I haven't found a topic about this so I wanted to share my knowledge so that hopefully somebody else can fix this awesome monitor too.
My Dell Ultrasharp U2412M turned dark yesterday and at first it appeared that just the power was gone cause nothing worked at all. No image, no led, nothing! I checked everything and the monitor was definitely getting power from the wall.
Due to the fact that the monitor is 4 years old, I dissasabled it (thanks to this guy for a nice video how to get it open: https://www.youtube.com/watch?v=5j2xMS8MvCc ).
After I opened the power supply, I checked all the capacitors, because they tend to loose capacity over time and therefore this is a common issue. But all of them were perfectly fine. But in this case I didn't suspect the caps to be faulty cause this often results in flickering, screen turning off etc. but in my case there was no power at all.
So the next thing was to check the other components. The component tester of my Hameg oscilloscope showed that there was no current at all getting through one of the UF5404 diodes on the board.
I also checked the diode after desoldering it with the diode tester of my multimeter but the strage this was that it seems fine there. The difference between the component tester and my multimeter is basicly, that the component tester actually simulates a real tester where an alternating current is send through the component you test so this is way more accurate.
The diode has a reverse voltage of 400V. I went to my local shop and they only had a UF5408 in stock but that doesn't matter it actually might be better to choose a diode with a higher reverse voltage because we don't know why it died in the first place. But a UF5404 UF5405 UF5406 UF5407 and UF5408 will work.
Hope this helps!
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Thank you for your post, very helpful.
I think I have the same problem with my U2412M, just lost power all of a sudden with no warning or flicker just went black.
Not being very electronically minded I think I have identified the diodes on the power board and putting the multi meter on them reveals that 2 of them (D702 and D703) don't work (well the meter doesn't buzz!). The other 2 (704 and 705) do seem to work.
The two diodes "seem" to be the same, would that be a very bad assumption? Or could I get two of the UF5408's and just see what happens?
Hoping to get this monitor going again as it is only 4 years old.
Thank you for your help
I'm glad you found my post. Unfortunately, I haven't taken any photos of the repair so I don't remember the diode names but D702 and D703 sounds right!
How have you measured the diodes? Has your multimeter a diode mode?
I do remember that there were two diodes next to each other and in my case one of them was faulty. The two were definitely from the same type. I know sometimes its hard to read but there should be a name printed on the diode. If you can read it, what is printed on the diodes?
If you have a smartphone/camera it would be awesome if you could take a picture and post it here!
I ended up de soldering the 4 diodes on the board and sure enough a uf5404 was dead. It was right next to an identical one, so I bet it was the exact same problem. I have just replaced it with a uf5408 and it is working again.
This is not my photo but this was the bad diode.
I also hope this helps someone in the future
great you found the defective diode and could repair it! After seeing the picture, I can tell that my defective diode was also one of the two you marked.
thank you so much for posting this, you saved me so much time. I had exactly the same problem and after quickly checking the UF5404 diodes I found out both were defect (the same as marked in Jeremy's picture). Without you I would have checked every single diode, since I only have a multimeter. After exchanging them the monitor runs fine again.
You Sir deserve a medal.
Thank you again for your help
UF5408 is a bit more robust in terms of peak reverse voltage, blocking voltage which might help. If you have to buy a new one anyway, I would go with the UF5408 but if you already have a UF5404/UF5405/UF5406/UF5407, just use it.