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820-00840-A not turning on

F

Fanfwe

Member
Hi,
I am practising with another 820-00840-A board that does not turn on.
There is no sign of liquid damage.

My charger gives 20V so I have PPBUS_G3H.
PP3V3_G3H gives me 3.41V

Then my S5 rails are all missing.
I figured that PP3V3_S5 reads 7 Ohms to ground. I suppose this means a short to ground. I know 7 Ohms could be fine for some rails such as CPU VCore, but I think this is no good for that PP3V3_S5 rail, right ?

Is there anything else I should check before I try to inject some voltage into that rail in order to find the short ?
 
2informaticos

2informaticos

Administrator
Staff member
That is very low value for 3V3_S5.
First lift one side of the coil and see where the short remains.
If is on the filtaer cap, then you can try voltage injection.
7 ohm may doesn't heat anything, as you cannot inject more than 4v there.
But may have luck and some chip enters into thermal avalanche.
Remove CPU heatsink before...
 
F

Fanfwe

Member
I already had the coil resoldered from before.
Pin 23 has 13 volts
Pin 12 has 0 volts
Pin 13 has 0 volts
Pin 22 has 3.3 volts
Pin 29 has 0 volts
Pin 21 has 0 volts

Looks like I'm missing the enable signals for the rails. I went on and checked the U7890 gate that controls PM_EN_PVXS5, it has power from PPBUS_G3H however its two inputs are low. One of them comes from the SSD connector, so it's probably normal as I do not have an SSD connected. The other one comes from the PMIC U7800.
Looks like that chip has two power inputs, one that comes from PP3V3_S5, but it can't be that one, since that chip must turn on the S5 rails, so...
The other power input that I see is PP3V3_LDO3V, and on that one I only get 1.8V, so something is probably wrong there I guess ? It reads 27 Ohms to ground.
 
2informaticos

2informaticos

Administrator
Staff member
That voltage is only used by PMIC itself.
Remove C7805/24 and check again diode mode.
If still low, change U7800...
 
F

Fanfwe

Member
Allright, the caps were not causing that issue.
It took me a while to get a new PMIC in order to replace U7800, but I finally got it yesterday and replaced the chip.
The board still doesn't boot, however I am now getting a lot more power rails.
In fact, all rails are present, except for PPVCCCPU_S0G and PPVCCGT_S0G. So all I am missing now, is essentially CPU Vcore.

The boards draws 240mA at 20 Volts for a little while, then down to 20mA for a few seconds, and then back to 240mA and stays there.
The CPU get just a little warm, not hot.

PPVCCCPU_S0G measures 7 Ohms to ground
PPVCCGT_S0G measures 4 Ohms to ground

I do not have a reference board to compare this to, so I'm not sure if this is right or wrong.
 
F

Fanfwe

Member
Yeah, PM_EN_CPUVCC is high, I forgot to mention that.
OK, so I will try to play with the BIOS.
I'll start dumping the existing one, just to be safe.

But then, what should I be doing ? Can I just pick a dump from another board just as is ? I guess this will have ME issues, but maybe it's still good enough to start with, in order to see if board will boot ? Then if that works I could look at preparing a clean image for this board. Right ?
 
F

Fanfwe

Member
Didn't manage to find a clean ME BIOS. I took a dump from a dead CPU donor board and flashed it on this board.
Same problem. No PPVCCCPU_S0G and no PPVCCGT_S0G. When plugging in the charger, PM_EN_CPUVCC goes high, then low, then high and remains high.
 
2informaticos

2informaticos

Administrator
Staff member
Something had to cause that problem on 3V3_S5 rail.
The low value disappeard, but possibly there is a chip with internal damage; that could be CPU.

VCCGT doesn't come on untill graphics load goes high.
 
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