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820-00165 Pulsing, started after SMC replacement, PPBUS_G3H drop slightly

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phuketmymac

New member
Hi,

I have that board which had a small liquid damage near SMC (L5001), Bios (the prob points on the right of the Bios, opposite clock chip) and Thunderbolt circuitry (U3210)
I've cleaned the area, replaced the Thunderbolt chip and L5001 but wouldn't start.
I did then replace the SMC with one from a donor board and the board started... Yeah! Took a screen to check for the flashing folder... wouldn't start anymore.

Sometimes, the board will reach .1A and stay there, like it is stuck in an S3/S0 states but then next restart .02A, S5 rails only.
However one important thing to notice is that PPBUS_G3H is sometimes dropping to 8V or slightly lower.
I believe I could have a partial short somewhere but I did measure every rail in diode mode and the values are very similar.

I wanna win that board but it smells the rabbit hole to me as I've spent many hours already.

Any idea what could cause PPBUS_G3H to drop like that?
 
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phuketmymac

New member
I've checked PPVRTC_G3H and I am getting a stable voltage there.
The bios area had a very tiny liquid drop next to R6116, not even touching it. That's the probes point next to it which got corroded slightly.

Why do you believe U1900 could be any issue?
 
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phuketmymac

New member
On U1950, all signals come for about 200ms each however SMC_DELAYED_PWRGD goes to 3.3V for about 50ms.
Is that normal?

Also if system is pulsing, can I assume there are not short on rails or it is a possibility?
That PPBUS_G3H dropping to 6.5V is a lead for sure but what could cause this?!
 
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phuketmymac

New member
Resistances on U7100 measure fine. U7100 has already been replaced.

Doesn't start either if battery is plugged. PPBUS_G3H is 8.4V with battery and dropping to 7.2V for a shorter time.
Sounds like an S4, S3 or S0 rail if pulling my PPBUS down isn't it?

The only slight difference in voltage drop to ground are on:

- PP1V2_S3 with .125 instead of .225
- PP3V3_S4_TBTAPWR with .3 instead of .425
 
2informaticos

2informaticos

Administrator
Staff member
Are you surely using SMC from 00165 donor board?
Recently I soldered by error 3476 SMC into 3437 board.
It started and PPBUS gone to 1V after 3s...
 
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phuketmymac

New member
Yes I am sure it is 820-00165 SMC. It did start even the first time after replacing it but then nothing.
I am thinking replacing it again but since PPBUS is dropping slightly in voltage I believe the problem lies there.

CPU Vcore is pulsing as all the signals connected to U1950 but at the same time it is pulsing you can see PPBUS dropping in voltage so I guess it is related.

I will measure again 3V3_S5 and see if it is stable or not.
 
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dukefawks

Administrator
I have no idea, PPBUS being pulled down must be because of an overload condition or bad sensing resistors. Resistance to GND on CPU Vcore?
 
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phuketmymac

New member
Resistance to ground for Vcore is 15 ohms.
With a battery plugged in or the charger, same thing, something is pulling down PPBUS at some point when starting.
It can't be a bad sensing resistors since the same thing is happening if I am bypassing the ISL. I've plugged my PS on PPBUS_G3H at the fuse and I can see the voltage dropping of 1V too.
 
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phuketmymac

New member
Yeah but still, not being able to find out what is causing it.

I assume it has to be a partial short or transistor on an S4, S3 or S0 rail opening and creating the short when the board is turning on, right?
Since I am bypassing the ISL disconnecting the Fuse, it can't be the current sensing from the ISL which is telling the SMC to shutdown, correct?
 
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dukefawks

Administrator
Power up in SMC bypass mode. Hold power button plug in AC hold down for 5 more seconds and release. Without battery of course.
 
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