Dell Latitude 5590 (motherboard: LA-F411P Rev 1.0) No Power (Liquid Damage)

[zklgfs]

Hi,

I have a Dell Latitude 5590 with motherboard LA-F411P Rev 1.0 that has liquid damage and doesn't turn on anymore when only the charger is plugged in. However, it does turn on when powering it from a battery. I couldn't find schematics for this exact board, but LA-F401P seems to be similar (that post says LA-F411P, but the attached schematics are actually for LA-F401P): https://www.badcaps.net/forum/troubl...78#post3262678

I have an identical laptop that is still working, so I can compare voltages, etc. between the two boards if needed. When the dead board is plugged in, the main power rail is missing completely (without a short to ground), but the DCIN pin on the ISL9538H charging chip (see attached image and page 59 of the linked schematics) does have about 18V. I couldn't find a full datasheet for that chip, unfortunately. I found that one pin on that chip had a much lower resistance to ground than on the working board (unfortunately, I don't remember which pin that was and what the exact resistance was), and I could still measure that resistance directly on the chip after desoldering it, so I decided that the chip was probably bad and got a replacement. The low resistance is gone now, but the main power rail still isn't there (unless the battery is plugged in).

The current sensing pins CSIN and CSIP both have about 19.27V, and the resistance between them is 4.7 Ohms, which is the same as on the working board. ADP is 19.24V, VDD and VDDP are 5V, ACIN is 3.47V, and ACOK is 3V. BOOT2, UGATE2, LGATE1, and BOOT1 are all about 4.3-4.7V. Finally, CMOP is 5V, and everything else is between 0V and 0.3V (except for ASGATE, which is 8V, but that seems to be floating anyways).

I haven't been able to find a short to ground anywhere. The dual MOSFET PQ905 (AOE6936) has 19V from the charger at pins 3 and 4 (drain 1).

Does anyone have any suggestions what else I could test? Do these symptoms point to the replacement ISL9538H also being bad, or does it sound like another issue? I don't really have much experience with these types of repairs yet, I'm just trying to practice on dead laptops that I've collected, so I appreciate any help you can give me.

[mcplslg123]

Check PU2 voltage on all pins.

[zklgfs]

Check PU2 voltage on all pins.

Thanks for the suggestion. VIN has 19.5V, all other pins have 0V. The VOUT pin has about 100K resistance to ground, so there's no short there. I'm guessing that voltage regulator is dead then? I couldn't find a perfectly matching replacement chip in my collection of dead motherboards. The closest thing I found was a G920AT24U, which is the right voltage but doesn't seem to have the same current handling capacity as the RT9058-33GX (see attached datasheets), so it's probably best if I just order the correct chip.

[zklgfs]

I must correct myself. I don't know what I messed up while measuring before, but now PU2 does actually have 3.3V on its output. However, PQ8 only has 3.3V on its drain (see page 50 of the schematics). If I understand it correctly, this MOSFET is switched on when 3.3V_ALW is present, which is powered by PU100 (page 51). For PU100 to work, PWR_SRC needs to be present, and that brings us back to the ISL9538H (PU901) and its surrounding circuit. Does that make sense, or am I missing something?

[MrBurritoMan]

Following this thread, I have the same computer with a similar problem...
I have some values that I measured off of a working system and will post them in a little bit.