[new topic for adding this to the Vectrix Collaborative Handbook?]
The MC Fan is regulating again. What i did is replace 3 components on the MC board, U4 (PS2801), R22 (1K) and Q2 (FDS5670).
U4 is an opto-coupler which receives FAN-PMW signal from the processor and controls the fet Q2. This fet drives the MC fan.
Components can be find on Ebay. Special thanks goes to David, who explained the logic.
For replacing the components, SMD soldering skills is recommended.
U4 and Q2 removed: