A1278 MacBook Pro with a backlight short? Did the fuse win this time?

okay today we have a 2011 8 12 78 with a customer complaint of no backlight so we can confirm this is no backlight by taking our microscope light and shining it back here where's beer right there and you can see there's an image there it's hard to see but there is an image there so no backlight typically with LCD displays you have your image and you have your backlight so the backlight is what illuminates the LCD displays you can actually see something there are several reasons why this cannot work one you could have a bad screen or two can have a board issue so when diagnosing a board issue there's a few voltages that we need to pay attention to one is backlight output because that's relatively going to tell us what's going on in the circuit so if we have 50 30 90 50 volts it's typically a bad screen or bad screen cable because there's no load on it if it's 25 to 27 volts that means it's working or if it is 8 to 18 volts that means it is not boosting so it's a DC to DC to DC boost circuit that takes the 12 or 8 volts and PPOs you through yot and changes it into 50 so we're gonna check what voltage on output is here so we have a reader here and you can't really see that because it clear move it there you go now I'm gonna check voltage on backlight output here which is gonna show up right around here and that is zero volt so it's zero volts typically means it's either there's a short or that the fuse is blown so what we're gonna do check for short so our meter is in continuity mode we are gonna check for a short on the back light output caps and we see we have a 10 ohm short to ground there so 10 ohms short to ground typically means something shorted so first thing unplug the screen because the screen is a very common cause of shorts so the screen is unplugged we're gonna check again and it's still shorted so we know what you definitely of a board issue can't really fix the board and it's in the computer so I went ahead and pulled it out so we could work on for anyone wondering I went ahead and checked back light output voltage and resistance to ground and these 2 caps right here these are our backlight output capacitors right before it goes into the screen so we do so you have a little bit of corrosion here now one of my main questions here is is the fuse gonna be blown so the backlight fuse as you know on this model or on these back books doesn't typically blow it's usually the connector melts or something melts so let's check this fuse and what do you know the fuses actually blown so the fuse actually did its job so this corrosion right here although this is corroded and you could say this is liquid damage right here this doesn't look too bad like this does not look anything that would concern me so let's go ahead and look so here's our backlight driver that's probably not it could be what kind of doubt it so let's look on the other side of the board now and let's look at the rest of the backlight circuit so that's gonna be somewhere around here mm-hmm what do we have here this doesn't look too good does it so this looks like we have corrosion here so let's go over to the schematic and the board view and see what this stuff does so switch over to display capture and let's do that okay we have our board view open here and we could see that this was fairly corroded this is looks like gonna be something for ppl v5s oh and the board actually turns on so I really don't care too much about this we also saw these two caps right here were coated and these are for PPV out SW LCD backlight that sounds kind of important here's D 7701 as well which is the back light output diode kind of important then we saw this chip was really corroded and especially this capacitor right here is corroded and this is for wood you know LCD backlight so this could definitely have an issue be related to our issue so if we look at the schematic at this we can see that this is going to be our backlight MOSFET so pull this up so ya so here's our input to the backlight Circuit PP + g3 hot here's our MOSFET here's a couple transistors here's our driver you know nothing too crazy and we see this is completely destroyed by liquid there's a couple things we can do here we could either inject Fulton and try and find the exact component that's shorted or we can just wipe out all this stuff cuz we kind of know none of this stuff is good so let's add some flux and let's wipe out pretty much everything looks crowded here so let's get our hot air on my soldering on there's a lot of dust here - that's okay that's not really gonna hurt anything these capacitors don't even look that bad so I don't think the short is on one of these capacitors could even be on the other side of the board I kind of doubt it though I think it's gonna be one of these guys this coil obviously we don't need that and obviously the coil is not gonna be shorted to ground but it still looked kind of corroded so we're gonna get rid of it yeah our MOSFET in fact quite enable resistors and caps or 1v5 and chip which is probably fine but I don't want to take any chances with it and there's that see there that that capacitor may have been the cause that issue see that see how that pad is it's all corroded that could probably do it so let's try now let's check and see if our short is gone so why are you out of focus microscope let's check again for short and we have no short to ground anymore so that is good so we're gonna go ahead and start