T480s random shutoff and then completely dead

[wangbreeze]

Hello Gurus,

I got a ThinkPad T480s. It was running fine but suddenly started random shut down. When it turned off, I had to push the reset pin to bring it back to life. But it got aggressively worse and became completely dead. No charging light, no life.

I did some initial diagnostics with just the board (no battery, nothing attached, and no CMOS battery as well)

Initial testing of the coil resistance seems to indicate no dead short. The only low resistance I measured was at PL701. It reads 38 ohms.
charger gave - 20V - ok

U101B
VREGIN20 - 20V - ok
BAT_VOLT - 7V - ok
TH_DET - 0.05V - ok
VCC3SW - 3.3V - ok
-pwrshutdown - 3.3V for split second when the charger plugs in then becomes 0V


VINT20 - becomes high for split second when the charger plugs in then becomes 0.01V
VSYS15 - close to 9V for split second then drops to 2V

I've tried to identify any shorts but I'm still a noob. Could anyone cast some light on this? Thanks.

[MOD EDIT] Link to schematic -> https://www.badcaps.net/forum/troubl...-t480s-nm-b471

[mon2]

Remove all power. Meter in resistance mode. Measure the resistance to ground of this VINT20_IN rail. If there is an excessive current draw, then the power adapter is shutting off. If there is an excessive current draw then some part is shorted on the same rail.

PS: Thanks for the schematic but it must be posted in the dedicated schematic forum so future readers can find them quickly.

[wangbreeze]

Thank you and sorry about that.

it measured 695k ohms.

[mon2]

No short here. Still with no power, meter in resistance mode. Proceed to test the resistance across the mosfet pins of the DUAL mosfet @ PQ101.

So you will be measuring:

source & drain (1st mosfet)
source & gate (1st mosfet)
gate & drain (1st mosfet)

Post each measurement. Then repeat for the 2nd mosfet. Checking for a possible short inside this dual mosfet device.

Datasheet for this mosfet is attached. You can use it to locate each of the pins. Note the weird bellyside pin that will need to be probed as well. This pin will appear around the body edge.

[wangbreeze]

Thank you. It's really hard to get a good probe of the mosfet. Here's the value:

pq101
source to drain: start at really low <10 k ohm and gradually increase.
5 and 6-8 pin: 140k ohms, steady
1 and 2-4 pin: >100k ohms and rising

pq102
source to drain: start at really low < 10k ohm and gradually increase
too small to probe the rest. I'm reading the source to drain from the nearby test points.

I'm also reading more about these dual mosfet and I think I did it incorrectly. They can't be measured in circuit, can they? Thanks.

I suppose increase in resistance is due to capacitor charging.

[wangbreeze]

What I don't understand is VSYS15 has about 2V even if VINT20 is 0.001V. Isn't VSYS15 generated from VINT20? Does it mean a short is present in VINT20 circuit and pulls the voltage down? Thanks.

[wangbreeze]

ok. scratch those measurement I posted yesterday. I took a closer look at the schematics and found traces of hidden drain/sources into the PU101 chip. I've take these measurement. Any inputs will be much appreciated! Thanks.

MOSFET measurement:
PQ101
High side:
gate(1) - drain(2): 542k
gate(1) - source(7): 125k
drain(2) - source(7): 317k

Low side:
gate(6) - drain(7): 550k
gate(6) - source(3,4,5) - 198k
drain(7) - source(3,4,5) - >~300k

PQ102:
High side:
gate(1) - drain(2,3,4,10): out of range > 2000k
gate(1) - source(9): 125k
drain(2,3,4,10) - source(9): 100k

Low side:
gate(8) - drain(9): 240k
gate(8) - source(5,6,7) - 285k
drain(9) - source(5,6,7) - 24k

The low side is suspicious but I took a measure of another working board. Its low side drain(9) - source(5,6,7) has 35k resistance.

[wangbreeze]

A little more discovery. Disconnecting fan seems to make the power rails return. Problem on the VCC5M rail?

[wangbreeze]

I want to put a closure to this post. I discovered water damage in the area of pq14 mosfet. No wonder why connecting the fan has an impact. I cleaned it up and everything started to work as intended.