Testing fuses on laptop

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kris_hm

kris_hm

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Feb 14, 2014
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Ok, I need help. I'm working on my MacBook Pro, mid 2009 with no backlight that no one is interested in helping with. I decided its a component on the board. It's either the fuse of the wled driver itself. I'm trying to test continuity but I don't get any reading on any of the fuses. So I'm obviously doing something wrong. I know how to test continuity and I've got my dmm set correctly, but every time I test a fuse I get nothing. All of them can't be bad. So can someone help me figure out what a the fuse looks like on the MacBook Pro mid 2009. I've already searched the internet and the small component with the white dot is the one I'm testing but it's not getting anything. I tested other fuses I found on the board, ones I think look like fuses, the small component with the orange middle. My meter is working, I touched the leads together and it works, and I tested other things too like the grounds on the motherboard, that worked, I tested a good piece of wire, that worked. So I think I'm testing the wrong component.
Help would be appreciated. I want to test the fuse before I replace the wled driver chip, assuming I can get my hands on a replacement.
 
Solution
J
I found that information in the datasheet for the LP8543SQX, and also on a schematic from another MacBook Pro who uses a LP8545SQX. So, I deduct that your motherboard is using also a coil in the power line for the LED. You wont see a physical difference between a coil and a capacitor, they are both black.
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J

jarotech

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Hi, the fuses have a white body, sometimes with a number on it. What is the number for the wled driver chip? To best way to test a fuse, you power up the motherboard and test the voltage on both sides of the fuse, the fuse is simply a short who will permit the current to pass.
 
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kris_hm

kris_hm

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The chip number i have is LP8543SQX, atleast thats what i found online. On the actual chip itself it says LP8543SQ,
but since its so close and in the right location and i cant find another one with the same numbers or in the correct location, i assume that is the wled driver chip. The fuse i read has a small white dot on it, but what i see, it is the size of a resistor, very very small. It has metal sides and a orange middle. i know sometimes parts are very small, but this doesnt look like a fuse and i cant test the continuity of it, or any component similar to it. But i know my dmm is working properly. I do not really want to try the voltage test because i dont have leads that are the size of the solder pads on the fuse or the wled chip. If i accidentally cross pads or touch 2 components together, im sure il kill something. Can you post a picture of what the fuse is supposed to look like? and Il take a picture and post it of what im looking at to try to show you. Its really small, and hard to see though, even in person.
 
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kris_hm

kris_hm

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motherboardpicture_zps990f6832.png.html


This is the image i took of the board. I have circled in red the wled driver chip and in yellow i circled what i have been led to believe is the fuse for the wled driver.
Hopefully you can see the image. I couldnt figure out how to upload it directly.
http://s66.photobucket.com/user/kris_hm/media/motherboardpicture_zps990f6832.png.html
 
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J

jarotech

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I don't think you circle a fuse, it's a resistor. If you look at the picture you post, at the upper left you have a rectangle with a orange center, that is a capacitor, on the right of this capacitor if the rectangle is white, that is a fuse. The very small components with the center orange are capacitors. You can also find capacitors with the center black, but nothing is written on them, no need to test them. You have the resistors, they are black but usually with number written on them. For the fuses, they are white and are bigger than the resistors.
 
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kris_hm

kris_hm

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Thats what i thought, but for whatever reason i found online that the small component with the white dot on it was the fuse. I thought it was way to small to be a fuse. I did test the white one in the corner of the image, and it did test good when i did the continuity test. Leaving the chip to be at fault if that is the fuse for the backlight. Now i just need to find a chip.
i would have replaced the board, but it seems so wrong to replace a working board, just one little component not working properly. Id rather attempt the repair.
 
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J

jarotech

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It could be a fuse, it depends which company manufactures that component, and also, it's from Apple. On the datasheet for the WLED driver, in the power line for the LED, you have a coil, if it's open you have no power going to the LED. But I don't know if on your motherboard you have a coil in the power line. The only way to find out is to look at the schematics for your motherboard, but I don't have them.
 
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J

jarotech

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I found that information in the datasheet for the LP8543SQX, and also on a schematic from another MacBook Pro who uses a LP8545SQX. So, I deduct that your motherboard is using also a coil in the power line for the LED. You wont see a physical difference between a coil and a capacitor, they are both black.
 
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