inspirion 15, Vegas/Turis SKL/KBL 15341-1 5/3.3v output issue

[Djosla]

Hello,

Looks like this one`s gonna beat me

Motherboard: Dell inspirion 15,
Vegas/Turis SKL/KBL 15341-1
91N85

Loptop came dead,there was some liquid damage,near cd rom,nothing major,but after fixing that,main power supply voltages are not rising properly.
Instead of 3.3v and 5v i am getting 2,5v and 3,6v.

I have checked and changed every component around and including pu4503 (tps51225),but problem persists.
I have checked feedback resistors,output capacitors,mosfets for leakage,LDO outputs no shorts,i have inspected all traces in between.
What am I missing ?
I am getting 19v at input,enable`s are at 0.75v,feedback is a bit lowered bacause of bad output.Vreg3 is 3,3v but Vreg5 is 3,6v(no short or leakage on that line)

schematic: https://www.badcaps.net/forum/attach...9&d=1591843490

[mon2]

Re: 5/3.3v output issue

Hi.
1) what is the resistance to ground for each rail?

2) enable voltage readings are at 0.75v? Are these active low enables? Otherwise sounds very low to enable.

[mcplslg123]

Re: 5/3.3v output issue

The replacement ic is likely bad also.

[Sametbey]

Re: 5/3.3v output issue

1 pin of TPS51225 is reference for VREG 5. Check the resistance value of PR4531. Give a voltage from 3V to 5V by separating the VREG5 output. Check for any heated parts. 3D3V_PWR and 5V_PWR voltages will be active when 3V is reached to 6 pins of the integrated. good works.

[Djosla]

Re: 5/3.3v output issue

Thanks for your replies.Well I managed to fix it,sort of

That enable voltages were indeed to low,but i was thinking that pwm would not work below threshold voltage.

Culprit was in active short on PURE_HW_SHUTDOWN# rail,which affects "enable" voltage generation over D4001 diode,so it was hard to figure it out. Since that line is connected to Q2602 and Q2605 (Thermal shutdown protection) which are not populated on this board (UMA),I have found that this line goes straight to KBC at pin 85 "CMP_VOUT0",which has active short.There was some corosion around Q2602 traces ,but it is unpopulated area,no components,but that must have damaged comparator output somehow.

Because I don`t have spare MEC1404 to try out, I`ve removed diode D4001 which eliminated voltage drop from active short, and then I got around 2v at enable and stable 3,3/5v output.
Laptop passed Dell diagnostics and several stress tests so it will work like that until KBC replacement arrives.