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How to know this burnt device on my laptop's motherboard ?

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eagle1109

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Hello,

I was doing some fixing to my Dell 7559 laptop frame because there were some breaks at the hinges spots and I was trying to fix it with the super glue.

But later I noticed that when I move the display it shuts off when I move it forward, and it comes on just with a slight more backward. I didn't realize that the reason the display shuts off because there is short going on !!

The last time I smelled something is burnt .. I knew then I burnt something with a short. And I think I shorted the stuff going to the display.

But I didn't know about it after the display went really dim. I opened the motherboard and I noticed this device that is burnt.

I think if I could replace this device that I can fix my laptop again. But I don't know how to find its number !

Can anyone help me how to get the number of this device ?

20200214_134202.jpg

20200214_134209.jpg
 

eagle1109

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Is there anyway to simulate the operation of Q7 ? just temporary until I receive the replacement mosfet from aliepxress.
 

d123

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Hi,

You'll know... If there isn't a spice model (quick search in Goggle gave no results, best link was to datasheet by what looked like manufacturer), sifting through datasheets, or quicker still, your simulation tool's models (parameters) of similar devices that are the closest match, and accept a non-ideal extrapolation of results; if it's a standard general purpose (and preferably 'old') transistor, might be easy to replace with e. g. IRF640 or similar commonly available spice model, etc. Good luck.
 

albbg

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Is there anyway to simulate the operation of Q7 ? just temporary until I receive the replacement mosfet from aliepxress.

It depends. From the small screenshot of the schematic is not possible to say if you can just try to short source and drain, using a mechanical switch. The mosfet is used as switch driven by the signal "LED_BL_ON". Form which part of the circuit is this signal coming ?
Anyway, before to replace the transistor you should investigate why it burnt. I mean to be sure has been a short circuit and not it is removed. In order not to have a second one burnt.
 

eagle1109

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Hi,

You'll know... If there isn't a spice model (quick search in Goggle gave no results, best link was to datasheet by what looked like manufacturer), sifting through datasheets, or quicker still, your simulation tool's models (parameters) of similar devices that are the closest match, and accept a non-ideal extrapolation of results; if it's a standard general purpose (and preferably 'old') transistor, might be easy to replace with e. g. IRF640 or similar commonly available spice model, etc. Good luck.

I ordered the mosfets from aliexpress, I just have to wait like a month until it arrives.

It's the ao3409.

I also searched for a replacement motherboard and it's really expensive; $500+. But I would like to try fix it myself and I hope that the only burnt thing is the mosfet.

It depends. From the small screenshot of the schematic is not possible to say if you can just try to short source and drain, using a mechanical switch. The mosfet is used as switch driven by the signal "LED_BL_ON". Form which part of the circuit is this signal coming ?

Yes I tried to short drain to source by 2 ohm resistor but didn't work, I think the gate signal is monitored somewhere. But the display is still very dim.

To me if the signal is dim it's a good signal that the fix could be at least not very expensive; because if the motherboard is burnt and I can't fix it with my simple troubleshooting experience, the motherboard is really expensive.

I stopped doing any efforts to fix it now, I better wait for the parts from china and try to replace it. If the problem didn't go away, then it's another story other than a mosfet.

Here's another bigger shot.

ckt.png




Q7 in brightness power circuit.

Complete document link:

http://sualaptop365.edu.vn/attachments/dell-inspiron-15-7559-rar.24600/

page: 29


Anyway, before to replace the transistor you should investigate why it burnt. I mean to be sure has been a short circuit and not it is removed. In order not to have a second one burnt.
 

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