Dell U2715H no power

[bangaham]

Hello,
so I got this Dell U2715H monitor that originally wouldn't turn on. I took it apart and started diagnosing. The power supply is supposed to give 20V (printed on silkscreen) and the output is 19.1V stable, all capacitors look good and are Rubycon brand so I assumed that was ok (don't have ESR meter to verify).

I then proceeded to the main board. Plugging in the power supply nothing happens, front panel touch buttons do nothing, laptop doesn't detect it via HDMI, but the main heat sink gets warm. Don't have the schematics but found the main voltage rails (5V, 3.3V, 1.8V, 1.5V) and they all seem to be good.

With all the voltage rails present I assumed bad bios (saw bad bios chips cause similar issues on other monitors) and unsoldered the main bios chip and read it with a reader and it had data on it and since I can't find another bios file to compare I assumed it was good.

Since the only thing left is the main chip (RTD2786B) and it was getting pretty warm, I decided to give it a reflow (don't have anything to lose at this point) and reflow the RAM chip as well. Preheated the board up, used lots of flux and reflowed both ic's. After letting the board cool down and cleaning up the flux I gave the board a try. I plug in the power supply and the main ic doesn't get warm anymore, the front pane touch buttons didn't work immediately, takes a couple of seconds and multiple tries to turn it on, but I got the power LED on. Sadly I don't get an image nor backlight (backlight driver doesn't seem to get an enable signal) but when plugged in to my laptop via HDMI and installing drivers it gets detected and I can set the proper resolution but no image and backlight on the screen.

Anyone have any ides to try or do I get a new main chip and replace the existing one?

[momaka]

Re: Dell U2715H no power

The power supply may be good, but check the smaller power circuits near the main chip. In particular, look for linear regulators like 1117, 317, 3084, an etc., as well as integrated step-down converters (should have a coil/inductor/toroid near them.) These can go bad. Or if there are SMD electrolytic caps... these can also go bad without any visual signs.

[bangaham]

Re: Dell U2715H no power

Hi, thanks for the reply,

I took more time with the board and all the voltages are present except the 12V which powers the LCD controller board. Unfortunately the switch IC has no datasheet (the only writing on the IC is AIV7JA) it has input voltage (19.2V) but
since I don't have a datasheet or main pcb schematic I can't determine if there is an enable signal coming from the main ic or the ic is just dead.

If anyone has any idea what the IC model is or has a datasheet I'd be thankful.

Added pictures of the main board and the switch ic for 12V.

[momaka]

Re: Dell U2715H no power

Looks like a buck regulator + FET IC of some sort, judging by the small inductor next to it. Perhaps try changing/checking that small electrolytic cap next to it. If it's bad, maybe that could be the issue.

[bangaham]

Re: Dell U2715H no power

I measured all already and all seem good, only C733 has a lowish resistance to ground(around 20 ohm) but I removed it and measured it still had that resistance (could be normal but since I don't have schematics I have no clue what the line is and what it does).
I bypassed the whole regulator, added my own buck converter from input(19V) and set it to 12V and plugged in the screen and still nothing so I assume the main ic is just dead because sometimes I can't turn it on, sometimes doesn't recognize laptop.

Next I'm just gonna replace the main IC, already ordered it just waiting for it to arrive.
Will update when I replace it.

[bangaham]

Re: Dell U2715H no power

Update time.
Got the new main IC, replaced it and it gets power, I get backlight, laptop detects it at full resolution, I get the 5V for lcd power(not 12V as expected), but the screen only flashes colors (red, green, blue, white, gray). After some research found out it is Dell burn in mode and I can not disable it. Tried all button combinations from google, nothing works, so yeah. One funny thing is I have no power indication light, might be my BGA soldering, but the power button responds, can't really check for other buttons since I can't really do anything.

Any ideas? I might try to reball and retry, but I am lost at this point.