Originally posted by jddwoods Unfortunately two SDM versions; DA 17-70 and DA* 55 are SDM only and cannot be converted. I will bite the bullet and send my 17-70 to either Precision or KEH and get the motor replaced. Hopefully the new motor will be a more durable design than the old one. Had it been convertible to screw drive I definitely would have gone that route.
The DA* 60-250mm f/4 is another lens where, unfortunately, screw-drien conversion is not possible. At least with the conversion available for the DA* 16-50mm, one then has a full-function lens. And there are many reports of the screw-driven version having faster AF, but maybe not with the DA* 16-50mm. My FA* 300mm f/4.5 has only screw-driven AF but its AF is very fast and sure on my KP. In fact, the AF of other screw-driven lenses I have, even the DA 55-300mm f/4-5.8 WR, have shown decidedly improved performance with this camera! Therefore, one advantage of the old screw-driven technology, in addition to reliability, is the possibility of upgraded performance via a new camera model! The premium Pentax Limited primes continue to be screw-driven.
I've seen many instances come up on this forum, including this thread, where the SDM was replaced only to fail again. The SDM issue, in addition to cost, was my reason for choosing the Sigma 17-50mm f/2.8 EX DC HSM instead. No WR, but I am very pleased with it in every other way.
Pentax has since switched to DC motors, which have proven to be reliable, or in some cases a new SDM motor design. So it mystifies me that there was no upgrade of motors in these older lens models, replacing the old failure-prone designs. The highly-regarded DA* 60-250mm f/4 especially beckons for such an upgrade, with its very fine performance capability, and also its adaptability for FF use. Developing a whole new DFA 70-200mm f/4 with a reduced zoom range instead of upgrading the AF motor in this fine lens was a decision that is beyond my comprehension.