A1708 (820-00840) power issue

[Deluccia]

Okay so I've had an absolute nightmare with an 820-00840 board.
Annoying because I'm usually pretty successful/lucky with my component level repairs.
If anyone can help me I would be so grateful and also would even pay for the gesture.

This board has had minor liquid damage on the far side corner where C6491 and many other PPBUS-G3H caps are and also liquid damage where the U7000/battery connector is.

So after ultrasonic cleaning and replacing all damaged looking or incorrectly valued caps/resistors I plugged in and immediately found that U7800 was getting unbareably hot right at around pin 136 (PPBUS-G3H-CPU) I assumed an internal short and replaced, no excessive heat to report.


Ive been testing at D6905 and have 0.3V at pin 1 (ppbus-g3h) 20V at pin 2 (DCIN) and 20V and pin 3.
I removed D6905 to test at pads and found:
Pin 1: 0V
Pin 2: 5.8V
Pin 3: 3.8V
Ive replaced D6905 and found that i just go back to the same voltages as before.
I have replaced almost all capacitors on the PPBUS-G3H rail as I had 0 resistance to ground when testing.
I have replaced U7000
I have replaced U6990
Have started looking at Q7030 and Q7040 but feel I'm being led astray.
I am missing TBA-LX1 last I checked.
Battery connector has no crossed pins or shorts.

Maybe the G3H CPU short in U7800 has killed the machine?
I feel like if i can restore PPBUS G3H I will get all rails back and have power.

Please can anyone shed any light on this?

Thanks in advance.

- Joey

Board-view and schematic attached if needed.

[piernov]

Re: A1708 (820-00840) power issue

So you have a short to ground on PPBUS_G3H right now? How much resistance?

20V on DC-in, 3.3V on PP3V3_G3H are ok on the other hand?

[Deluccia]

Re: A1708 (820-00840) power issue

Hello,
Thanks for your reply.
Yes thats correct, positive to ground beeping at 061 on PPBUS_G3H
DCIN is 20.3V
PP3V3_G3H is 3.4V.

I am looking through schematic to see if there is any components that take in PPBUS_G3H and convert it to a computing power rail, sort of like the retinas did with S5_HS_COMPUTING.

EDIT: Found it, PPBUS_G3H converts to PPBUS_PMIC at R7835.
PPBUS_PMIC has the same 061 beeping to ground.
U7800 removed, still 0.1V on PPBUS_G3H

[piernov]

Re: A1708 (820-00840) power issue

Resistance in ohms, not diode mode. 0.061 seems too low anyway, so you may have a partial short to ground somewhere on PPBUS_G3H, or a shorted high-side MOSFET.

[Deluccia]

Re: A1708 (820-00840) power issue

Hi, Yes tested in OHMS not diode.
Just tested in diode and found 0.061 anyway.
Sorry didn't specify that previously.
Any ideas where to start?
Really pulling my hair out here, I have a donor board so happy to try everything.
Thanks again for helping me

[piernov]

Re: A1708 (820-00840) power issue

So it is 61.0 ohms in resistance mode?
That's indeed too low. But finding a partial short to ground like this is not easy.
First I'd check if any high-side MOSFET connected to PPBUS_G3H is shorted, by checking resistance between PPBUS_G3H and the output of buck converters. So measure resistance between PPBUS_G3H and L8030, L8060, L7960, L7900, L7730, L7690, L7670, L7430, L7370, L7220.

[Deluccia]

Re: A1708 (820-00840) power issue

Correct,
I'll boot up board-view and check now!

[Deluccia]

Re: A1708 (820-00840) power issue

Okay,
Interesting..

L8030: 238
L8060: 205
L7960: 0
L7900: 31
L7730: 220
L7690: 369
L7670: 444
L7430: 34
L7370: 40
L7220: 36

PPBUS_G3H still 0.3V

[Deluccia]

Re: A1708 (820-00840) power issue

Okay Q7960

Pin 3: 0.3V
Pin 4: 0.3V
Pin 5: 0V
R7969 fuse is good

[piernov]

Re: A1708 (820-00840) power issue

Time to remove Q7960 and pray a lot that the CPU isn't fried.

[Deluccia]

Re: A1708 (820-00840) power issue

Q7960 removed tested and replaced.
Now have PPBUS_G3H at 12.95V!!

going to put it in and see whats working

[Deluccia]

Re: A1708 (820-00840) power issue

Not working, no fanspin.
All_SYS_PWRGD missing.
I'm going to pull my own lips off

[piernov]

Re: A1708 (820-00840) power issue

You have to go down the list of power rails and see which ones you have and which ones you are missing.

[Deluccia]

Re: A1708 (820-00840) power issue

Yeah I know I'll be back to it tomorrow.
These newer models Im not as familiar with to be honest.
I have written and printed a check list i go through for the Air and retina boards but I am unsure with the 2017 and newer boards in terms of order to check and from where they derive from.
I'll get there In time.
Thank you so much for your help.

[befaoim]

Re: A1708 (820-00840) power issue

Hi everyone,
I'm facing a power supply issue on a 820-00840 motherboard. It seems that someone was visiting this board before me. U3100 and U3200 are respectively a CD3215B03 and CD3215C00. I know that both chip must be identical. Does someone know what is the correct reference value?
Best regards,
Fabrice

[piernov]

Re: A1708 (820-00840) power issue

Replacing both chips with C00 should work.

[befaoim]

Re: A1708 (820-00840) power issue

Hi Piernov,

Thanks for the reply.
I tried to replace both by C00 but I still don't have the 20V at PPDCIN_G3H. I have 5.2V.
I look for I2C communication between U3100/U3200 and the SMC and I can see both I2C acknowledge so I think both circuits seems to work correctly.
I'm confused between B03 and C00 because I read that this board need B03 here: https://boards.rossmanngroup.com/for...not-turning-on
and the inverse here: https://store.rossmanngroup.com/inde...cd3215c00.html
So I'm lost.

Fabrice