I opened up the mouse (you need to remove the feet and there is another screw under the G500 sticker) and immediately realised that the lens in front of the laser was loose. The way it was mounted to the mouse was a very poor design in that it was essentially just floating in place.
I used a drop of superglue (cyanoacrylate) to firmly fix the lens and then tested the mouse - everything seemed to work great!
The next morning the scroll wheel stopped working. On investigation, as the superglue dried, it released fumes that had fogged up the optical mechanism that tracks the scroll wheel.
I returned the mouse to the vendor and then got a new one. The new one also had a loose lens with the same rattle and poor tracking symptoms. Either I had a bad batch, or this was a common problem. A bit of Googling and it turns out that this is a common problem (example thread below).
Rather than return the second faulty mouse, I decided to fix it with a less volatile glue (hot glue) and now it works perfectly.
If you get a mouse with this problem - it's probably best to just fix it... since your replacement mouse will probably have the same problem.
Keep the following tips in mind:
- Carefully remove the slippery feet with a pocket knife. They will be easy to replace. You will need to remove all the screws under the feet.
- Feel the G500 sticker to locate the hole for the last screw which will also need to be removed.
- The top shell is attached to the main chassis by a wire (to connect the LEDs and top buttons). If you can avoid it, you should leave this attached - it's a pain to reattach it if you remove it.
- Remove the screws that hold the rest of the mechanism together and you will find the loose lens.
- Use a hot glue (or other non-volatile glue) to glue the lens to the circuit board. This is best done by gluing the plastic pins on the lens.
- Reassemble and enjoy!