Why isn't my backlight on?

MacBook Pro 15 22 Unibody 2.53 GHz Mid 2009

I have a mid 2009 15" Macbook Pro 2.4 Ghz Intel Core 2 Duo. the computer boots up. There is a chime and the screen boots up. But no backlight. So the only way I can see my screen is by holding a flashlight to it. I know the computer works. I plugged it up to an external display and it worked fine. I know it's not software because I booted from an external HDD which I know works.

I read on another forum that these models use LEDs and don't have an inverter.

So my question is: What makes the backlight work? What do I have to replace to make this screen visible?

Any general info about the screens or inverters or whatever is helpful. Thanks

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I found this other post with similar symptoms. I am good with a soldering iron and have a microscope available. If it is possibly a blown fuse that is causing the problem, I'd like verification before I buy a whole new display that I don't need.

Again. I am good with a soldering iron, but I'm not a computer science major. Please spell things out as simply as possible. I can borrow a multimeter and test components if I need to. I just need to know what they are supposed to read. Thank you very much

This is the url of the other relative question.

Detailed Logic Board Question. No Backlight..

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There is a fuse to the circuit. It is a 402 SMD, 2A, 32V, fast acting and has a white dot on it. The 402 SMD are 1mm long and half that wide. If it is good it will have continuity. If it is bad there will be infinite resistance. That test would be preformed with the system off. The circuit carries between 14vdc and 27vdc, depending on where the brightness level is. The system must be on to measure voltage. You have to be very careful if/when you take voltage measurements to not allow your probes to touch anything other than what you mean them to or you can/will short out components. I soldered a sewing pin on each of a set of probes to do this kind of work. You can buy a set of special probes for this kind of work for around $60USD. I don't have your board. Click here, for a picture of a board with the same circuit in it. The components may be placed differently on your board, but will look the same.

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I changed this to an answer because it is. You may want to put it back to a comment but as it's not trying to gain more info or clarify a point I don't think it has any need to be a comment. Hope you don't mind, :-)

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Thank you very much. I didn't test resistance, but there was no continuity and the voltage, while on, was 0. I'm going to take that to mean the fuse is blown. I will test resistance next. That is ohms right? On the picture you added, you said test impedance to ground for all pins. Where is the ground so I know where to put the black lead? Also, for the 74 rectangular thing you said test voltage. is that supposed to be in between 14v and 27v as well?

Lastly, where can I find a replacement fuse? Thanks.

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Resistance is measured with OHMs. Voltage while on 0 on both sides of the fuse? Yes the diode (rectangular thing with 74) is a good place to measure voltage, I believe it is marked in the picture - measure backlight voltage. What country are you in - depending on where you live some places will be better than others to acquire a fuse.

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I replaced the fuse and the continuity is good now. But the backlight is still off :( Any other ideas?

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Do you have proper voltage on the marked side of the diode? The square chip marked measure impedance in the picture I provided. What are characters on it?

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