Dell U2412Mb repair

[Zynch]

Hello

I have a Dell U2412Mb monitor that decided to give up on me. It started giving issues a while back - after being unplugged, when turning it on again it doesn't show a picture. I can see a glow, which suggests, that the backlight still works, but the annoying thing is, that it doesn't even show the OSD menu. And after a while it started to work correctly again, without issues untill unpluging. But now, after a few days, it still doesn't want to start displaying anything.

Disassembling it and taking a look, nothing looks visually dead/blown up on the PSU and display boards. So I would like some suggestions, if it is worth to try and repair it.
I have another display, and will be buying a new one when I can decide on what to actually buy, which means I don't really need to get it working, it would just make a good secondary monitor, if I can get it working again.

So should I try and do it, and if so, any reccomendations on where to start?

[Rasz]

U2412M was covered here, plenty of caps to go bad, bad caps match description of your fault (starts working after plugged for a long time, doesnt after prolonged unplugged etc) https://www.eevblog.com/forum/repair/dell-u2412m-schematics/
but no idea how it differs from U2412Mb

[Zynch]

I saw that topic, but it was about identifying components.

I have replaced the caps with new ones, despite the old ones not measuring too bad in my opinion.
It did not help at all and the issue is still the same.

Are there any other recommendations as to what to try next?

[PKTKS]

I saw that topic, but it was about identifying components.

I have replaced the caps with new ones, despite the old ones not measuring too bad in my opinion.
It did not help at all and the issue is still the same.

Are there any other recommendations as to what to try next?

All power readings is your next step

Paul

[Zynch]

Well it has been more than half year since I posted, but I left the monitor aside as I got a new one, and now had a bit of time and decided to check it out more.
And since I repaired it, here is a short description in case anyone else has the same problem.

As recommended, I checked the voltages going from the power supply to the video board. The readings were:

Pins 1 and 2 - 0 V with backlight off and 2.8 and 3.2 V respectively with backlight on (presumably backlight enable)
Pin 3 - 7.18 V
Pins 4,5,6 - 5 V
Pins 7,8 - 3.3 V
Pins 9,10,11 - GND
Pins 12,13 - NC (goes to some unpopulated stuff)
Pin 14 - GND of the 12 V connector
Pin 15 - regulator enable of the 12 V connector

Well the 7 V on pin 3 were a pretty weird voltage, and absolutely unexpected so I started tracing it out.

The pin goes to the output pin of the KIA278R12PI, which is a 12 V regulator. So the output is not right at all. And on the video board, it goes via a presumably transistor into the LCD panel. Looking around a bit more nothing else seemed to be able to be a problem, and the input was decent at 12.8 V, so I decided that the regulator is probably faulty.

Thankfully the power supply had another of this regulator that was fed by the same input and was used for powering the 12 V connector. The connector was used for powering a soundbar that I don't have, so it's not used and became a donor.

Having replaced the regulator, the monitor started working properly again.

[noting000]

Hello Everyone,

sorry for digging out this kind of old thread but I also own a U2412Mb with a power board failure. No LED activity no nothing... and it happened all of a sudden.

I disassembled the unit and checked the voltages at the connector to the mainboard. The strange thing is there is a fluctuating voltage on Pins 3-8. The Voltage goes from about 1.5V to 2.3 V and back down and up again... nothing stable there.

I read about broken diodes, caps or regulators but i have no clue where to start. Are there any reference voltages that I can refer to to narrow down the problem? (For instance whats the voltage drop across the diodes 704/705 supposed to be? they seem to be the culprit for alot of people with similar issues) Or does the fluctuation of the voltage rather point to a different problem source?

Thanks for any advice!