The mainmotor had lost power too from all those flying hours. So I've replaced the main motor too. This is an easy job done within 5 minutes. I took off the heat sink and cut the strip that held all the wires a bit together.
I first unscrewed the main motor. It is held in place by two screws at the bottom. By rotating the main gear the second screw becomes visible through one of the holes at the surface of the main gear.
The wires of the main motor are attached to the "4 in 1" receiver. You can easily plug them out.
The new main motor. It comes with two new screws.
The new motor screwed on and wires plugged in. Old screws next to the skids.
The bottom looked like this. Time to make a test hover. The helicopter made a strange noise and had almost no lifting power.
After a close examination I found out that the teeth of the motor made poor contact with the gear. I loosened the screws a little and pushed the motor more towards the gear. Very tight this time.
This also resulted in a strange noise while rotating and the heli had also almost no lifting power.
The third time the grip was not too tight nor too loose. The middle way. (sounds like Buddha ;-)
This time it worked perfectly and I had an outside flight of 8+ minutes.