Lenovo flex 14 no display short, da0st6mb6e0 Rev :E

Previously when the laptop was turned on it responded and the power light came on but then turned off.

1. With lesson 7 it turns out to be short in PL8, PL4, PL3.
2. I checked the PL8 line, it turned out C291 was burnt.
then I took it off and tried to turn it on . it turned on with the fan spinning (maybe it was already booted) (with new ram ok, with old ram won't turn on).

The problem is PL4, PL3 are still short and now the LCD is just dark, there is no display. I tried disconnecting PL3 and PL4 but there was a short in VGPU_CORE.

How do I check the short section with lots of capacitors going to VGPU_CORE? And does capacitor C291 need to be replaced? or just let it go?

Codetrr napisal/-a:PL4, PL3 are still short

At GPU power path you should expect very low resistance to ground, even below 1Ω, depending on GPU chip model. What is the actual resistance on PL3/4 to ground (measured at Ω lowest scale, not at diode scale)? Please take into consideration the resistance of your probe cables. And what is the marking on GPU chip?

Codetrr napisal/-a:How do I check the short section with lots of capacitors going to VGPU_CORE?

The answer is: SHORT TEST. Please read Lesson 3 carefully: post5699.html#p5699

pl3/4
Resistance is 39 ohm
Voltage is 0.905 volt

Marking on GPU is N14m- GE-A2
Is it safe to leave c291, because I don't have changer for this capasitor.

Codetrr napisal/-a:pl3/4
Resistance is 39 ohm
Voltage is 0.905 volt

So it seems it is all good here.

Codetrr napisal/-a:Is it safe to leave c291

It should be okay with just the one capacitor missing on that power path.

Measure the voltages to ground on pins 1, 8, 9, 28 and 30 of CN3 connector when the logic board is turned on. Best would be to disconnect the battery and HDD/SSD before taking measurements (just the power supply and screen connected).

Cn3:
1. 20.5v
8. 370 mV
9. 470 mV
28. 3.28 V
30. 0 V

After I check on schematic and board, U1 not present on board.

Codetrr napisal/-a:U1 not present on board

Correct, +LCDVCC voltage comes from U7 (RTD2132R) pin 15 via R13 jumper. Check if you have any picture on external HDMI monitor while the internal LCD panel is disconnected from the logic board. Do not forget to disconnect all the power before you disconnect or connect the LCD panel's cable.

I tried with external HDMI monitor but nothing is displayed.
After waiting 3 hours the CPU fan is still on.
Pl3 and pl4 little bit warmer than other PL.
What else should I check?