The trickiest part about transplanting parts from a parts mouse to a working mouse, is keeping track of which one is which.
Third, fully-functional mouse for added confusion.
Backstory: one of my Sculpt Ergonomic mouse's scroll wheel died. A new set is $100-150, and a mouse with no dongle is $30 shipped on ebay.
Putting the oscilloscope on the scroll wheel sensor's pins tells me one channel on the broken mouse has died, and the donor mouse is fine.
This little three pin part is the optical sensor. It has no part number printed on it. It contains two infrared sensors, which receive light through gaps on the scroll wheel. The relationship between the pulses from the wheel spinning, and how the spacing of the two sensors works with the spacing of the holes in the mouse wheel, allow the mouse to sense speed and direction of the scroll wheel moving.
If one sensor is broken, it can tell neither speed nor direction. This is what happened to mine
With this one part swap, the scroll wheel on my mouse works again. I'll use some parts from the parts mouse to fix up the case of my other mouse. I like these so much, I have two sets.
Here is the scroll wheel mechanism installed. You can see the clear IR LED at the bottom, and the optical sensor on the other side of the mouse wheel. On the left are two arms that rest on two buttons - these are for when you tilt the wheel left and right for horizontal scrolling.
Randy Glenn