820-00850 PPBUS 3.9V

No corrosion on board.
At the start there was 5V and 0.1A from charger.
One of the CD3215 had burn mark and then I found PPV3_UPC_XA_LDO was shorted to ground.
I replaced that IC, short gone and then I found other CD3215 got short as well on PPV3_UPC_xx_LDO.

At this stage PPBUS at F700 was 0.1V (no short) and I removed F7000

I replaced other three CD3215 a got 12.25V on F7000/1
F7000/2 in diode mode .338

And now when put back F7000 PPBUS got short spike to around 12V and then stays for a few seconds at around 3.9V and then again short spike to 12V and drop to 3.9V.

PPBUS in diode mode now got .369
Big coils near CPU got low resistance to ground but not shorted.

Any idea?

Looks like overcurrent is detected and T2 resets power on sequence.
Compare diode mode readings on all coils with good board, or reference table.

Did you test with battery connected too?

Will check rest of coils soon.

With battery connected got 20V and 0A from charger with short spike to 0.025A.
Got 9.60V on PPVBAT_G3H_CONN so will test with charged battery soon.

OK, today there is 5V and 0.35A from charger.
Removed F7000 and on F7000/1 got still 12.55V and F7000/1 got .022 in diode mode.

During comparing coils in diode mode found short to ground on L7660.
Injected low voltage to PP3V3_G3H and U7800 was getting warm.

Removed U7800 and short on PP3V3_G3H is gone.
F7000/1 got now .050 in diode mode.

While U7800 is removed check coils around it and found that (diode mode):
L7812/2 .017
L7810/2 0.115
L7806 .007
L8100 .07

So.... T2 faulty?

"F7000/1 got now .050 in diode mode"
Isn't that pin 2 in fact???

These readings are from a board which doesn't actually work.
L7600 405
L7660 427
L7702 061
L8100 096
L8102 084
L7270 011
L7210 002
L7410 002
L3500 497
LB700 497
L7030 482
L8410 826
L6900 356
L9080 352
L9020 382
L9030 354

L7806 015
L7810 350
L7812 048
L7813 016
L7815 245
L7816 066
L7820 120
L7821 012

Is an i7 with 16GB RAM and 1TB SSD.
Please, give me the readings from the other coils of your board.

T2 coils should have good readings, as it completes DFU revive.

""F7000/1 got now .050 in diode mode"
Isn't that pin 2 in fact???"
Yes, pin 2 - sorry for mistake.

L7600 401
L7660 265
L7702 040
L8100 082
L8102 091
L7270 018
L7210 004
L7410 003
L3500 473
LB700 479
L7030 470
L8410 840
L6900 190
L9080 299
L9020 239
L9030 286

L7806 011
L7810 270
L7812 030
L7813 332
L7815 223
L7816 039
L7820 244
L7821 137

Thats 2TB board.

Check resistance (ohm scale) between PPBUS_G3H and L7702.

Remove R6900 and check again diode mode to ground at PPBUS_G3H.

"Check resistance (ohm scale) between PPBUS_G3H and L7702."
3.97 kilo ohm

"Remove R6900 and check again diode mode to ground at PPBUS_G3H."
When was checking R6900 I found crack on U6903.
After U6903 has been replaced I got high value in diode mode on PPBUS_G3H.

With soldered F7000 I got 12.26 on PPBUS_G3H.
On charger 5V 0.29A so with this current it should switch to 20V.
Thermal camera - only warm is T2 at 32C.

Checked all LDO outputs od CD3215 chips and all good in diode mode.

Try DFU...

No luck

What means that?
Can't enter DFU, or you can't complete it?

Cant enter DFU.

Do you get expected SLPS2R and AWAKE voltages on U7800's coils?

L7810 0.78V
L7812 0.82V
L7811 0.84V
L7809 0.75V
L7808 0.75V
L7807 0.75V
L7806 0.76V
L7815 1.09V
L7814 1.10V
L7816 0.90V
L7817 0.89V
L7813 1.79V

L7819 0V | .240 in diode mode
L7820 0V | .240
L7822 0V | .133
L7821 0V | .133

Force it manually into DFU...

Forced method works.
Did 'Revive" during 'Installing System' around in 1/3 process with error:
"Gave up waiting for device to transition from DFU state to DFU state.
[com.apple.MobileDevice.MobileRestore - 0xFCA (4042)]

Tested with know good battery with 60% of charge.

Data is the most important here.
Is there any trick to force in to 'Target Disk Mode'?

No way.
Authorized Apple service can retrieve the information in DFU mode.
Not sure if is possible, if revive can't complete.