If the LED would be short-circuited, I could imagine that it would destroy its driver transistor. With the power off, you could try to measure with an ohm-meter on the LED, while in-circuit. This method is of course unreliable, but if you would get 0 ohm, then it is at least an indication, and then you might consider removing it. But it seems unlikely to me that the LED would be the cause? It rather seems like an overvoltage spike, causing all the damage, including destroying the LED?
I read that a lot of switched mode power supplies cannot generate a stable output voltage if there is no load. Thus this applies to almost all modern supplies with tiny transformers running at high frequencies, instead of the old huge and heavy transformers running at 50Hz. These modern switched mode supplies need a minimum load to work well. So I could imagine that the output voltage would have been way too high, due to no load, and that your board got an overvoltage spike when you inserted the plug, and the overloaded capacitors discharged into your board? This is just guessing, since I don't know the circuits of the UM-board nor the supply, but it might be an explanation.
Some of the better switched mode power supplies have provisions to switch off the output if there is no load. Or they have an internal resistor over the output, to always give it enough load to be stable (the old Delta Electronics supplies had this, around 1990). But maybe yours doesn't?
But normally both the power supply and the printer board should have overvoltage protection diodes (transzorbs or zener diodes?), to absorb such spikes. I don't know if the UM board has these? Or maybe they weren't up to the charge?
It is best to only connect such power supplies with the main power off, and the supply discharged before connecting. To discharge it, place a suitable lamp over the output, for example you could use a suitable car or truck lamp, and solder wires on it. (Not a LED, unless you put a suitable resistor and protection diode in series with it, or unless you put two LEDs in anti-parallel, and that in series with a suitable resistor.)
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conny_g 251
Researching about it I find that @neotko killed these transistors a few times, so it seems to me they are in fact a weak point of the board.
Thinking about what might have happened.
The power adapter - the chinese one with 360W, not the original - might have significant output capacitance, maybe it even causes a voltage surge when plugging it in.
So possibly it sent that surge across the board on the 24V „rail“, that would hit all components exposed and surely these BC817 and the step down.
Then there is this damaged LED on the stripe, it could be dead due to that as well.
Or the LED dying actually killed the transistor? But that would not explain why the step-down died.
Wondering if that chinese power converter will kill my board more often. But on the other hand I am using the same model on my other printer for 1 year without issues...
Anyway, some more fun to be had with dissassembling the printer and repairing the board another time the next days. Would just ignore the light for a while, but the model fan missing is fatal. Had a really bad print due to that today. That triggers the repair.
Edited by conny_gLink to post
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