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4 Posts
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19761
U2412M, Repair a defective diode
Hello guys,
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!
cryptos10
4 Posts
5
May 22nd, 2017 05:00
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.
DELL-Chris M
Community Manager
Community Manager
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54.3K Posts
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October 5th, 2016 09:00
Thanks for posting this!
jeremywoodham
4 Posts
1
February 25th, 2017 18:00
Hello cryptos10.
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
Jeremy Woodham
cryptos10
4 Posts
0
February 26th, 2017 02:00
Hello Jeremy,
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!
Regards,Jens
jeremywoodham
4 Posts
6
March 2nd, 2017 22:00
Hello Jens
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
Cheers
Jeremy
cryptos10
4 Posts
0
March 3rd, 2017 09:00
Hi Jeremy,
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.
Regards,
Jens
wirthiwirth
1 Message
0
March 31st, 2017 13:00
Dear Jens,
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
Simon
addepat
4 Posts
0
May 22nd, 2017 00:00
Hi, I have the same problem. Is it better to replace them with the UF5408 or should i buy the UF5404?
addepat
4 Posts
0
May 23rd, 2017 10:00
Ok Thanks
astmario
3 Posts
0
June 17th, 2017 08:00
Hi,
I opened my monitor that had same problem, found out that LD7750RGR circuit is dead and also named UF5404 diode. I was full of joy for finding problem, but closer inspection showed that 2 SMD resistors around LD IC are burnt out and I cant read their values.
If anyone can post R611 and R612 resistor values I would be very grateful!
jeremywoodham
4 Posts
1
June 17th, 2017 16:00
www.eevblog.com/.../
Maybe that will help?
jeremywoodham
4 Posts
1
June 17th, 2017 16:00
Hello astmario.
If your eyes are anything like mine I have to de-solder and look at it under a magnifying glass to get values.
If I do open up my monitor again I will try to find those values.
Cheers
J
astmario
3 Posts
0
June 17th, 2017 23:00
Thank you for reply!
My eyes are not much better but those values can be without desoldering just by using magnifying glass or by list taking picture of them and zooming on screen ;) I gave a look to topic you linked but I don't like that post thay says that you can just use safely 10R just like that, those resistor in my knowledge regulate voltage so using wrong one can have great impact on on output voltage so can damage something on processor board. And I looked that damaged board picture and one resistor it totally missing, another one is blurry lol :)
So far I changed LD7750RGR circuit, 2SK4108 FET and UF5404 diode and today I will recap the board with new capacitors to ensure I don't need to repair it for a longer time, I just don't trust those Elite caps. Everything else is double checked, so recap and finding R611 and R612 values is only thing left to do!
astmario
3 Posts
0
June 20th, 2017 07:00
I found a working monitor of same version in my office, so I was able to open it and read values, so I will post it here for anyone else with same problem. A friend of mine with good camera took a picture, so here it is.
Also to mention burned resistor R615 on another side of the board, but it is normal and can be read easy.
SatWgat
1 Message
0
December 3rd, 2018 13:00
I'd like to express a HUGE thanks to cryptos10 (and others who contributed) for this info. I replaced a pair of (what looked like burned) UF5404 with new ones and it worked!