820-00165 PP5V_S5 and PP3V3_S5 fluctuating

[No-Clean]

Board was repaired a few months ago with SMC replaced and some corrosion clean up, now is back.

PPBUS 8.6V
I get green then orange light on original charger.
I can trace the fluctuations as early as SMC_PM_G2_EN this fluctuates around 1.1V to 1.8V.
Diode reading in all related rails are ok and no shorts found elsewhere on board.
Tried SMC bypass, clean BIOS, different SMC and new U7501 with no luck.
PP5V_S5 and PP3V3_S5 fluctuate around -1V from their normal values.
U7501 Pin 23 PPBUS_S5_HS_OTHER_ISNS is 8.6V

 

[2informaticos]

Continuous fluctuations, or you note them pulsing every 3-5 seconds?
Any pulse at big coils, ALL_SYS_PWRGD?

 

[No-Clean]

For example PP5V_S5 goes from 5V down to 2.3V in about 1 second then repeats, other mentioned rails do the same but with different voltage.

ALL_SYS_PWRGD is missing completely

 

[2informaticos]

Remove R7551 and short its pad 1 to ground.
Test again mentioned voltages.
If no changes, do the same for R7552; test again.

 

[No-Clean]

With R7551 pin shorted to grown I get same pulsing, with both R7551 and R7552 PP5V stays steady but board does not boot and 3V3_S5 goes missing.

 

[2informaticos]

If you remove R7551 and short its pad 1 to ground, 3V3_S5 is stable?
If yes, don't touch R7552.

 

[No-Clean]

With R5551 removed and its pin 1 shorted to ground 3V3_S5 and 5V_S5 is still pulsing same as before

 

[2informaticos]

Seems to have a problem with 3V3_S5, or 3V3_SUS.
Check diode mode to ground on both.
Be sure all components related with L/Q7560 and U7501 are good.

 

[No-Clean]

3V3_S5 and 3V3_SUS diode mode is on par with known good board.

Replaced L7560, Q7560 and C7524, C7564, C7518 and C7588 also measured all test points around U7501 as well as all resistors, all measure fine too, but after all this I still get same behavior.

 

[2informaticos]

Peak voltages repeats every 3-5s, or how?

 

[No-Clean]

As mentioned before for example PP5V_S5 goes from 5V down to 2.3V in about 1 second then repeats, other mentioned rails do the same but with different voltage, so basically the voltages dont restart they fluctuate up and down fast and continue to repeat constantly.

 

[2informaticos]

Keep 5V_S4 disabled on U7501 and remove U8020.
If still fluctuates, you have a problem with 3V3_S5 power supply.

 

[No-Clean]

Doing that makes 5V_S5 and 3V3_S5 stable

 

[2informaticos]

Then seems to have a problem with 3V3_SUS.
Check diode mode there.
If good, replace U8020 and try again.

Keep 5V_S4 disabled for the moment.
Enable it if you get stable 3V3_S5/SUS.

 

[No-Clean]

Thanks so much, there was a bad cap in PP3V3_S4 that was bringing the line down a bit, replaced it and done!