Scroll-wheel encoder replacement in a Genius NetScroll 120

After taking a holiday, I decide to fix a scroll-wheel issue which has been annoying me for some time. Follow me down the mouse-hole and discover the secrets of cheap rotary encoders, as well as some ideas about replacing unknown components without a datasheet or schematic. 0:59 Fault description 2:14 Thoughts on the parts\repair 8:14 Note on the given measurements 11:00 Investigating the circuitry 22:05 Confusing readings of the new parts 23:19 Disassembly of a new encoder 30:11 Disassembly of the old encoder 35:54 Test fitting, modifying and installing new encoder 46:22 Does it work? 48:53 Conclusion and more thoughts Search link: https://www.ebay.com/sch/mouse wheel encoder Available sizes seem to be 5, 7, 9, 11 or 13mm, and this dimension refers to the distance from the top-side of the PCB, to the centre of the shaft hole (See diagram at 9:15).