00840 with 1.8V on PP5V_S5

[captainmac]

Need some help on this one.
This one had extensive liquid damage around the usual CD3215 and U7650 area, but also around U7100 and U7800.
Initially, the USBC chips weren't good so swapped one of them from one from a donor, this made it ask for 20V again.
I've replaced U7800 because it was shorting PP3V3_LDO3V to ground (and the chip was getting hot). Have replaced it with one from exact same donor board, and short is gone.

I tried seeing in what state the board is now, but i'm not getting in S5. My 5v_S5 is measuring 1.8V. Anyone know how this is possible? There is no short there.

 

[2informaticos]

Do you get PM_EN_P5VS5?

 

[captainmac]

I get nothing on pin 12 of U7650. that's what you mean right?

 

[captainmac]

all resistors for that enable are fine, so i guess u7800 isnt outputting the enable?

 

[2informaticos]

Check basic signals and voltages for U7800.

 

[captainmac]

checked if its getting its VDDs, but pp3v3_s5 is shorted to ground (0.8ohms)

 

[2informaticos]

That is low enough to heat something.
Start injecting 1V and go high slowly, no more than 4V.
Let PSU current limit at max...

 

[captainmac]

Yes i was injecting up to 2V @ 0.9Amps, but can't find anything getting hot, which is strange.. i'll keep trying tomorrow and report back.

 

[2informaticos]

2V divided by 0.8 ohm means 2.5A.
Discounting multimeter probes resistance, you should have 0.5 ohm aprox in fact.
That means closer to 4A even.
I'm very very curious how did you get injecting ONLY 0.9A with 2V in 0.5-0.8 ohm.
We may need to re-write Ohm law in such case...

 

[captainmac]

Yeah you’re right so I guess my resistance to ground reading is a little off. Will check tomorrow.

 

[2informaticos]

Remember to let PSU current limit at max...

 

[captainmac]

doublechecked everything, not trying to rewrite nature's laws, but:
The short to ground measures at 0.76Ohms (my Brymen MM leads have 0.14Ohms resistance).
Still when i inject voltage (from a Riden PSU with 6A max) at 2V the board's taking 1.006A..

regardless of what 's causing this, something should be heating up at 1A but cant find any hot spots.

 

[2informaticos]

2V x 1A = 2W, not easy to find what heats with so low power.
You should get more than 4A going close to 4V; maybe 3.5V will be enough.
Just a simple formula, supposing 3A for 3V, that means 9W; should detect the heat for sure.

Anyway, remove CPU heatsink...

 

[captainmac]

ok so i am putting 3.8V into this thing and the only thing heating up is the PCH.... shit outta luck i guess..

 

[2informaticos]

Looks like case closed...

 

[captainmac]

yup.. thanks again mate!