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UMO nozzle fan not working

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Hello! This is not my first time here but I could not log in with my former user as it has an ! in it and it is not a valid character anymore...

Yesterday my nozzle fan started failing on and off while printing. I realized it was the connector in the nozzle side. I had no time to make proper repairing son I just cut and soldered to continue printing. The fan no longer spins...

I checked the fan works with a 9V battery and revised the wires which are ok. Measured the voltage in the wires and it is always around 14V, no matter if the fan is supposed to be off or at 255 speed. Is that normal? Could I have broken something in the electronics while repairing?

Thanks in advance!

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If it spins at 9V and it is now 14V it will certainly spin! So if you see 14V the fan must not be connected.

I would ohm out the wires one more time end to end (under the printer to fan) - each wire. Just to make sure you repaired it properly.

But it's probably the darlington transistor: Q4 - this fails easily and can fail in two possible modes: always on, always off. You may have it failing in the "always off" mode. Or you didn't properly repair the wires.

Part number BD679.


schematic here:

The circuit diagram, and board layout are here:


There is a zip file at the top. It contains the "brd" file which is the layout. Also the "sch" file which is the schematic.

Both files can be opened by eagle software which is free:


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I just disconnected the blue and purple connector from the PCB and connected it to the electronics fan connection; the fan spins so the wiring is right from one side to the other.

I measured the voltage at the nozzle fan connector in the PCB and it is 19.4V when fan speed is 0, and 14.4V with speed at 255. As far as I know the fan should be spinning in any of both cases, right?

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That's the voltage with no load. Once you put a load on it (the fan) there isn't a good pathway for enough current to get the fan spinning. I'd say 99% chance the problem is Q4 now.

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