Toshiba Satellite L750D - 19v missing on the main power rail

[Vesko356]

Re: Toshiba Satellite L750D - 19v missing on the main power rail

Yes yes, what i tried to say is that maybe 0.3v (instead of the 1v that "we asked") is low for burning the bad cap or the bad component xD.

I will try with a bit more amp as you said, thanks!

We are not trying to burn anything but to locate (by the dissipated heat) the component that causes the short circuit in the line, if it was certain that it was a capacitor we could easily increase the voltage, but we are not. The short circuit can be caused by a consumer of some switching power supply, and if the upper mosfet in that power supply is shorted for example, and we apply a higher voltage to the consumer (through the shortened mosfet) from which it is designed to work, it can burn out. So we will have to limit the voltage to safe 1v to make sure it won't get damaged if this is the case eventually.

[AidenX]

Re: Toshiba Satellite L750D - 19v missing on the main power rail

We are not trying to burn anything but to locate (by the dissipated heat) the component that causes the short circuit in the line, if it was certain that it was a capacitor we could easily increase the voltage, but we are not. The short circuit can be caused by a consumer of some switching power supply, and if the upper mosfet in that power supply is shorted for example, and we apply a higher voltage to the consumer from which it is designed to work, it can burn out. So we will have to limit the voltage to safe 1v to make sure it won't get damaged if it's the case eventually.

Hey! I was kidding about the burning thing! xD
And for the 1V yes, i read about it in the beginner's guide here!

With 2A, the voltage is now 0.8V, near to the 1V.
I asked about more amp for this, just to be near the safe 1V, not for put more of that! ^^

Anyway, this time maybe i found something really hot! And is nothing that i tested until now. x.x
It's near the keyboard flat connector, but seems more than one cap.... O.o.

I will desolder one and test it!

[Vesko356]

Re: Toshiba Satellite L750D - 19v missing on the main power rail

Anyway, this time maybe i found something! And is nothing that i tested until now x.x

Sounds promising

[AidenX]

Re: Toshiba Satellite L750D - 19v missing on the main power rail

Sounds promising

FOUND IT!

PC42, left side of the flat keyboard connector (missed in the photo n4).

I will put a photo tomorrow xD.

I was so focussed on that part of the board near the ram slots (the ram itself had a burning capacitor!), the mosfets, the hot area with the power supply connected....but the problem was in another part x.x.

And when a tried with the bench supply, probably i did it wrong. xD


So, that capacitor has 0 ohm, dead.
The "ground part" of the first mosfet is gone, like all the other things near it.
All the capacitors that i disconnected before with double ground sides now have only one side to ground!

Also, with the plus of the bench supply in that point of the mosfet, now no more voltage drop etc.

It's really only that capacitor the problem? x.x


NOW, i read isn't safe to put the dc power with a disconnected component....how can i know the farad value of that dead capacitor?
Testing the others with the same size?

Again thanks and tomorrow i will add the photo of the "new little problem" area!

[Vesko356]

Re: Toshiba Satellite L750D - 19v missing on the main power rail

FOUND IT!

PC42, left side of the flat keyboard connector (missed in the photo n4).

I will put a photo tomorrow xD.

As I assumed, and as I told you in most of the cases this is the reason, especially when the resistance in the line is so low as in your dead short main power line case.

..the hot area with the power supply connected....but the problem was in another part x.x.

And when a tried with the bench supply, probably i did it wrong. xD

I don't have a diagram for the model, but since you took a good pictures on both sides of the board (although in some places your markings were a bit confusing) I was able to figure out where the problem came from. Your initial mistake comes from trying to apply voltage to the main line before the input mosfet, which heats it up because it lets the voltage to pass throug it in a dead short line. In addition, the design of this particular laptop is quite strange, I rarely see such a design in which the current resistor is placed before the input mosfet. so applying the voltage as is usually done at the current resistor will lead to the same mistake.

So, that capacitor has 0 ohm, dead.

As expected.

The "ground part" of the first mosfet is gone, like all the others things near it.
All the capacitor that i disconnected before with double ground sides now have only one side to ground. Also, with the plus of the bench supply in that point of the mosfet, now it didn't take any current, no more C.C and voltage drop etc.!

Of course it was this way because they are all connected in the main power line, which was dead shorted.

it's only that capacitor the problem? Really thanks for the big help!

You're welcome. Congratulations on your first repaired laptop!

NOW, i read isn't safe to put the dc power with a disconnected component....how can i know the farad value of that dead capacitor?
Testing the others with the same size?

Again thanks and tomorrow i will add the photo of the "new little problem" area!

Size doesn't matter this time. The surrounding capacitors may have the same size, but this does not mean that their parameters are necessarily the same.
In this case there will be no problem even if you leave it like that, there are enough filter capacitors in the line, one less will not be fatal. Without a circuit diagram there is no way to know what parameters it had.

[AidenX]

Re: Toshiba Satellite L750D - 19v missing on the main power rail

As I assumed, and as I told you in most of the cases this is the reason, especially when the resistance in the line is so low as in your dead short main power line case.

