3.3v question for Microsoft Surface 3 1867

[Darren Hibbert]

Hi.

I'm trying to fix a Microsoft Surface 3 1867 laptop that is dead.

I only have a USB-C charger at the moment for it. When I check the board, I am indeed getting 5v over the board, but the 3v Inductor and other places on the schematic where I should have 3v are showing 0. It maybe a silly question but should I still be getting 3,3v in the laptop even when using a 5v USB-C instead of a 19v supply?

[mon2]

1) Use a suitable power delivery charger to properly diagnose this laptop.

How to charge the Surface Laptop 3 via USB C? : r/Surface (reddit.com)

Surface Laptop USB-C Charging: We tested all chargers we have - SurfaceTip

2) 5 volts is the basic output voltage of all USB C chargers. This laptop requires a charger that can support PD (power delivery) to select the best power delivery contract (voltage & current) to power this logic board. Often, it is the 20V PD contract that is used by laptops. What are the voltage & current power delivery contracts marked on your power adapter?

3) With no power to the board, meter in resistance mode. Measure the resistance to ground of the areas with 0 volts. If the resistance is too low, the fault may be due to some shorted component onboard.

Regardless, do not continue till you have a suitable power adapter for this board.

[Darren Hibbert]

Thank you for this information.

My USB charger is 3A.

I have ordered a proper charger for it.

[Darren Hibbert]

My charger has arrived.

Tested resistance with no power to board -

1 inductor = 200 ohms
1 inductor = 140 ohms
1 inductor = 141 ohms
1 inductor = 3.6M ohms
1 inductor = 27K ohms
1 inductor = 27 K ohms.

[Darren Hibbert]

Just to add.

With the power applied, I still get 0V at EVERY inductor.

[mon2]

Link the schematics but only if they are posted inside of the schematic forum.

Reference the silk screens of the logic board here if the schematics are not available to you. Each regulator will have an enable signal used to turn that rail ON / OFF. The ENABLE pins for each must be also reviewed. Best to start with the usual power path from the power adapter to the main power rail used by the logic board. Respectively, need to review which charger IC is onboard. Then we can study the datasheet or a similar schematic to understand how this logic board operates.

[Darren Hibbert]

https://www.badcaps.net/filedata/fetch?id=3292055

[Darren Hibbert]

These Microsoft motherboards are horrendous to work with.

[mon2]

Yup. Crapware.

Measure the voltage to ground on

a) the dual mosfet @ Q6302 (1-2-9) ; any of the pins will be ok - checking if the adapter voltage is reaching the charger IC.

b) L6301 inductor

c) Q6304 dual mosfet (1-2-9) ; this is the main power rail to the logic board (+VSYS)

Post each measurement.

Where are you measuring the missing 3V? Reference the inductor from the schematic labels.

[Darren Hibbert]

Many thanks for this. It might be a day or two before I can do these things but will let you know as soon as I'm back.

I really appreciate your help 🙏

[Darren Hibbert]

ok.......Q6302 Pins 1, 2 & 9 = 0V

L6301 = 0V

Q6304 Pins 1, 2 & 9 = 0V

[mon2]

See below for the power path diagram from the external power adapters to the battery charger IC.

U7902 is the PD controller that accepts the power from the external PD adapter after communicating over the CCx interface pins on the Type C connector. The selected power delivery (PD) contract is then passed onto the charger IC using the EXT_DC_IN netname.

If indeed you have 0 volts @ EXT_DC_IN which is confirmed with the measurements at mosfet Q6302 then the PD controller (U7902) is a suspect.

Review this part for damage. Is this part heating up? Check the voltage to ground of the LDO rails this component is supposed to output on pin 35 and also pin 9.

The firmware for this PD controller is most likely stored inside of the flash IC @ U7901. Pin # 8 should be at 3v3 if it is being powered by the LDO rail created by the PD controller. Confirm this detail.

[Darren Hibbert]

U7902 does not look damaged. It does not heat up either (I have a thermal camera)
I have attached a picture of this chip and I don't know if it's just me, but those pins along the top look a bit suspect. They look like they are joined together! Maybe they are meant to be like that or maybe the chip got very hot and melted the solder 🤷♂️ Although looking closely, the chip pins seem to be covered in some kind of wax.

Pin 35 = 0v
Pin 9 = 0v


U7903 Pin 8 = 0v

Also to note, I am using the main 15V charger and not the USB-C charger.

[Darren Hibbert]

I am able to inject voltage if that's any use?

[mon2]

No injection needed yet. Follow the power path from the connector you are using (15v) to the charger IC. Report back where the power stops.