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820-2936 PPBUS at 1.4V

bjf

bjf

Member
I'm getting killed by this board. It has 1.4V on PPBUS and will work on battery. It had a bad trace on pin 20 of U7000. Other than that nothing looked damaged and the board is very clean. It has Magsafe light, 3V42, ACOK. I replaced U7000 twice. No change either time. I replaced all 4 sense resistors and R7020 and R7050. All traces are good in those circuits. All components around U7000 are good. All pins on U7000 are measuring normal voltage-drop to ground. If I plug in a battery and then plug in the adapter it will stay turn on. But without the battery, it stays at 1.4V. This isn't adding up for me. Any idea what I'm missing?
 
A

aprendiz

Moderator
Obviously the problem is on ISL side...

What is the voltage on pin 2?
Resistance betwenn pin 17/18 and 27/28?
 
bjf

bjf

Member
Yes, it's the ISL circuit, but everything's there. I can't find anything off so far. It's baffling. This should be straightforward. Pin 2 is 16.3V. 17/18 is normal at around 2.5Ω and 27/28 is at 20Ω. They measured that before I changed the resistors, but I changed them just to make sure there wasn't something weird happening. No change after that.
 
bjf

bjf

Member
Normal, it's somewhere in the 2MΩ range. I don't have the patience to wait for it to settle. Voltage drop to ground is normal at .440. I've covered all the obvious stuff, something else is happening here.
 
bjf

bjf

Member
I tried that too. It doesn't. It stays stable at 1.4V. And I already replaced both FETs. No change. I feel like I've done everything, which is why this one has me so baffled.
 
D

dukefawks

Administrator
Must be another corroded trace then. Check the traces from U7000 pins 21/23/24 traces to C7025.

Is 16V present on R7020 even?
 
bjf

bjf

Member
Those 3 traces are intact. 16V on R7020. CHGR_ICOMP (.2V) and CHGR_VCOMP (.6V) are both low and are bouncing around a little. CHGR_BGATE and CHGR_UGATE are both at 1.5v. CHGR_LGATE is at 0V. I'm not sure if that's telling me anything.
 
D

dukefawks

Administrator
I am out of ideas as for a simple fix. It is now down to checking each and every trace to/from U7000 and the components they need to go. Also advisable to replace pretty much everything in the area including the 6259 again.

But before you do all this test if the board power on from a charged battery, U7000 can be missing and it should still power from battery.
 
D

dukefawks

Administrator
Ah ok. Well then there is no other way as to wipe and replace U7000 area and check all traces, it is in the U7000 circuit somewhere......
 
Bouba148

Bouba148

New member
Hi BJF, DUKE,
I had the exact same problem ( ppbus at 1.3V and booting on battery )
Solved by redoing the soldering job at U7000, make sure all pins are properly soldered and are in contact with the motherboard.
Hope that will help.
 
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