So if the voltage drop isn't of 0.x V but something more high like 2V+, the % to be a faulty mosfet will be more?

... (although in some places your markings were a bit confusing) I was able to figure out where the problem came from.

Sorry for that! But that pcb wasn't clear and so confusing. xD

In addition, the design of this particular laptop is quite strange, I rarely see such a design in which the current resistor is placed before the input mosfet. so applying the voltage as is usually done at the current resistor will lead to the same mistake.

YES!
Can we say that to be the first "try to fix the mainboard" it wasn't a so normal and easy board for a beginner ? xD

I watched many videos and tried to do the same tests (in particular for the mosfets!) but everytime i didn't find a similar initial power line (like the usual dc power - first mosfet - second mosfet - current resistor) and that confused me!

So it started to be a "test random stuffs" near the initial power part and all that ground zone, but the dead component wasn't there x.x

In this case there will be no problem even if you leave it like that, there are enough filter capacitors in the line, one less will not be fatal. Without a circuit diagram there is no way to know what parameters it had.

Hope so! Still want to understand the functionality of that cap.

There should be a connection between the shorted cap on the ram stick and that one on the board (but apparently not near the power ram area).

I will post the photo later!

Thanks again "for showing me the way" and for clarifying how to test things correctly! ^^

[AidenX]

Re: Toshiba Satellite L750D - 19v missing on the main power rail

Sooo, the bad capacitor (PC42) is connected with that near cap PC35 (and probably others) and, in the opposite side, with PC138 cap and the first CPU mosfet PQ24.

Is still safe to power on the board without replace the cap? x.x

(There is also a photo with no edits this time xD).

[Vesko356]

Re: Toshiba Satellite L750D - 19v missing on the main power rail

Is still safe to power on the board without replace the cap? x.x

Yes, don't worry.

[AidenX]

Re: Toshiba Satellite L750D - 19v missing on the main power rail

Yes, don't worry.

Can you answer at my first question of the other post?
Just for understand how it works. xD

I desoldered the burned cap also from the ram (photo n1 first post), now it's not shorted anymore. Can it be used again without replace the cap?


EDIT: Oh, the board it's alive!
Leds turn on, hope that also the cpu, gpu, etc are fine. x.x

Will try all later!

[AidenX]

Re: Toshiba Satellite L750D - 19v missing on the main power rail

Well...bad news...it's turn on, but is not POSTing... :'c
Just another led on, fan it's ok but that's all.

External display doesn't work also.
The notebook is on, also the hard drive, keyboard etc, but blocked before the bios.

Without the ram, it beeps (and it's normal), could this mean that the cpu is correctly detected (and it's fine) ?

I don't have any other cpu for that socket to test, but with different ram memory it still not POSTing (and no beeps).

From the guide here:

Check if laptop is POSTing: pressing caps lock key triggers caps lock LED, activity of USB drive blinks when turning on, audio plays, HDD is being read, etc… If it's not POSTing:

  Check RAM compatibility
  Reseat RAM and clean RAM slots
  Measure exact voltage on CPU VCore
  Check CPU Reset signal
  Follow power sequence if it exists in the schematics
  Follow list of power rails if it exists in the schematics

Can you explain me better how to do the tests #3 and #4?

At least now it's less dead than before, there is just a new problem to fix! xD
(Are we sure that the missed cap isn't a problem anymore?) x.x

Thanks! ^^

[Vesko356]

Re: Toshiba Satellite L750D - 19v missing on the main power rail

Well...bad news...it's turn on, but is not POSTing... :'c
Just another led on, fan it's ok but that's all.

External display doesn't work also.
The notebook is on, also the hard drive, keyboard etc, but blocked before the bios.

Without the ram, it beeps (and it's normal), could this mean that the cpu is correctly detected (and it's fine) ?

I don't have any other cpu for that socket to test, but with different ram memory it still not POSTing (and no beeps).

From the guide here:

Check if laptop is POSTing: pressing caps lock key triggers caps lock LED, activity of USB drive blinks when turning on, audio plays, HDD is being read, etc… If it's not POSTing:

  Check RAM compatibility
  Reseat RAM and clean RAM slots
  Measure exact voltage on CPU VCore
  Check CPU Reset signal
  Follow power sequence if it exists in the schematics
  Follow list of power rails if it exists in the schematics

Can you explain me better how to do the tests #3 and #4?

At least now it's less dead than before, there is just a new problem to fix! xD
(Are we sure that the missed cap isn't a problem anymore?) x.x

Thanks! ^^

Without schematic it's almost impossible. You can measure the resistance (battery and adapter must be disconnected) and voltage on all of the coils.
Also this laptop is very old with ATI graphics, maybe the chip is faulty - you can try to heat it up with hot air for about 30sec. and try to power on to see if this will do the trick. And yes, the missing cap is not a problem.

