Lenovo laptop burned/shorted power rail repair

[stuntR]

First thread here! I've got this Ideapad 720s laptop to fix it has no power or charging so immediately after disassembly I found a cap that looked burned, and on the other side of the board a layer was found in this state.




I believe this is the main power rail as it is on the battery input mosfets. When I plug in the charger and nothing else there is small sparks when it is pressed with a plastic spudger (oh god).

I have taken pictures of the burned area and also removed some capacitors on the reverse to have a better look

Can I peel and scrape away the short? The layer must have burned into a ground track inside and the burned pcb can't be helping things

If you need any more information please let me know!
Thank you for any help!

[Fyod]

Re: Lenovo laptop burned/shorted power rail repair

Geez.
Luckily it's a large ground, so you might get away with digging down into the board to remove any burned material/copper. Keep checking continuity until there's no short.
You'll also likely have to replace the FETs because I doubt the cap caused this, more likely one (and consequently more) of them failed. Remove them until there's reasonable resistance to ground all around that area.

[Fyod]

Re: Lenovo laptop burned/shorted power rail repair

Also check/replace that diode PD403 and the transistor next to it.

[stuntR]

Re: Lenovo laptop burned/shorted power rail repair

Will do thanks, its the first time i've had to deal with damage like this and just wanted some input before I go scraping away at that pad xD

do u reckon those small holes by the caps I removed just go straight through to the other side of the board where the burn is? Hopefully they didnt put any other tracks in between or around that high voltage line?

[Fyod]

Re: Lenovo laptop burned/shorted power rail repair

Yes, the vias go straight through.
The problem is that there could be traces in the 4 or so layers between the top and bottom that run close to the vias.
If such traces were affected then repair is almost impossible.

[stuntR]

Re: Lenovo laptop burned/shorted power rail repair

Ok Ive managed to remove the short! I measure the resistance of the power rail to ground and I get 30k ohms and increasing, and in diode mode i get 0.365V. Is this normal?

[stuntR]

Re: Lenovo laptop burned/shorted power rail repair

Here's a picture of my handiwork


I soldered the components back on and get around 0.31V in diode mode on the rail

[stuntR]

Re: Lenovo laptop burned/shorted power rail repair

After plugging in the laptop i get 20V at the drain of the first mosfet (AON 7408) but then about 100V at the source! Surely this isnt right? Also it was slowly getting warm and warming the inductors too (nothing burning hot). Any ideas?

[Duranitron]

Re: Lenovo laptop burned/shorted power rail repair

After plugging in the laptop i get 20V at the drain of the first mosfet (AON 7408) but then about 100V at the source! Surely this isnt right? Also it was slowly getting warm and warming the inductors too (nothing burning hot). Any ideas?

when you power up the unit did you see any led light ,,cpu fun spin..check all 20v power rails specially the 5vALW and 3.3v ALW try to get a good meter..when measuring the voltage place the black lead grounded then the red lead is use for poking a schematic can help a lot...

[SMDFlea]

Re: Lenovo laptop burned/shorted power rail repair

After plugging in the laptop i get 20V at the drain of the first mosfet (AON 7408) but then about 100V at the source! Surely this isnt right? Also it was slowly getting warm and warming the inductors too (nothing burning hot). Any ideas?

100V isn`t right,check the battery in your meter is good.

[stuntR]

Re: Lenovo laptop burned/shorted power rail repair

I've checked the power rails, on one side of the charger input mosfet I get the 20v, but on the source of it I still get around 100v, and this is then feeding into the big inductor next to it which is getting warm, but not really hot. Also on this rail after the mosfet I am getting around 10V, so there is a leak somewhere as it should be all at 20V from the charger.

I have also found a large row of caps shorted to ground above the GPU - this could be my partial short on the 20V rail

I have the 5v and 3.3v which is good but no lights

Sometimes when I plug in the charger I get a quiet ticking noise coming from the board, and instead of 100V on the source of the mosfet, I get 35V fluctuating slightly

Totally lost here

[stuntR]

Re: Lenovo laptop burned/shorted power rail repair

Yeah the source pad of the 3rd mosfet up from the bottom is shorted, which must then be connected to the caps on the GPU power rail which are also shorted on both sides. But nothing on the board is getting hot in that area.

[Duranitron]

Re: Lenovo laptop burned/shorted power rail repair

is this the first time the board has been repaired or someone just give up on this unit and end up in you hands. because its a mess... those caps will never have that much solder. try to clean them up and try to power it again..

[stuntR]

Re: Lenovo laptop burned/shorted power rail repair

It hasnt been attempted before me and those caps were soldered back by me, it looks bad but its not causing a short. I had to put more solder because the pads had been damaged due to the heat from the cap shorting originally.

The problem with the burn hole is fixed now, I'm trying to figure out why i have a line of caps shorted to ground now and why my mosfets are giving out 100V when not turned on

[Fyod]

Re: Lenovo laptop burned/shorted power rail repair

Sounds like the mosfets are shorting to ground giving you 5x20V in series.
I would remove all mosfets and take measurement, then replace the mosfets and make measurements after every replacement. They have to be gone after such a blast, imo.

You could theoretically find the mosfets' datasheet and check them after removal based on the pinout. It is always more reliable to measure parts off the board.

30K to ground seems reasonable.

[stuntR]

Re: Lenovo laptop burned/shorted power rail repair

Yeah I've given up at this point haha, I removed all the mosfets and theres still a short on the pad itself which then shows up on the caps above the gpu, so it must be fried or something

Most of those mosfets were destroyed too when i tested them off the board with the meter, I'm going to take it into a local repair shop to see what they think because its very difficult and expensive to buy the individual replacements

Oh well xD

Edit: Im just glad I managed to fix the big explosion on the back of the board TBH haha