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How to know if backlight short is fixed?

I have a 15" Retina Display that got some moisture in it (visible as bright spots in lower corners of screen) and apparently shorted the backlight, causing the backlight fuse on the logic board to blow. I subsequently put the display clamshell in a plastic bag, along w/ 2 large desiccant bags. That was several days ago. When I have the logic board replaced or repaired, is there any way I can check the display first to make sure it's not going to pop the fuse again?

Thanks,

Fred

EDIT: I have been able to get the fuse replaced (thanks, James!), but found that the backlight short was still present. After another fuse replacement due to popping the first, I'd like to know if there's any way w/ a multimeter (perhaps on the display cable MLB connection?) or other tool to check to see if/when I get the short fixed.

Related question: Since the Retina Display is essentially impenetrable as far as repairs go, can anyone offer tips on how to get any remaining moisture (from isopropyl alcohol) out of it and fix the short?

Answered! View the answer I have this problem too

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Fred turner, do you have a (the) model number of the LCD panel itself? BTW isopropyl alcohol will evaporate rather quickly and should not have stained the display. It does sound more like stains from other residue....

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Hey oldturkey! GSX shows the part # as: "661-6529. Part substituted to 661-7171." Does that help? I can try to find further identifying markers on the display itself if you are looking for something specific. Thanks! Fred

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Hey, just saw this comment (not getting emails again)...did you need me to do something further, or are you saying you're looking at the schematic for that p/n? Thx, FT

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Hey OT-- I don't believe I can get anything from the back of the LCD, since the Retina units are built right into the clamshell. Is there something specific you can point me to? Thx, FT

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Okay, thanks, I'll check it when I get home. Is pin #1 closest to the hinge? Do I need to ground to a certain other pin, or just somewhere on the chassis?

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Sadly, it sounds like your display has gone. Time to replace the part.

While its possible moisture is still present water alone is not the enemy as many people think! You could dip your system is distilled water wait a day or two after leaving it in the sun to warm and power it up! I don't recommend you try this though ;-} Its the stuff in the water that kills you.

The corrosion caused by the ionic action of the salts and other contaminants in the water likely has shorted the circuit so waiting any longer most likely won't change things here. Sorry ;-{

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Thanks for the reply, Dan! I'm hopeful that the situation isn't as dire as this...the actual liquid was 90% isopropyl alcohol, so at least it shouldn't have the "stuff in the water" that presents the greatest threat. I'm hoping it is just some residual alcohol in a very small area that somehow didn't evaporate...

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The quality of the Isopropyl alcohol can be a factor here. The lower quality stuff has more water and can have other stuff that could leave a conductive film. If you only got Isopropyl alcohol in the lid you maybe lucky here - Got my fingers crossed ;-}

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Firstly never turn it on again or bother replacing the fuse without finding out if the short is fixed. Which board is this, is this 820-3332-A or a newer one?

You tell if the short has gone away by putting your mulitmeter into ohms/resistance measuring, or diode mode. Put the red probe on ground, being the metal around the screwholes that hold the board into the machine, and the black probe on where backlight would be present. I like to grab the huge chunks of the ceramic caps right next to the backlight circuit. if you tell me which board you have I can tell you where to measure.

Right now you have no idea if the short to ground is in the board, the screen, the LCD connector, or the screen cable.

Also let me know if you have an old piece of junk PC you don't mind blowing up that I will explain to you how to turn into a short finder so you don't have to spend $1000+ on an infrared camera & lab power supply. Near the CPU on a PC motherboard you will see some transistors, one of which supplying something between 900 mv to 2v to the CPU. Solder a piece of wire to where you measure this voltage,and we're going to attach the other end of the wire to the backlight power rail of the board if it IS shorted to ground. Then we attach the ground of the lenovo to the ground of the laptop, and turn the old clunker computer on. So let''s say you attached the 1.7v from the old comp to the backlight rail on the board, you SHOULD see 1.7v on the logic board where backlight power would be would be, but something is shorting backlight power to ground, so that power will go to ground. Now since you attached the ground of the old computer to the ground of the board, that 1.7v will have a DIRECT path to ground through the shorted component. That shorted component will get very hot and you will know exactly what to remove/replace.

The only circumstance under which nothing will get hot is if the short is inside the LCD connector. The LCD connector is just bare metal and will not get hot.

Update

Block Image

This is backlight. Measure from here to ground, red probe on ground, black probe on the top of this capacitor here in diode mode on your multimeter. What do you get?

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From previous postings by the OP it was determined that the board is a 820-3332-A

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Thanks, Louis! I have been tied up w/ my newborn son and trying to get back into a routine now. I intend to study your response more closely and check the backlight circuit you suggest w/ multimeter in the next day or so, and I'll let you know what I get. I can probably round up an old PC to function as you describe, although that might not be until I'm back at my main office later next week. Thanks again! -Fred

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Fred Turner will be eternally grateful.
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