820-3476 Board gets warm, no fan spinning, not booting up, Corrosion around U7501

[2informaticos]

"PPBUS_G3H is still showing 60 Ohms"
---
"showing the same resistances as PPBUS_G3H (which is showing 35Ohms)"
Can you clarify the things please...
Also post resistance between PPBUS_G3H and L7520 too.

 

[SamMac]

Okay my multimeter must've glitched on me, disregard that bit - "showing the same resistances as PPBUS_G3H (which is showing 35Ohms)"

The resistance values between PPBUS_G3H and L7520 is 60 Ohms

Taking a reading with PPBUS_G3H to ground is also showing 60 Ohms
When taking a resistance value from L7520 to ground it is showing 2 Ohms

 

[2informaticos]

So you have the sort to ground on L7520.
Only 2 ohms will allow you find the culprit injecting voltage there.
Be aware, do not overtake 5.5V and keep ampers below power lab supply limit.
Start with 2V, that should mean 2W dispation and increase until get a hot element.
4-5W should be easy detected with the finger...

Remember, do NOT power up the board before resolder U7501.

 

[SamMac]

should i remove L7520 before injecting the voltage or do it with the coil attached?

Many thanks for everything!

 

[2informaticos]

No matters L7520, just remove the burnt Q7520...

 

[SamMac]

Shorted chips Q7630 and U7600 removed both and the shorts where gone from everywhere and resistance values normalised.

Replaced the chips (including Q7520) and short still gone. (blessed), made sure everything was still good and then plugged it in.

PPBUS was showing 12.6 Volts for awhile then dropped back to 1.150V.

L7520 and the whole rail isnt showing short anymore to ground or when checked between the coil and PPBUS_G3H

But PPBUS_G3H is showing short to ground and also showing short with L7560 (PP3V3_S5)

 

[2informaticos]

Did you put back U7501?

 

[SamMac]

Yeah I did I put a new U7501, Q7520, Q7630 and U7600.

Many thanks.

 

[2informaticos]

I hope you didn't power up the board without U7501 in any moment.
That is a good reason short U7560.

As you was able to eliminate the low values on 5V rail, you can do the same on 3.3V.
Be aware, this time PCH, or SMC could be burnt...