Thinkpad L540 5v/3.3v output issue

[baddjavo]

I have similar problem. Lenovo laptop came wont turn on on pwrbtn. Shortly after inspection found that missing 20v Vin in 3v_5v regulator. Probably burned track. So i shorted 20v from mosfets supply and get laptop working. Day after customer reported that laptop wont turn again. Figured, that voltages on 3.3v is 1.3v and on 5v is 2.1v, and ENC pin is 1.24v whitch is quite low like Mon2 said, but still giving output.
I have read this thread and did same like Djosla said. Now i have correct voltage but laptop wont turn...

[baddjavo]

Re: 5/3.3v output issue

img

[baddjavo]

Re: Thinkpad L540 5v/3.3v output issue

Update, after remove diode (diode.jpg) i got correct 3.3v and 5v.
After a while TPS51123RGER regulator died.
I have put back diode and changed TPS51123RGER with new.
Now iam on starting point again.
Question is, why ENC pin is on 1.27v ? Probably pure_hw_shutdown# signal after diode is bad.

i have schematic from thinkpad L430 and boardview L530

[mon2]

Re: Thinkpad L540 5v/3.3v output issue

What is the voltage to ground of PURE_HW_SHUTDOWN# ?

Are the DCin mosfets bypassed / shorted to force the logic board to power on ?

[baddjavo]

Re: Thinkpad L540 5v/3.3v output issue

DCin mosfets are good.

PURE_HW_SHUTDOWN# yesterday was 0.7v, but right now is 0.8v, 0.1v increased, and 3.3 and 5v outputs are 1.4v and 2.3v, increased also. ENC pin was 1.27v and right now is 1.34v. Looks like some component changing characteristics.

[mon2]

Re: Thinkpad L540 5v/3.3v output issue

3.3 and 5v outputs are 1.4v and 2.3v

Remove all power. Meter in resistance mode.

Measure the resistance to ground for each of the above power rails.

One meter probe on 3v3 rail; other to ground (shield is ok).

What is the resistance ? Post each measurement.

Do you have an oscilloscope? Perhaps the ENC pin is oscillating / pulsing and causing the above outputs to also be swinging on/off which with a meter will read to be a lower value.

[baddjavo]

Re: Thinkpad L540 5v/3.3v output issue

mon2, after removing diode d3601 on picture all voltages are ok, ENC ok, 3.3v ok, 5v ok ... but pure_hw_shutdown# going to 9x plases, one is pnp transistor q2701 that cut ECRST from 3.3v to 1.4v which is needed in power sequence. Ill try to remove all components connected to pure_hw_shutdown# one by one to figure which one causing voltage drop.
resistances on rails are: dcbatout 4M, ENC 95k, 3.3v 570ohm, 5v 30k , looks good, didnt notice overheating.
No oscilation, checked with oscilloscope.

[baddjavo]

Re: Thinkpad L540 5v/3.3v output issue

update

tracing pure_hw_shutdown i came to U2802 (op amp) and removed
R2815 ( 0ohm ) which connecting pure_hw_shutdown to U2802. After that motherboard starting. Laptop working.

Question is why U2802 outputting wrong voltage and causing pure_hw_shutdown# voltage drop ?

Something about FAN or some of PTC network failed ?

[mon2]

Re: Thinkpad L540 5v/3.3v output issue

Good job !!

Measure the voltage to ground on each pin of U2802 comparator.

[baddjavo]

Re: Thinkpad L540 5v/3.3v output issue

u2802
before bwrbtn
only pin 5: 5,11v
after bwrbtn
1: 3.65v 2: 0v 3: 5.11v 4: 0v and 5: 5,11V

customer call me, so ill leave this state. Laptop is working.