iPhone 6 backlight

[Jimbob]

iPhone 6 with no backlight.

Came in with a VCC_MAIN short, removed C1552, C1597 and it's gone but now no backlight.

Checked all filters, all good.

The problem is when I have a screen attached and plug in cable i'm not getting 16V on PP_LCM_BL_ANODE
I've done backlight circuits before. Full strip and rebuild but this one is confusing me.

I have a good working board but now that's doing the same thing too..

Here are my notes while doing the repair

On a working board 4.2V is at the Anode side of Diode and 16V on the Cathode side.
On my dead backlight board I only have 4.2V on both sides but i'm missing C1513.
I'm going to remove this from the working board and see if I still have backlight.
OK
Still 16V without the cap
Will remove the diode on good board and see if the cathode is being fed the 16V from the chip
First lets get readings from the diode on both boards - - - ALL GOOD
It's the chip
Changing again..
So my question really is - - - Where does the 16 come from.. Coil, Chip, Diode?
I'm pressuming it comes in from coil to chip and goes out to other side of diode from chip too..
Removing chip now, checking if voltage is present under chip
Underneath chip I have 4.2V
Going to remove coil now and see whats going on.
Then work on good board, remove chip and see if 16V is under the chip when removed
Ok.. Without coil, were not getting the 4.2V now under the chip, going to put on a new coil and see
But I still don't know if on a good board, the 16V is under the chip without the chip, it should
Ok..It doesn't, going to put on another chip and if still no 16V, ill remove chip from good board and see
Ok.. new chip and still no joy, getting the 4.2 again. Could it be the two caps VCC caps removed thats causing problem C1552, C1597
Lets remove them from good board and see if we lose the 16V
One removed, still have it
Next one
Still there..
Lets remove the other three and see if we have it underneath them
One removed, still have it
3 removed and it's gone, hardly damaged chip.. lets put on again and see if we get it back
One back on, now I've 4.2V - lets put another one on
2 back on and still only 4.2V - hardly damaged the chip, nah... let's put last one on and see..
3 on, surely 16V on em..
Nope.. 3 back on and no 16V
Confused.com

Any ideas?

When I was removing the caps from the good board, i wasn't using any heat, just giving them the aul scandanavian flick so I doubt I damaged the chip.. but, something has to have changed..

Thanks for the time, I really do appreciate it.

[Jessa_the_Professa]

Let's clear this up.
1.) measuring backlight voltage is always a waste of time. If you have no problems in the system you will get full voltage. If you have any problem at any point in the system then you will get main voltage only.

2.) What is the MOST LIKELY way that the backlight circuit has failed here?
A.) There is a short on one of the backlight caps just like there was a short on the adjacent main caps
B.) The backlight driver got nicked and damaged from knocking of the vccmain caps
C.) The filter blew
D.) One of the cathode return filters blew
E.) The diode went bad
F.) The coil stopped being a coil
G.) There is damage at the connector causing it not to make a good connection from screen to board.
H.) This phone is actually in DFU mode which doesn't have backlight
I.) I long screw damaged the phone during testing and now I have no backlight or image because I tore the screen reset line


Any of those are perfectly possible, your job is to decide which is MOST LIKELY and come up with a plan to rule them out one by one until you find your answer.
Let us know what you find.

[Jimbob]

Ok cool. Will get at it and report back.
Thanks Jessa.

[Jimbob]

Quick question..

If one of the backlight caps were short and I removed it..Lets say I remove all three and every other component is working in the circuit, should I have 16V on the cap pads or do they have to be there to get that voltage?
I know you said testing for 16V isnt a great method but it has to be there for the backlight to work..

Thanks again..

[Jessa_the_Professa]

So you are STILL trying to measure backlight voltage? You gotta stop that.

IF you insist, then here's an easier way to test for backlight voltage--put a screen on and see if it lights up.
It if lights up you have 16v, if it doesn't you don't. Also know that you will never have backlight voltage without the screen (remember the entire circuit must be working for it to be there and that includes the screen, otherwise you have no enable signal due to missing screen and therefore BL is not active you will just have main voltage,

This makes voltage testing for backlight super easy, no multimeter required! Also not very useful, so can we agree to abandon that method?

To answer your question here: If you have cleared a short on backlight anode line, then you will ONLY have 16v on the cap pads if you have a screen connected and the phone powered on, image circuit is working, and there are no other problems in the backlight circuitry. You will need to put those caps back or your backlight will flicker.

[Jimbob]

Ok, I replaced the caps with 3 new caps and still no joy, 4.2V on them.
Diode has been replaced, it's reading

- OL
- 0.179

[Jessa_the_Professa]

Please tell me you are testing for backlight with a screen installed and not with a multimeter hoping to see backlight voltage magically appear without a screen at the connector. (I get cranky when you ignore the advice I already gave you....)

[TVtechcellphone]

I had a defective generic low cost screen die on me while replacing shattered screen on i6 and prevented booting. Got another screen from parts wall fixed that and made customer wait 10 minutes longer. I rather prefer quality job over hasty job.
The second replacement screen had pressure spot too, I checked everything that could press on the screen from behind and find none but fortunately customer does not care, as it is only 70 dollars total. I rather use quality screen.

I handle ESD properly.

Cheers.