820-3635 105mA current draw upon boot, no CPU voltage

djdblaster

New member
History: It had some minor issue on PP3V42 and then it was working good. Suddenly shut itself off, then was working day after and again shut off. Turn on, shut off. I found some solder balls around U3740, but they could be a result of previous technician work since I see that CPU, SMC and BIOS were reworked on. (CPU and SMC missing glue, BIOS probably replaced). However not looks burned on the board. It's actually pretty clean.

PP3V42, PPBUS, PP5V_S0, PP3V3_S0 present.

PPVCC_S0_CPU not present. Barely 0.008V.

DCPS shows around 105mA draw upon boot.

Seems like CPU is not being told to turn on.
 

Alan.L

Member
820-3662 board?? these boards have a common fault where "U7310/20/30" will stop working, you can try to replace all FDMF6808 with new FDMF6708.
 

djdblaster

New member
U1950:
ALL_SYS_PWRGD - 0.069V
SYSPWROK_R - 0V
PM_S0_PGOOD - 0V

Y1915: (don't have an oscilloscope)
PCH_CLK24M_XTALOUT_R - 0.34V
PCH_CLK34M_XTALIN - 0.42V
 

2informaticos

Administrator
Staff member
Strange to have S0 voltages without ALL_SYS_PWRGD signal.
Confirm if this is the case...
Check if all voltages involved in ALL_SYS_PWRGD generation are present.
You can start removing Q7950.
 

djdblaster

New member
Q8150 is now removed:
pin 1 - 3.2V (PP1V5_S0 comes here, should be 1.5V, right?)
pin 2 - 0V
pin 3 - 0V
pin 4 - 0.07V
pin 5 - 1.09V
pin 6 - 3.3V
pin 7 - 1.06V
pin 8 - 0V

Other resistors connected to ALL_SYS_PWRGD measure correctly.

PP1V5_S0 - 0.600 resistance (12.5kOhms)
 
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djdblaster

New member
I found U7870 pin 6 looked like it has some extra heat absorbed since there was a solder ball there. Removed U7870, laptop starts, draws around 550mA. Question mark on the screen, probably would boot into OSX if I plugged in external drive.

When I measure its' voltages:
pin 1 - 3.2V then drops to 2.9V.
pin 2 - NC
pin 3 - 3.3V
pin 4 - 3.3V
pin 5 - GND
pin 6 - 1.8V

When U7870 was installed, on pin 6 (PP1V8_S3) I had around 3V if I remember. Pin 1 (PP1V5_S0) still measures high since it is around 3V, instead of 1.5V?

U7870 is 1.5V S0 LDO. So it seems that there is a problem on the PP1V5_S0 rail.
 
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djdblaster

New member
Continuity test showed PP3V3_S0 connected to PP1V5_S0. Resistance to each other 13 Ohms. Since CPU was reworked... I guess it could only be connected under CPU because pins are next to each other?
 

2informaticos

Administrator
Staff member
Check for short between pins 1-4 of U7870.
If yes, remove U7870 and check again between its pads on the board.
If you still get 3V at pad 1, then you have a problem with PCH, or some balls molten under it...
 

djdblaster

New member
I have not idea. It boots, but I haven?t checked further.
PP1V5_S0 is responsible for some audio stuff as far as I can see.
Probably it should work.
 

djdblaster

New member
Unfortunately, still connected to each other after LIO flex was removed.

In order to make stuff work, I'd probably have to replace resistors with other values so that it brings down voltage on certain rails needed for Audio stuff?
 
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