Backlite is gone, not display issue

Question

iPhone 3GS

Revamped version of the iPhone 3G with faster processing speeds. Repair of this device is similar to the 3G, and requires simple screwdrivers and prying tools. Model A1303 / 8, 16 or 32 GB capacity / black or white plastic back.

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Anri @intenditore

Rep: 71

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Gepost: 27 jun. 2016

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Backlite is gone, not display issue

Hi!

Some time ago I replaced a screen on 3GS. That unit was bad as I discovered later, any pressure applied to top right corner of the screen was shorting backlite led + and - than backlite turns off. After lock/unlock it restored, but only untill I touched that corner again. So once again it didn't do back. I replaced the screen, no luck. Than I went with this scheme

and tried to replace 6R8 coil, no changes. Than I wired 3rd display connector pin with third pin on ic (if I call it right). Nothing changed. Than I wired 2nd display connector pin to the contacts on the back. And nothing again. I jumped all resistors and filters I could but no backlite :|

Guys, please, help my find out what's wrong there and how to fix it?

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Anybody?..

I'm without a phone at all :\

28 jun. 2016 door Anri

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Vantas @outraged Rep: 13 2 · Answer 1 · 2016-07-08T03:37:13-07:00

Since you know which pins to short, you probably have the schematics, so here are the basics: the backlight circuit in 3GS consists of a power inductor, a transistor driven by the power controller chip, and a small filter inductor. The first two form a DC-DC converter which rises battery voltage to a level sufficient to power backlight (don't have an oscilloscope right now to tell you the exact value), shorting the coil is not a really good idea, the converter simply won't work.

The coil usually dies first, but if you're adamant it's OK, you can also try replacing the transistor (on the right to the coil on your pic, marked with yellow "2"), it's FDFMA3N109 by Fairchild Semiconductor, a diode and an N-FET in a single case. They're still in production, you should be lucky. Please be advised: it has 2 additional pads on its "belly", so soldering's gonna be fun if you never tried that before.