DELL G5 5587 LA-E993P Power on, but no video

[robante15]

I have a DELL G5 5587 LA-E993P here that after a maintenance process (changing thermal paste, thermal pads, and general cleaning) turns on for a few seconds, but just before showing video it turns off.
When connecting it to the power source it indicates normal consumption, and when pressing the power button the fans spin to maximum, and after a few seconds it turns off.
I can see that the screen turns on the backlight for a fraction of a second and then turns off along with the entire computer, and the boot process restarts.

If I remove the RAM then it shows color codes on the LED indicating lack of RAM.

In another shop they flashed the BIOS so I don't have the original dump, I flashed one that I found here that they reported works but nothing.

When measuring voltages, the only thing I'm missing is VCCGT, which I understand is for the processor's integrated graphics, but in the new generations, that power supply doesn't turn on until there is a higher power requirement.

I have no idea where else to look for the problem, since everything seems to be correct for displaying the image, but when I turn on the screen it turns off. Any recommendations or suggestions on what to check?

[mcplslg123]

Was the maintenance done without disconnecting the battery?
Does the system turns off when screen is not connected?
Status of DRAMRST#/PLTRST#/All sys prg good?

[robante15]

Was the maintenance done without disconnecting the battery?
Does the system turns off when screen is not connected?
Status of DRAMRST#/PLTRST#/All sys prg good?

In order to perform maintenance on this equipment, the battery must be removed, so I don't think that's where the damage comes from
Yes, when removing the screen it keeps turning off at exactly the same moment

Regarding DRAMRST# and PLTRST_CPU# I am attaching captures from the oscilloscope, the first is DRAMRST#, both are from the moment the equipment is turned on, until it turns off by itself

Regarding the PGOOD, I haven't measured them all, but I have VCC_CORE, VCCSA, 1.2V_DDR, 3V, 5V, GPU_CORE

[mcplslg123]

Yeh difficult issue. i had a HP LA-E802P with very similar symptoms-the mb turns on with good variation on dc supply,DRAMRST#,PLTRST#,ALL SYS PWRGD,VCCSA,VCCORE evrything ok.Checked even pulses on VCCGT but the mb keeps on rebooting without display in a loop.there was slight flickering on screen also but no display. After measuring all whatever i can,went ahead replacing the SOC and got display.
So may be SOC is faulty but might be something else.

[robante15]

Tomorrow I will try to measure all the sources and the PGOOD, if that does not work I will have to give up.