Toshiba P50-C won't charge or run of adapter but works of battery

[mission]

Hello everyone.

I'm having a very annoying problem. While I'm sure that most would find it a rather hilarious reading, I shall cut the long story short...
The laptop got water damaged and HDD failed. As the battery was fully charged I figured after I installed new HDD that perhaps the adapter died as the laptop worked perfectly fine but wasn't charging. After I got a new adapter it still wasn't charging the battery. However, in the meantime the battery ran out of charge and died. It then transpired that adapters are fine but somewhere in the charging circuit between power socket and motherboard something wasn't right.

I'm trying to figure out if it is possible to fix this, however I am unable to find any diagrams or references for the motherboard in question so i can't establish what may be causing the issue.

the first image is that of the motherboard itself (sorry about huge images, resizing them didn't work and .. well.. I didn't have time to upload new ones)


next image is of what i believe to be an issue part of a circuit


as you can see, first MOSFET is working and showing 19.34V on all S and D pins. the tiny (?? can't even see exactly what it is, is reporting power, but nothing toward battery terminals is reading any power. Since the battery is now dead and I don't have any others to try it on with I can't check which circuit sectors being fed from the battery itself are working.

I really need to figure out the problem and try to get it fixed. Laptop is just over a year old and it will be some time before i can either afford to get a decent replacement or buy a new motherboard. In the meantime, I am all up for troubleshooting, soldering, rebuilding or whatever I have to do to get this puppy going again.

[amyk]

As I can't find a schematic for this model, you'll have to ID the ICs that make up the power section and look at their datasheets, as the reference design often matches what's on the board.

[flolic]

Carefully check all components for water damage.
Check all MOSFETs in power/charging section.

I had few cases with the same symptoms where problem was in a shorted MOSFET.

[xavier60]

In many laptops the 19V has to go through 2 MOSFETs to get to the VBAT rail. Or a schotky diode and a MOSFET. These MOSFETs often suffer from Gate to Source breakdown. The charge control IC usually controls these MOSFETs. It senses the presence of the  19 V from the AC adapter on a pin called ACDET or something simillar.

[mission]

Thank you all for suggestions, I will check why is the second mosfet not showing any signs of power (I would at least expect that it had power at the source side even if nothing was passing through, that would make my job way easier) this is why i'm thinking something else may need replacing to get power to it (not that it's ideal, but continuity test tells me that the mosfet itself is fine)

Both mosfets are 6414A I checked their pinouts SSSG/DDDD One is fully functional and the other is just not getting anything at all, which is why I was thinking the issue will be somewhere between them.

On the other hand, battery fins, from the + side 3rd has 3.1V and 5th 0.7V (left to right on pics) i'm thinking 3rd is thermistor and 5th possibly data comms so some power is fueling the circuit

Don't get me wrong, I have difficulty telling apart the little light yellow/brown SMDs how can I check them without resorting to removing them from the board? for most if not all as i recall continuity says they should be fine, but i can recheck each and every one again.

[xavier60]

Is that MOSFET being intentionally turned on with the Gate being more positive than the Source?

[mission]

Do forgive if I sound dumb but I am not entirely sure what you are referring to.I am new to working with these type of components, my electronic skills are for the most part on a single layer circuit board with through-hole components.. simple and fun.. this is very challenging for me as I have quite a lot to learn to reuse my 'basic' knowledge and apply it to such complex boards.

When I plug the power into the board I am able to trace power in all S and D pins on the lower MOSFET at 19.34V and on the upper I get 0V on all pins

I did the several tests on the MOSFET chips:
S1,2,3 to D1,2,3,4 on diode setting returns 0.501 on one and 0.466 on the other MOSFET (as they are n type red probe on S and black on D)
any other combination of S&G or D&G returns OL
From what I was able to ascertain this means that both should be fine? If it comes to it I don't mind pulling them out and testing outside the circuit, i'm just trying to avoid it for now.

Now when you say Gate being more positive than Source? what does that mean? is there a reading I can do to check this?

[xavier60]

Those readings indicate no obvious problems.
With power applied, measure the voltage between S and G. An N MOSFET needs the G to be more positive than the S to be ON, as little as 2V will start to turn on some MOSFETs, the actual normal voltage in this situation is typically 10V.

[xavier60]

On page 6 there is a possible example. The 19V needs to get to the +VBAT. The +VBAT feeds all of the Point of Load Buck regulators, where ever you see 2 MOSFETs near a large square inductor.

[mission]

Thank you. That explains and makes sense. I'm away for work today and will check if G is more positive tomorrow.  I will also read and inspect the file, thank you much, to figure out how to trace it around.

i shall post more findings tomorrow. This is like a cool, if on occasion frustrating, investigation. Onwards and upwards!