replacing the stuff and see if we get back light first things first we need to clean this area wick it and put some fresh solder down before even attempting to put new components on cuz soldering on top of all this junk would not be good that's not a quality repair we're just going to clean this it's great wait some of these pants how this is no good but if we scrape away this layer of layer of oxidation we could easily solder on top of it no issues at all most this nasty stuff will clean up in the ultrasonic - probably gonna want to put a little solder on that via it's not too bad next thing I'm gonna do is just grab my iron some flux and just run it around everything to pick up any remaining dirt and stuff and all that lead-free solder that we don't want then we can go ahead and put wicked away in put the fresh components down you don't generally want to mix lead-free and let it it's not a big deal if you do if you do but it tends to be more brittle so it's best to stick with one type occasionally I'll do it it's not a big deal but try and avoid it you don't want brittle solder joints if there's that scrubber wick let's wake up all of this stuff another common thing I get is when people take away the conformal coating they think they bridged it that's not a bridge you're just you just took away the coating it's not a big deal whatsoever don't worry about it you know like this looks like a bridge but it's really not it's just just a conformal coating on the board that you're taking away not a big deal at all makes you feel better you can put a little green stuff after you're done but it really doesn't matter it's not gonna hurt a darn thing being like that so you're good looks pretty good we'll go ahead and start putting our fresh Center down stay in focus microscope there's that okay let's start by the MOSFET working through the coil last because the coil will melt if we put any hot air on it so that's another thing when you're taking it off from the donor board do not use hot air use low melt and pull it off the donor that way because it will you'll ruin it the other way so let's grab our MOSFET basically we're just putting everything back that we took off add a little bit more flux whatever reason something doesn't look modern percent right we could always touch that up with the iron when we're done I really just want us to go in place we're good the coil we're gonna do last looks like we have two capacitors here when we're done and the diode can forget the diode and that just ran away okay we'll put back what ran away because you saw that cap took off that little coil there not a coil a filter yeah similar thing dr. filter this is actually an inductor looks like a filter but it's a little inductor line that it's a matheran it seems a piggy one wants to run away this is even on low air I know I'll get comments saying you need to lower your airflow but this is actually pretty low there we go that's down now for our capacitors there's to some water flux in here and now the diode now the coil so we put some solder there we want to wick away that solder and here's why that coil needs to sit flat for us to solder it right so we're gonna wick away the existing solder and we are going to put it on flat and then we'll add solder to it grab a coil a little bit's fine might even help you a little bit but you don't want it like sticking up all lopsided that wouldn't be good would work it just wouldn't look good there's one side that's right if we melt it a little bit sometimes you may want to consider a bigger tip too so I have this little J tip on there right now cuz that's what I prefer but if whatever reason we don't think it's uttered well it's always best to switch to a bigger tip just to confirm I want to make sure you're doing a good job and not a half way job come on get hot there we go that should be soldered we'll check it just to check something then get trolled by it later so we're good to go so this area looks good everything's here no obviously this isn't gonna work because our fuse is blown so another thing I'm looking at is a backlight driver and that looks okay I'll let that stay for now but this fuse has got to go so one time an Apple fuse actually blew now happens more than once but it happens fairly often at least on this model it does on the airs no the errors the connector always burns before the fuse will blow but on the a12 seventy-eights it does tend to blow you'll get a lot of these if you if you keep blowing backlight fuses and you're plugging in the screen of the battery connected not good which the scope would stay in focus but it doesn't looks good through my eyepieces but on this camera side not so much this is a it's not the M scope adapter either it's a expensive zoom adapter some solder there let's grab a fuse little backlight fuse okay now it's time to test okay so we have our motherboard back in the enclosure we're go ahead and plug this in and let's see what we get so we have a green light on the charger we should get our fan spinning here shortly actually no books save the battery plugged in let's go ahead and hit these power on pads that are right around here there we go my fan is spinning and let's see what we get and look at that we get a working backlight and an Apple logo so this is fixed thanks for watching and I hope this video helps you

Channel: TCRS Circuit

Views: 6,047

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Id: 4Fd-6Tqy6ng

Length: 18min 57sec (1137 seconds)

Published: Fri Feb 07 2020