Surface Pro 4 dead - Main 8.5v power rail is 2v - Have good board to compare

[mattpd]

Please ignore the red circle in the pics. I stole it from the internet.

This board was pulled from a working unit and tested for a week before I sold it on ebay.

It was returned saying that it "worked for 10 minutes and died"

I found the main 8.5v power rail to be 2.15v. Not shorted. Same diode mode reading as the good board.

On the good board if you measure the coil with an Oscope you can see the 15v pulsation on one side of the coil and a flat 8.5v on the other side.

On the bad board it is a flat 2.15v on both sides of coil, no pulsation. It seems like the full 15v is not making it to this area of the board for whatever reason and it's not even attempting to pulse the voltage.

But right next to this area is the 3.3v standby rail area and it is working on both boards which also uses the 15v rail. I can see the 15v being chopped up into a smooth 3.3v, even on the bad board, so the 15v rail is present on the board but just not able to power the 8.5v rail.

At least that's what my amateur brain is telling me.

Not sure how to proceed since there are no schematics... Any idea of what could be happening?

[khaahk]

Re: Surface Pro 4 dead - Main 8.5v power rail is 2v - Have good board to compare

Check diodes inside the red circle first.

[mattpd]

Re: Surface Pro 4 dead - Main 8.5v power rail is 2v - Have good board to compare

Check diodes inside the red circle first.

All of them test OK and 15v is present on all of them.

[guruji]

Re: Surface Pro 4 dead - Main 8.5v power rail is 2v - Have good board to compare

check the io chip ite8527 that starts the charging

[khaahk]

Re: Surface Pro 4 dead - Main 8.5v power rail is 2v - Have good board to compare

Same housing(battery)?
Without battery are there changes?
Use charger datasheet for reference
https://cdn.badcaps-static.com/pdfs/...95b04c6146.pdf
Good board most likely charging battery, bad one not, and battery discharged? (2.5V)

Standard errors are described here:
https://www.youtube.com/watch?v=kOScqCSul6M
https://www.youtube.com/watch?v=Qrdfd3Rojxw

[mattpd]

Re: Surface Pro 4 dead - Main 8.5v power rail is 2v - Have good board to compare

Same housing(battery)?
Without battery are there changes?
Use charger datasheet for reference
https://cdn.badcaps-static.com/pdfs/...95b04c6146.pdf
Good board most likely charging battery, bad one not, and battery discharged? (2.5V)

Standard errors are described here:
https://www.youtube.com/watch?v=kOScqCSul6M
https://www.youtube.com/watch?v=Qrdfd3Rojxw

No battery or housing. A good board will put out the 8.5v rail with only a charger connected.

This bad board has twice randomly started working again though. Both times while I was probing voltages around the FETS. For no apparent reason I started getting the expected 15v around the FETS, 21v on the gate pins, and the 8.5v rail came up.

But then it died again, but instead of having 1.8v around the FETS like I used to, I now have around 5v.

[khaahk]

Re: Surface Pro 4 dead - Main 8.5v power rail is 2v - Have good board to compare

Still i would start with measuring the BQ24770 VCC and ACDET etc. Also the FETs at the lower left corner, gate-drain-source - any shorts or low resistance?
Does the bad board work from charged battery? So you can rule out parts after the charger and power source selection circuit.

[mattpd]

Re: Surface Pro 4 dead - Main 8.5v power rail is 2v - Have good board to compare

Still i would start with measuring the BQ24770 VCC and ACDET etc. Also the FETs at the lower left corner, gate-drain-source - any shorts or low resistance?
Does the bad board work from charged battery? So you can rule out parts after the charger and power source selection circuit.

No shorts or low resistance on those FETS and the bad board does not work with battery.

Thanks for the PDF. I checked VCC and it's around 1.8v. Good board has 15v adapter voltage at VCC.

This caused me to look elsewhere on the board to try to figure out why the 15v adapter voltage wasn't making it to the charger IC. I assume that they should almost be directly connected...

So I found two more FETS that were under a shield that I had ignored earlier. These FETS appear to be the first stop between the charger and the rest of the board. I'm thinking this may be the root of my problem.

On the good board, on the FET to the right, there is 15v on the source 5v on the gate and 15v on the drain.

On the bad board there is 15v on the source but there is also 15v on the gate, and 0v on the drain. The FET to the left is all 0v.

So I guess now I am trying to figure out why this FET isn't passing 15v. I assume because the gate is too high...

[mattpd]

Re: Surface Pro 4 dead - Main 8.5v power rail is 2v - Have good board to compare

I see what's causing the 15v gate now.

