I have the similar issue, from my searches it is about the plastic coupler broken between motor and gear. They described some solutions, I opened the camera removed the plastic coupler (it was broken) but could not fix the spring to gear shaft back. Now it is waiting in a box, for me to spend some more time and fix it. I hope to fix it in future, but I need to remove the motor and it is difficult. I need some new tools to be able to remove the motor (I need soldering iron and need to learn how to solder first). There are lots of sources in internet and many people discusses same issue (in Flickr, some websites, etc...).
Inside of the SX70 camera is quite impressive and complicated for me. I hope I can fix it as I love SX70 cameras. I can post some photos of it if needed.