Released on September 19, 2014, this 5.5" screen iPhone is the larger version of the iPhone 6.

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No backlight on iPhone 6 plus

Iv got an iphone 6 plus in that im rather puzzled with, it came in with no backlight in the top right corner of the screen. I checked all the ferrites and they were all just fine. I probed TP2520 and got 11 volts, and then probed TP2559 and just got the battery voltage, so checked out both diodes, both diodes were fine so changed out U1580 with a new ic.

Once I had replaced that I then had no backlight at all. I checked the two test points again and now neither chip was boosting and I was just getting battery voltage on both test points. So I now changed BOTH backlight drivers and also the lcd driver ic. I am still having the same problem though, display, but no backlight at all, both test points show the battery voltage and arnt boosting at all.

The backlight caps seem to be fine, no shorts there, measuring across the coils seems to give the same reading as a known good board.

I noticed then that this board also has long screw damage, so I repaired ap_bl_l2c1_sda and possibly broke lcm_to_chestnut in the process, but now fixed that too.

Im getting 1.8v on all of the lines in the hole that I can see, just no voltage on ap_bl_l2c1_sda , but not sure if this is a data line and I need an oscilloscope to probe it ?. If tested in diode mode it gives a similar reading to a working board.........

Soooooo im really puzzled now, as iv had both backlight ic's off and checked all the pads that I can with diode mode, and continuity all the way back to the connector pins and they seem fine.

As I said the test points are reading the battery voltage, so the backlight ic circuit isnt boosting for some reason. I did look into finding a datasheet for the ic's to see if I could jerry rig something so that I could maybe fool the circuit into enabling, by pulling a pin low or something, but it looks like it talks to the cpu or chestnut to turn on and isnt I hack I can do.

So yeah im really confused where to go next now.....

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Changing U1580 killed backlight completely so did you try another U1580?

With backlight I always find the least thought method and simplest is to change everything lol but there's always a possibility that one component is bad or just a crappy reballed ic

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Solución Elegida

As C9679 points out, backlight issues are sometimes best resolved by clearing out the whole circuit. One thing I have found is bad coils do occasionally pop up. Sometimes they are cracked but still "measure" correctly but don't really behave as inductors. The only way to know for certain is to measure them with an LCR meter. I also had one situation where the chinese supplier I had ordered replacement coils from sent me the wrong ones. With no markings on most of the coils, you never really know what you are getting. In this instance, they were 1uH instead of 15uH and that messed up the circuit.

As for the I2C lines, they are data lines so you need a scope to wee what they are doing. However, measuring them in diode mode can at least give you an indication of their overall state, such as shorted or no.

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