling by pulling it straight up, it should come off easily.
Remove four black screws that hold the rear part of the lens housing. Remove housing by pulling up.
Notice the black gasket that belong between the rear part and the rest of the lens body. This is a part of the weather sealing of the lens. Put it aside and remember to put it back on when mounting the lens back together.
Now for the good stuff: using the screwdriver, try to twist the back part of the SDM motor back and forth. It is (and should be) rather tight. Don't apply more force than you're comfortable with. Hopefully this movement will unjam the mechanics inside the motor and once again have working autofocus.
Remember the gasket? Put it back onto the rear housing before attaching to the lens. This can be quite tricky as it twists and wants to fall off. Be patient. Don't put the gasket on the lens itself, as it will most probably not sit right when you screw the lens back together. When you finally manage to hold the gasket onto the rear housing, put it back onto the lens and screw in the 4 screws that hold it in place. Don't screw them too tight though, as it will make your zoom ring very tight and hard to operate. You can try to adjust the stiffness of your zoom ring by adjusting these screws.
Add the aperture coupling back onto the lens. The lever/pin that goes into the lens body should be aligned into the part marked in red in the following photo:
Using tweezers, reattach the spring that hold the coupling to the lens body. The spring (blue) should attach to the hook (red).
If you've done it right you should be able to stop down the lens by carefully twisting the aperture coupling. When released the aperture blades should be open.
Put back the golden lens contacts. The one without a tip belongs to the hole that is slightly raised compared to the others.
Carefully add the lens mount, making sure the golden lens contacts fits through their holes in the mount. You may have to twist, tilt and tap the lens and mount a bit to finally make it all align. Check to make sure all contacts are correctly aligned before you screw back in the five metallic screws (marked in blue) that hold the mount. Also check that the gasket around the mount sits as is should. This is also a part of the weather sealing of the lens.
Add the SDM contact module. The pins on the back should fit the two golden sockets inside the lens. Make sure they align and don't apply force, risking the contacts getting bent. Screw back in the two small screws that hold the SDM contact module (marked red).
Check that you have no parts left on your desk that you've forgotten to put back onto the lens. Check that there are no loose parts inside the lens. Check that the mount looks aligned and otherwise appear normal.
Now it's time to see if the repair worked. Put the lens back on your camera. Keep your fingers crossed and fire it up. If you're lucky like me, and several others - your lens should once again be autofocusing using and SDM enabled camera. Congratulations - you've just saved $400 in repairs!
You may want to tell others about your success or failures following this procedure. Please feel free to do so in the following thread @ pentaxforums: http://www.pentaxforums.com/forums/pentax-slr-lens-discussion/130429-diy-supersonic-drive-motor-repair.html