Hi,
Since my last post i have asked to TTS about that defect. They talked about infrared LED.
I can use a solder so i tried to open one. I have discovered that they have 1 infrared(IR) LED + 1 infreared photodiode for each wheel and they use them as an encoder (that is to find the wheel position ref). They face each other with a gear in the middle. I buied some new components (3mm diameter and 940nm wavelegngth like these).
I replaced all LED and photodiodes and now they are working. One of those can't go straight even after the repair, but i have discovered that it can happen if the LED+Photodiode are not well inserted.
I have now repaired 4 of them in this way, and one does not go straight.
I hope this can help you.
Feel free to ask me if there was something not clear
