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820-00840 short on PPBUS_G3H

J

joshAU

Member
Hi all.
I've got a dead 820-00840, won't power on, no fan.
The board has no signs of liquid ingress or other damage.

PPDCIN_G3H has 20.1v, tested with two different chargers.
PPDCIN_G3H_CHGR also measures 20.1v

However, there appears to be a short on PPBUS_G3H, as diode mode reads, with red on ground, and black on F7000, i measure 0.002.

I removed F7000, and retested in diode mode.
Pin 1 measures -0.638 (PPVBAT_G3H_CHGR_REG)
Pin 2 measures 0.002 (PPBUS_G3H)

I measure 13.08v on pin 1 with F7000 still removed, and obviously 0 volts on Pin 2.

Any help appreciated.
Thank you.
 
2informaticos

2informaticos

Administrator
Staff member
Check which big coil has the same value to ground.
Start checking the ones near CPU.
Switch to ohm scale, for better resolution.

Do NOT inject voltage on PPBUS_G3H!!!
 
J

joshAU

Member
Hi, again, 2informaticos, and thanks for your help.
With the multimeter on the ohm scale, ground to pin 2 on F7000 is 2.1 Ω
Testing all the coils near the CPU, 3 show the same value.
These are L7410, L7420 and L7430 all show 2.1 Ω

No worries re not injecting voltage on PPBUS_G3H.
I don't want to release any magic smoke. :)
 
J

joshAU

Member
Hi again.
OK.
So, I removed them one by one, and of course it was the last one where the short disappeared.

So, checking resistance to ground on Pin 2 of F7000
With L7430 removed, resistance dropped to 1.7 Ω.
With L7410 removed, as well, resistance dropped to 0.8 Ω
With L7420 removed, as well, resistance shot up to 64 Ω, and is gradually climbing.
EDIT: With all three now removed,the resistance too ground on Pin 2 of F7000 has now gone weird... sometimes measuring O/L, even on the 20MΩ, setting, then other times fluctuating values, increasing or decreasing depending on whether red is on ground or black is.
A bit later, in 200Ω setting, with red on ground, pin 2 measures around 70.3Ω
Diode mode with red on ground measures .112.
A tad confusing... I suspect some component in the circuit is confusing me. :)

With all three removed now, I measured the resistance to ground on Pad 2 of those coils, and all measure 1.6 Ω .
All three pads seem to be directly connected to each other, as their resistance to each other is 0.2 Ω - actually, looking at the schematic, they are all connected by the very low value (0.00075 Ω) resistors R7410, R7420 and R7430, so that would make sense.

Pad 2 of F7000 (PPBUS_G3H) shows a short to Pad 1 of L7420 (GTVR_SW2)
The only thing the board view says should be connected to Pad 2 F7000 is Pins 21 & 24 of U7420 and to Pin 1 of R7424 (which isn't populated on this board).

So I assume U7420 is the issue here???
 
Last edited:
2informaticos

2informaticos

Administrator
Staff member
U7420 is shorted, change it.
After that, solder F7000 and check diode mode on it; ALWAYS red probe at ground.
If no short, crossfingers and apply power.
 
J

joshAU

Member
Thanks 2informaticos.
I changed U7420, and the short from PPBUS_G3H to GTVR_SW2 is gone!

However, should Pad2 on L7420, L7410 and L7430 still be 1.6 Ω to ground?
Its PPVCCGT_S0G_PH3.
Is it normal for it to show such a low resistance to ground?

Just thought I'd check before proceeding further.

I haven't soldered the 3 coils, nor F7000, yet.
 
J

joshAU

Member
Ah, OK, that makes sense.

I resoldered the 3x coils and F7000, checked in diode mode - approx 0.400.

So fingers crossed, I applied power.
I now get the CPU getting hot, and the fan coming on, but no display.
The system appears on, as the touchpad clicks until I hold the power button, and then starts working when I press the power button again.