The component labeled "DP" has something to do with controlling the gate.

The top center pin should have 3.3v. It is 0v on bad board.

If I inject 3.3v onto this pin, the gate drops to the normal 5v, and the FET opens. But still no main 8.5v power rail coming up.

So my guess is that 3.3v is missing elsewhere on the board too. I can measure 3.3v elsewhere on the board though, so the rail isn't completely down..

[mattpd]

Re: Surface Pro 4 dead - Main 8.5v power rail is 2v - Have good board to compare

check the io chip ite8527 that starts the charging

Reflowed it and now it's working.

I'm guessing the person I sold it to must have dropped it.

[khaahk]

Re: Surface Pro 4 dead - Main 8.5v power rail is 2v - Have good board to compare

I suggest testing it for a while just in case, by reflowing you might have heated something else on the board that made it to start working.

[mattpd]

Re: Surface Pro 4 dead - Main 8.5v power rail is 2v - Have good board to compare

I suggest testing it for a while just in case, by reflowing you might have heated something else on the board that made it to start working.

Will do, but I am pretty confident on this one.

The reason I reflowed in the first place is because I had another bad board here that has CPU problems, but was at least able to bring up all it's main power rails. I decided to remove the EC from the board to see what would happen. The results were the exact same as the board I was working on, 8.5v rail dropped to 2.5v.

[SPBR]

Re: Surface Pro 4 dead - Main 8.5v power rail is 2v - Have good board to compare

Hi everyone,

Very glad I found this thread. I got a surface pro 4 mainboard here with similar problems but know what caused it (me). I replaced the battery on this surface and got sent a faulty battery that didnt output any voltage. Trying to figure out why it wasn‘t working I probed around and accidentally shorted the taller capacitor (marked green) that is on the 15V rail. It smoked the diode close to it and possibly more.

I replaced the diode and now I am getting only 14.5V on the 15V rail when plugged in and the 3.3V rail shows around 3.1V which slowly drops. I am not getting anything around the FETs on the left or the capacitors.

The FETs on the charger input are in the same state as on mattpd‘s bad board. Also no voltage on the 3.3V pin of the „DP“ component.

What I noticed is that in the area of the shorted cap the board starts to heat up when its plugged in to the charger. I marked it in the pictures.

Any suggestions are very welcome! Unfortunately I don‘t have good board to compare.

Cheers!

[jozman]

Re: Surface Pro 4 dead - Main 8.5v power rail is 2v - Have good board to compare

Hi!
I have similar problem

[vgcrepairs]

Re: Surface Pro 4 dead - Main 8.5v power rail is 2v - Have good board to compare

Have one here with same issue - I replaced the battery, the IT8528VG and 8v voltages returned. I have already replaced both diodes and they also test good - but still this SP4 will not power on

[mon2]

Re: Surface Pro 4 dead - Main 8.5v power rail is 2v - Have good board to compare

As per above, check the voltages on each coil and post their values. If the voltage is not logical for that coil (ie. is too low to be practical) then that local circuit must be reviewed.

You should also power down and remove the battery, with your meter in resistance mode, check the resistance to ground for each coil. If the resistance is too low, you could have a bad cap causing that voltage rail to not turn on or remain on.

[sicilianul]

Re: Surface Pro 4 dead - Main 8.5v power rail is 2v - Have good board to compare

Hello. I have one of these and I have ~8V from battery, I press the power button and 3.3V and 5V appears as in the photo below. But the main 3.3V becomes 2.7V. PCH is heating and other voltages aren't appearing. Ideas ?
Also I have marked some of my measurements on photo.

[mon2]

Re: Surface Pro 4 dead - Main 8.5v power rail is 2v - Have good board to compare

Remove all power.

Meter in resistance mode (ohms).

Black lead of meter to ground.

Red lead of meter to one side of the referenced coil.

What is the resistance to ground on this coil ?

Good idea to check the resistance to ground on other coils as well.

[sicilianul]

Re: Surface Pro 4 dead - Main 8.5v power rail is 2v - Have good board to compare

Measured and attached a photo.

Also I have removed the diode from below (near lcd connector) and I still have shord to ground and diode is good.

[christian100]

Re: Surface Pro 4 dead - Main 8.5v power rail is 2v - Have good board to compare

Measured and attached a photo.

Also I have removed the diode from below (near lcd connector) and I still have shord to ground and diode is good.

Hallo Friends.
i hade same Problem and i could sove this Problem. I had made a video about this Problem
https://youtu.be/6ydT_RxYt-k
i will hope that this will help you