submerged Macbook A2141 - no power on, no voltages

[Sanevent]

Hello there.
I'm new to this Forum and I don't know if I'm in the right place to ask.
I'm working as a forensics examiner for smartphones. I'm very familiar with Smartphones but not with Macbooks, so please be kind to me as I'm trying to learn to repair macs now.

My colleague dropped a Mac on my desk which was bathed in Tea. When I opened it up, all traces of corrosion I expected had already been cleaned off.

With that in mind, I dried the device in a dry box for 2 days and then tried to power it on - with no success.
I removed the logic board and probed the PPVBUS_USBC_X which was 5.124v with no significant change.
I expected 20v here, but that did not happen.

After studying the schematic I tested the voltage lines around the cd3217b12 USB C controller.
Voltages here were:

PP3V3_UPC_XX_LDO = 3.3v
PP1V5_UPC_XX_LDO_CORE = 1.5v
PP3V3_G3H_RTC_X = 0v

It was consistent on all 4 of them.

Please correct me if I'm wrong here, but based on the schematic of the A2141 I assume, that the 3v3_RTX line powers the USB-C controller, which then negotiates the voltage with the charger. This line is generated by the U6960? which is enabled by the CHRG_EN_MVR line. I tested this line, and it was 0v.
I traced back the CHRG line to the U7000.

From her on I lack experience. The only suspicious line I found on that chip was the PP1V8_SLPS2 line, that connects to some kind of enable pin on that U7000. But that line was also missing power.

I tried to get as far as I could, but now I'm not quite sure how to proceed.

I also tested PPBUS_G3H which is also around 0v measured on the F7000 Fuse, in Diode-Mode I measured 0.032v, also on F7000 to GND. The resistance was about 15 ohms.
I'm not sure if this even makes a difference now because the other voltages are missing in the first place - so I assume the main power will not be generated if the other ones are missing. I don't know if this is dependent or not. I also don't know if 15 ohms (it seems pretty low to me) are already shorted or if this is fine.

I would be very grateful if someone could assist me or give me a hint on where I should look next or even what is wrong with the device.

 

[2informaticos]

13 ohm for L7330 should be good enough.
However, you must get higher reading on PPBUS_G3H; usually 0.450 (approx) in diode mode.

 

[Sanevent]

I suppose that's with the U7370 soldered back onto the board?
Because without the IC I got 0.08v to PPBUS_G3H and 0.02v to GND in diode mode.
Currently, I can not check with the IC in place, because it takes some time for the chip to be delivered.

Edit:
I guess you meant PPBUS_G3H to GND - there I get 0.06v in diode mode (also with the IC not back in place).

 

[2informaticos]

0.060 in diode mode is still bad, for PPBUS_G3H.

BTW, don't write end "V" for diode mode readings.
Avoid any confusion with voltage readings.

 

[Sanevent]

Ok got it.

Do you have any advice for me, what I can try to do next to find the error on PPBUS_G3H, or should I wait until the new IC arrives to replace it first?

 

[2informaticos]

I hope you didn't apply power (nor even battery) on the board without U7370.

Check if any big coil gets a reading close to 0.060 in diode mode to ground.

 

[Sanevent]

Nope, no power was applied to the board. I would not even dare, until I'm not at least 70% sure that nothing blows up or you advise me to try.

I rechecked all big coils. The only 2 candidates that came close were L8102 and LA850, measuring 0.052 to GND. Everything else was far beyond or above the 0.06.

 

[Sanevent]

It is 86 ohms for the L8102 and 89 ohms for LA850.

 

[2informaticos]

So short doesn't come from that side.

Did you check at L9080/580 too?

 

[Sanevent]

I checked L9080 and L9580. Both of them have the same values:
0.51 to GND in diode mode
0.46 to PPBUS_G3H in diode mode
2.5 mohms to PPBUS_G3H in resistance mode
4 mohms to GND in resistance mode

If i measure in resistance mode, the Resistance decreases slowly. E.g. if i measure and have 4 momhms to gnd, then i measure 30 seconds later i only get 2 mohms. Could be normal behaviour, i just wanted to mention it.

 

[2informaticos]

If you are really sure, the lowe reading doesn't come from any big coil, then is safe to inject voltage there.
Just in case, remove both F7000/1 and check again in diode mode.
Also check removing F8400.

Anyway, start with just 1V.
Be aware, if you forgot to check some secondary power supply derived from PPBUS_G3H, can fry the board increasing the voltage!
BTW, checked L3500 & LB700?

 

[Sanevent]

Sorry, I did not reply earlier, I could not access the site on the weekends.There was an SSL error or something.
Anyway .... in addition to the F7000/1 fuses I removed the F8400 fuse too and checked L350 and LB700. Their values are roughly the same:
0.5 to GND in diode mode
1.5 mohms to GND in resistance mode
2 mohms to PPBUS in resistance mode.

I measured on the 2 points of F8400
PPBUS_G3H still has 40 ohms in resistance mode to GND and 0.060 in diode mode to GND
The other side of the fuse has an open loop an no connection at all.

When injecting voltage, do I just look for spots that heat up with my thermal camera or is there something else I must do (measuring things or something like that)?

Edit: I still got no replacement part for the U7370. There is still no IC in place. Is that a problem when I try to inject voltage or should this be replaced first?

 

[2informaticos]

As per schematic, U7370 includes the power MOSFETs, so no problem even if you apply power on the board.
Anyway, you can't apply main power yet, because of the PPBUS_G3H issue.

When inject voltage, start with 1V and slowly increase.
Set lab PSU current limit to max first.
With low voltage, small temperature difference can be detected with IR camera only.