The whole fan test and repair as follows.
You will need a hot (lead free) soldering iron, a solder sucker and normal hand tools.
Remove cover from fan.
Locate the three large soldered connection to motor winding in the centre of the board and unsolder these.
Look at each power transistor and identify the thin track to each - this is the gate drive and NOT the terminal to test.
Measure the resistance between the other two 'thick track' contacts on each transistor in turn. The broken one will measure a very low resistance.
Transistors fitted are type 2SK3755, you can get these from RS or Amazon but long delivery when I wanted some.
Transistor used was a TK72A12N1, (RS 896-2432) slightly higher thermal resistance but lower on resistance.
Remove the heatsink from the board and transistors. This involved digging solder from cross head screws but easy enough.
Unsolder the faulty transistor, preform the legs of the new one and carefully solder it in making sure solder does not bridge to other tracks.
Refit heatsink, resolder motor windings, cover on and all done.
Enjoy