I tested on an external USBC monitor, with the internal display disconnected. and there is no display.
However, it does seem to detect a signal, as, if I turn off the Macbook, then press the power button on, the external monitor status light switches from orange to white, for a few seconds, then the display comes up reporting No signal, then turns back to orange.
When I turn the macbook completely off, the status light turns off completely on the monitor.

So, it does seem to be working.
The system seems to run on battery alone, and on AC adapter alone.
Power button works.
Touchpad click thingymajiggy works.

I tested PPBUS_G3H at F7000.
It now reads 13 Volts, with a 65 Watt AC adapter connected, the battery plugged in
It reads 12.5v with just the battery connected.

(The customers AC adapter must have died, as I get nothing from that, with the battery disconnected).
 
2informaticos

2informaticos

Administrator
Staff member
Did you try known good USB-C hub?
Test on both ports, both directions.

Any voltage at J8500?
3V3/5V_S0SW_LCD must appear at leats in the first seconds.

PRAM reset tried?
Caps Lock reacts?
 
J

joshAU

Member
Hi again 2informaticos.

Did you try known good USB-C hub?
No... but I shouldn't need one, should I?
I'm going directly from USB-C, to the USB-C on a Asus USB-C panel.
I've tested it from both USB-C ports, and it has the same symptoms... light on monitor changes from orange to white when I turn on the Macbook.
Then after a few seconds it displays "No Signal", then the white light goes back to orange.
With the panel unplugged, no light is displayed.

I tried the USB-C panel with a known good same model Macbook.
Its a bit silly how I have to actually log into OSX before it will display on the ext. panel... however, I put the good hard disk into this faulty Mac, booted up the amount of time it would take to get to the login screen, typed in the correct password, and the screen should then have come on... but didn't.
I assume with the internal screen disconnected, it should automatically go to the external panel though, without having to login.

I tested it multiple times and compared it to the behaviour of the known good Macbook.

I then disconnected both the hard disk, and the internal screen, from both systems, and tested with a linux boot disk, which went straight to the external monitor on the good Macbook, but doesn't display with this faulty Macbook.

Any voltage at J8500?
3V3/5V_S0SW_LCD must appear at leats in the first seconds.

Is this with the screen disconnected?
I tried, with the screen disconnected, then, while measuring it, plugged in the USB-C charger.
No change from zero volts on PP3V3_S0SW_LCD.
Also no change from zero volts on PP5V_S0SW_LCD either.

PRAM reset tried?
Tried PRAM reset & SMC reset.

Caps Lock reacts?
No. it doesn't do anything.

Could it be a bad solder job I may have done replacing U7420?
Its no longer shorted of course, and it looks visually flat, and well aligned, but wonder if I should try re-soldering it again?

Thanks so much for your help so far 2informaticos.
I was really hoping that it would just work after replacing U7420 and seeing the fan come on.
 
2informaticos

2informaticos

Administrator
Staff member
Overvoltage occured on CPU, caused by bad U7420.
CPU can be dead now, even no short there.

"Is this with the screen disconnected?"
Doesn't matter, you should catch voltages in the FIRST FEW seconds anyway.
If screen is correctly recognized (HPD signal appeared), PCH must maintain the voltages.

The board doesn't boot, if you cannot confirm USB activity.
No reason to continue searching for image/backlight in such case...
 
J

joshAU

Member
Hi again.
That doesn't sound great. :(

I plugged in a USB mouse, via a cheap Keweisi branded USB volt/amp meter, and I get a light on the mouse.
The voltage is 5.10, The amp draw is 0.29A.

The external screen, connected to either usb port, does light up the white LED to indicate it detects a signal, then turns amber after a few seconds.

Not sure if that means much, or anything?

The board doesn't boot, if you cannot confirm USB activity.
Does that mean any usb activity, or specifically video activity?

Would you suggest I call this a loss?
 
J

joshAU

Member
Tried with an external SSD adapter, that flashes when plugged in.
The light comes on for a second, then turns off and stays off.
Rechecked 3V3/5V_S0SW_LCD - both zero volts.
 
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