820-3437 Intermittent backlight issue

gabewhite

New member
Hi, I've read several posts about these board backlight but can't figure where my problem fit and I'm new at this so don't be too quick to roast me please.

Macbook had a broken LCD, changed the screen and when I got to test it there was no backlight. I inspected and saw that I plugged it the wrong way and thought I might have damaged the assembly. I tested with another known good screen housing and problem was still there so I figured it's a board issue.

I replaced the LCD connector, U7701 and also F7700 (was shorted) & L7701 (weird readings) from donor boards.

I'm getting intermittent backlight now, it's flickering on startup and stabilizes once booted only if I'm near maximum brightness. Voltage and continuity seemed fine but I reflowed U7701 as someone with similar problem did it and corrected the issue. After reflow, no more flickering BUT brightness controls didn't affect the backlight level so even at minimum it would light up and wouldn't shut down unless I closed the lid. I left it last night and today it's back, if I turn down brightness below 60-70% backlight is gone and there is still no gradation when it lights up.

I haven't tried anything else and figured I would ask you guys before sending it in and/or messing something.

Thanks !
 
Last edited:

2informaticos

Administrator
Staff member
First of all, welcome to the forum!

"F7700 (was shorted)"
I'm not sure what this mean; you may probably want to say open, not shorted...

Your issue looks like bad solder (corroded) at A5 ball of U7701; change it.
 

gabewhite

New member
Thank you very much, I look forward to learn more and share knowledge with others here. I meant blown, fuse wouldn't allow current through it.

I guess you were right because every reflow made it better but got worse over time, I'm thinking cold solder joints. I replaced U7701 again and tried to align it better, mine wouldn't move easily with flux like other chips I've done like U2 Tristar and Touch ICs on iPhones. I had to up my 861DW temp to 400c and 70 air because the solder wouldn't melt as easily as my other repairs. It now works good and allows brightness to be higher and lower. I'll update tomorrow if it stops working but I'm pretty sure it was a bad solder on U7701 like mentionned.
 

gabewhite

New member
You are correct, I didn't think I needed to remove CPU heatsink. Thanks for the advice on settings, I'll try this next time !
 
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