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UMO - Darlington transistor, maybe motherboard replacement

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Good morning to all, my first post here.

I have acquired an older UMO, it has been working fine, however recently the hotend fan began to act up. (I think an intermittent connection at the connector).

I decided to wiggle the wire, got the fan to run, however the machine then shut down, would not reboot for several minutes, then thankfully when I came back later it seemed to boot, except that the hotend fan is now running full from the moment of power on.

I have read up on this, I suspect the darlington transistor (BD679) and have ordered two of them. Of course, I am wondering, due to the age of this machine, if the motherboard itself has been damaged or may be approaching end of life. I have spent some time looking for alternatives, but am unsure what motherboard versions will support the UMO and, once that is answered, what sources have proven to be reliable.

At this point I would prefer a swappable unit as close to the UM OEM standard as I can get.

Thanks in advance for your help!


(Ottawa, Canada)

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I did the same on my UMO (shorted the fan wires) and the BD679 blew.

I ordered a few and carefully soldered on the replacement, everything is perfectly working ever since.

For replacement board, you can buy a new from a reseller but they are expensive.

I think Neotko have upgraded his UMO with the duet wifi board, of course it's not OEM

any longer but I think it's a cheaper and more advanced board (built in wifi, 32bit CPU, web interface, and your step motors are dead silent when it's printing)

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Thank you for your help with this. I soldered in the new Darlington transistor today and it appears to have done the trick!

One question, when I went to test the fan speed, I could not get it to run until I set a value over 200 (I think it was around there). After that I could set slower values and it responded accurately. Does it make sense that I need a minimum initial setting to get it turning, then can set speed after that, or is there still something amiss?

Thanks again


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Amedee that's reassuring, thank you. I also appreciate the link to your firmware. Once I have things stable and know what I need, I'll take a peek.

Honestly, still on a journey of discovery here. The print quality is already better than printers I have owned before, but due to the mileage on this unit, I suspect there will be a number of things to address in the short term. The challenge for me will be knowing what is normal and what is not.

For example, after system boot, I get this slow rising note for about 45 seconds, then it stops (or rises to a pitch I can't hear). It almost reminds me of the sound the old camera flash units would make as they charged, but a number of octaves lower.

So far I haven't had a chance to isolate the source. Pretty sure it is not the motherboard cooling fan (unless it is some weird harmonic), I know it is not the nozzle cooling fan. Any thoughts?

Much appreciated!


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