820-3115 - Short on PPBUS_G3H ?

[kronos23]

Hi Guys.
Working on a 820-3115 with no power no green light. Liquid damaged in the U7000 and Q5480 area.
I replaced both of those components and surrounding caps/resistors.
Found a short on PPBUS_G3H that I tracked down to CHGR_PHASE on Q7035, when I removed it the short went away. I also replaced Q7030 & Q7080 for good measure.
Connected adapter and got green light but PPBUS_G3H was low 6.34V. It slowly went down to 0V and the short returned.
I replaced U7000 again to rule out bad placement. Same symptoms.
Here is what I am noticing now....if I test for a short at CHGR_PHASE on Q7035 the short shows for a few seconds then is gone. I also get these results if I test PPBUS_G3H on R5481 as well .It is like the short is "moving" from one end of the circuit to the other.
PPBUS_G3H shows a short then CHGR_PHASE doesn't and vice versa.
I am stumped. Any suggestions ?

 

[PhilipC]

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[dukefawks]

Uhm this is a PPBUS short and has nothing to do with the sponge cap.....

You need to replace U7000, Q7035/30 all together at once. But it does sound like the gate of Q7035/30 has no continuity to U7000 so you can "activate" the FET with your multimeter and cause a short because you enabled the FET by measuring on the gate. I suspect corroded trace to U7000.

 

[kronos23]

Found R7005 to be blown. Replaced it and the board powered on and then died with in 10 seconds. Green light faded away and PPBUS_G3H short returned until I removed Q7035 again. Something on the circuit is blowing Q7035 and creating a short but I cant figure out what it is.

 

[dukefawks]

I can only advice to check all traces and resistors around U7000. Traces to Q7030/35 are still suspect are traces to C7025. U7000 will probably be dead again and both FETs too.