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Old 11-10-2019, 02:25 PM   #1
bangaham
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Default Dell U2715H no power

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?
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Old 11-14-2019, 07:25 PM   #2
momaka
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Default 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.
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