iPhone 7 Plus No Touch When J3801 is connected

[GuruFix]

Hello community,

I have been lurking for a long time but now its time to join the community. Hello everyone! I got a brain-cracker for you guys.

Phone: iPhone 7 Plus 820-00249-A
Fault: No touch
History: Not known, we got this phone from another repair shop.
Water damage: No
Previous mainbord repair: No
Known good original proximity and display are being used.
Homebutton/TouchID is original.

For starters the phone boots ok with a normal DCPS boot current. So no shorts or abnormal behaviour.
When the phone is booted the touchscreen does not respond at all. The homebutton/touchID is original and working great.
After some research i discovered that there is no PP1V8_TOUCH_TO_MAMBA_CONN and there is no PP2V75_MAMBA_CONN at J3801
Diode mode on J3801 in compare to a known good board is OK. This is when i concluded there is no PP1V8_TOUCH which normally creates the missing voltages at J3801.

This is the moment when it is getting weird for me.

If i do not connect J3801 PP1V8_TOUCH is present and so is PP1V8_TOUCH_TO_MAMBA_CONN and PP2V75_MAMBA_CONN.
And i've got touch fully functional, but of course no homebutton/touchID.

I decided to inject 1V8 into the PP1V8_TOUCH line and connect everything as normal. PP1V8_TOUCH_TO_MAMBA_CONN is present and
PP2V75_MAMBA_CONN is created by U3801. But i do not have touch

I'm guessing the fault is in a data line at J3801?? But in fact i'm pretty lost at the moment.

[GuruFix]

[darkwintercloud]

Did you tryied to make a diode mode when the flex is connected and the board is off?

On the PP1V8_TOUCH_TO_MAMBA_CONN and PP2V75_MAMBA_CONN, the impedance remains normal?

Also, theres a note on the schematic that says that the "impedance must be > 0,005 ohm on the output", the output for U3801 is PP2V75_MAMBA_CONN on pin1 and caps C3823 and C3812. The ampedance is correct when the flex is connected on those?

[GuruFix]

The impedance is exactly the same when i connect J3801 or when i do not. PP1V8_TOUCH .455 and PP2V75_MAMBA_CONN .510 in diode mode.

[crea2k]

I'm guessing you've tried another screen ?, I only ask as it's odd how those lines vanish when you plug in a screen.

[GuruFix]

@dark no i didnt diode when J3801 is connected. I will try that! But there is still no 1v8 touch to enable

crea2k yes i have tried 3 screens. All original and known good.

[Jessa_the_Professa]

Start ruling things out--like taking off R3805 and see what happens to your 1v8 touch.

[GuruFix]

Thanks Jessica! I've removed R3805 and now the missing voltages are present and so is touch!
Still don't know why there is touch when J3801 is not connected, but im happy so far!!
Can u share your theory?

[Jessa_the_Professa]

Stare at the schematic and you tell me why I suggested taking off R3805.

[GuruFix]

R3805 is a in line component and in fact no more than a wire.
If the resistor is removed, a variable is also removed. That's all i have..

[john]

Removing that resistor just isolates that part of the circuit. I don't see how that made touch work?

[GuruFix]

I'm lost to, but i can ensure you touch is back! s
PP1V8_TOUCH and PP2V75_MAMBA_CONN are present. That means removing R3805 doesn't isolate that part of the circuit.
PP1V8_TOUCH_TO_MAMBA_CONN cannot possible be present but touch still works.

We have looked at the schematics for hours, Jessa_the_Professa free us!