If you can open it, you can explore the problem, look for a fix - but these days it's very likely you will break the plastics in trying to open it. THAT is the value of this page, it reduces the risk of broken plastic tabs etc. because someone has already "been there". I've just rescued my (otherwise perfectly good) M180 by cleaning out the gunk that had collected inside the wheel - which was interrupting the optics that count wheel rotation. I didn't need a guide for that part...