Broken hotend fan (mosfet?)

@Mechanikus im hoping you might be able to help me given all your time working with the mainboard as part of your Duet replacement project (thank you for that pinout!).

My right hotend fan got ripped out due to ‘compounding factors’ from a bad bed level, and while I soldered it back on fine at the hotend PCB, I think it must have shorted a mosfet in the drama, as it now runs permanently at 100% speed, right from boot.

I actually have a spare mainboard (long story), so I’m hoping to swap out the right mosfet and see if it fixes it - but I’m pretty lost as to where it might be. Do you happen to know? I’m at the edge of my experience level with this issue and don’t want to just wade in recklessly (if avoidable).

Any help would be greatly appreciated.

@sjm88 Sorry, but that is something I fear I cannot answer as well. Since it was clear for me that the board would be replaced soon with the Duet I focused pretty much on the pinout, and finding no visible security measures (fuses, sufficiently powerful resistors or such to cope with e.g. a 24V short in the heat block against the level sensor, anything…) at all on the J1 board convinced me that might be indeed a good idea.

The only thing I saw was that the board is multilayer despite its considerable size which makes it difficult to trace signals without the schematics.