Voltage Injection on 3.3v rail

[ROONEY50]

I am repairing a laptop that has a short on it's 3.3v rail. I injected 1v on the rail and it was pulling 200mv but can't find any heat anywhere with thermal camera or alcohol. What else can I do to find what's shorted?

[stj]

go to 2v or 3v to get it to draw more current maybe.

[ROONEY50]

Went to 3v and still no heat source found.

[lotas]

Went to 3v and still no heat source found.

Now increase the current (amps) on the laboratory power supply.

[ROONEY50]

When I turn it up it won't take any more because it's only 200ma the board is drawing. I can't force more amps in.

[lotas]

That's strange, but what is the resistance on this bus then, is it just a short circuit?

What kind of power supply do you have? Can it output 5-10A? If yours is rated for 1-3A, it's too weak for laptops.

[stj]

maybe the short is behind a mosfet that has not been turned on,
a secondary buck psu maybe.

[harp]

Maybe is some lab psu with short circuit protection kick in... what model of psu you have?

[ROONEY50]

That's strange, but what is the resistance on this bus then, is it just a short circuit?

What kind of power supply do you have? Can it output 5-10A? If yours is rated for 1-3A, it's too weak for laptops.

I've got a 10amp bench supply.

[ROONEY50]

3.3v coil - 25 ohms
5v coil - OL
The other 3 coils on the board are showing

250 ohms
45.3 ohms
4000 mega ohms

I'm assuming the 3v is shorted maybe?

When I power the laptop on, it shows as dead. No CPU heat, no picture and no fan. However I noticed the 3.3v and 5v coils are showing the right voltages.

No 3.3v is showing on the BIOS chip pin 8

Voltages on the other 3 coils are showing 0v

[harp]

You have 3.3v and 5v on coil, but also 3.3v and 5v ldo from pmic. What is resistance of ldo 3.3v and 5v to ground? Oh, you are all over the forum... I see that laptom has common fault on usbc connector, have you check if voltage reach board without battery?

[ROONEY50]

To be honest, I wouldn't know where to find the LDO. That's a new term for me.

I am aware this particular model has lots of problems with the USB-C connector. It's pulling only 5v through the USB-C charger but will check if it's reaching the board without battery and get back to you 👍🏻

[ROONEY50]

Yes the 5v it's pulling from the charger is reaching the board, it's passing through a couple of MOSFETs and capacitor's.

[harp]

As example of ldo in attach, it is useful to check what pmic you have and if it have ldo 3v3 on power button, sometimes with addition to 5v ok and they are working ok. Be systematic in repair, so do nor just repair on first sight. If is usbc data lines is broken, the pd controler not let charger to switch to 19v, nor enable batt charging... so, changeing pd controler before check continuity pin-to-pin endpin it may not solve the problem. Check on every mosfet if not shorted also, it is routine check like checking rails...

[ROONEY50]

I think this is the pmic and on testing, I think it might be faulty. However, I cannot find ANY identification on the chip. I've cleaned it etc but still no ID. I've attached a picture and the schematic and wondered if you could find the chip on the schematic so I can order one 👍🏻

[lotas]

I think this is the pmic and on testing, I think it might be faulty. However, I cannot find ANY identification on the chip. I've cleaned it etc but still no ID. I've attached a picture and the schematic and wondered if you could find the chip on the schematic so I can order one 👍🏻

You can't see how many pins she has, most likely one of these.

[truclacicr]

https://www.anpec.com.tw/product-download/0/APL3552-1/

https://www.badcaps.net/download/?id=8b5f226f

[ROONEY50]

Thank you both.

[harp]

"I think this is the pmic and on testing"... why you think that? So, say, if turn it is not: "I think it might be faulty"... please explain why you think that ic is faulty? ?................. on picture is sot23 package nearby, so that little chip is not likely to be pmic. Ldo is about 150mA I guess, so must be ic with ground plane... search on schematic for ldo, and pwrbtn, then find this ic on mb...