[Vesko356]

Re: Toshiba Satellite L750D - 19v missing on the main power rail

I reminded myself of one more thing - because this laptop is quite old model, see if there is a Nec Tokin capacitor on the back of the board under the CPU socket, if so it is very likely that the problem is coming from it. This capacitor is notoriously problematic.

https://www.youtube.com/watch?v=V96hp6Ok2U0

[AidenX]

Re: Toshiba Satellite L750D - 19v missing on the main power rail

Without schematic it's almost impossible. You can measure the resistance (battery and adapter must be disconnected) and voltage on all of the coils.

Just for be 100% sure, i have to do that measures with multimeter - to ground and + to one pin of the coil? Or with both on the coil pins?

I reminded myself of one more thing - because this laptop is quite old model, see if there is a Nec Tokin capacitor on the back of the board under the CPU socket, if so it is very likely that the problem is coming from it. This capacitor is notoriously problematic.

Oh the same capacitors that can be faulty on the old PS3 FAT BC (backwards compatible). I know about them! xD

Mmm... there are just the 4 "big" capacitors PC45-46-47-48 (marked "330 VPK") that you can see under the bad cap i removed.
All the others are normal smd caps.

[AidenX]

Re: Toshiba Satellite L750D - 19v missing on the main power rail

Here some coil to ground tests:

-Mainboard with cpu and ram-

PL3   43 Ω   0.043 V
PL4  < 4[B][U]M[/U][/B] Ω   2.8xx V
PL5  3.3 Ω   0.005 V --- [B]CPU[/B]
PL6  3.3 Ω   0.005 V --- [B]CPU[/B]
PL7 322.5 Ω   0.378 V --- [B]RAM[/B]
PL8 > 16[B][U]K[/U][/B] Ω   1.043 V (near the ram but ???)
PL10 > 30[B][U]K[/U][/B] Ω   1.264 V (near the ram but ???) 

PL9  10.8 Ω   0.013 V --- [B]GPU[/B]
PL11  10 Ω   0.013 V --- ?

-Mainboard with only the CPU (no ram)-

Just one coil is different.

PL7 553.6 Ω   0.445 V --- [B]RAM[/B]

[AidenX]

Re: Toshiba Satellite L750D - 19v missing on the main power rail

Hi! I forgot to say the last voltage values was with the diode mode of the multimeter.

So i did the coil to ground tests again but with the DC power on this time:

-Mainboard with CPU and RAM-

PL3  1.002 V
PL4 < 0.900 V (starts from 1.1xx V and then slowly drop)  
PL5  0.957 V --- CPU
PL6  0.957 V --- CPU
PL7  1.509 V --- RAM
PL8  3.354 V
PL10  5.086 V

PL9  1.113 V
PL11  1.112 V

Also is normal for this mosfets to have a low voltage (input to output) instead of 19V ?

Test between pins Source to Drain:

PQ18  [B]18.86[/B] V --- first main power mosfet
PQ24  [B]17.91[/B] V --- first cpu mosfet
PQ31  [B]17.75[/B] V --- first gpu mosfet

Thanks!

[Stevedb]

Re: Toshiba Satellite L750D - 19v missing on the main power rail

And take bigger cables when you injecting voltage.

[AidenX]

Re: Toshiba Satellite L750D - 19v missing on the main power rail

And take bigger cables when you injecting voltage.

Mmm ? I only injected 1V - 2A days ago for find the bad cap thanks to Vesko's help! I didn't use the bench supply anymore after that.

The tests i did yesterday (post #35) was with the original notebook power supply connected....maybe i wrote it wrong, sorry ^^"

Also this laptop is very old with ATI graphics, maybe the chip is faulty - you can try to heat it up with hot air for about 30sec. and try to power on to see if this will do the trick.

I did it at 200° C but still nothing! :c

[AidenX]

Re: Toshiba Satellite L750D - 19v missing on the main power rail

@Vesko356 Good news! Finally after a loooong search, and many strange and unreliable sites (with paywall etc..), i found the schematic for this board model!

I'm not 100% sure that is the same revision, but better than nothing...

I hope it can help to understand the new problem on this unlucky notebook... x.x

Thanks! ^^

[Vesko356]

Re: Toshiba Satellite L750D - 19v missing on the main power rail

@Vesko356 Good news! Finally after a loooong search, and many strange and unreliable sites (with paywall etc..), i found the schematic for this board model!

I'm not 100% sure that is the same revision, but better than nothing...

I hope it can help to understand the new problem on this unlucky notebook... x.x

Thanks! ^^

OK, good news indeed. I'm pretty busy these days, tonight or tomorrow I'll look at the diagram. At first glance, without looking at the schematic, the measurements you took look more or less good. But I'll take a closer look at all the things, and we'll talk again. I'll also briefly explain some basic principles for taking measurements.

[AidenX]

Re: Toshiba Satellite L750D - 19v missing on the main power rail

Hello, in this days i found also the bios for this notebook.

Flashed it but still no display / no POST...

At least now i'm sure that the bios chip is fine and the problem is something else... ^^"