How to fix the 4.3V output supply of a NXT 2.0?
Finally I've found a solution. It is not perfect since I wasn't able to find the schematics of the NXT-2.0 nor could I reverse engineer it. But this workaround seems to work pretty well.

This workaround works at the following conditions:


You have an NXT-2.0 (not a NXT-1)
The 4.3V voltage for the motors and sensors is no longer there (Voltage at C61 is zero.)
The 5.0V voltage VCC_RS485 has still 5.0V (Voltage at C57 is 5V)


As you can see in the image below I soldered a 1N4007 rectifier diode between the positive terminal of capacitor C57 and the positive terminal of C61. The cathode of the diode has to touch C61.
Ensure not to touch the test-point T41 close to C57 since this should have 3.3V on NXT-2.0 (while it had 5V on NXT-1).

A silicon rectifier diode causes usually  a ~0.7V drop. So it makes 4.3V from the correct 5V source. If you don't have an exact drop of 0.7V don't worry: The NXT motors work also with the Lego-EV3 and according to the spec. of the EV3 that brick sends even 5V directly to the motor-sensors. (So even a wire instead of the diode might work - but I used the diode to stay on the safe side)

To solder the diode you have to open the NXT and to disconnect the battery holder, which requires some desolder work.

Dependent on your skills this might be some tricky work. Please don't blame me if this kills your NXT totally. Only do it if your feeling qualified to do it and if you've understood the explanations above.