[SOLVED]820-4924 no power at all,missing pp3v3_S5

[roy.ruan]

Having a 820-4924 board with no power. board looks perfectly good condition. no liquid spilled or previous repair attempts.
green light. PPBUSG3H , PP3V42 all present and normal.

missing pp3v3_S5, which has only 0.3-0.4v.

Checked U7501 pin23 vin=around 12v pin 29 and pin 22 =around 1v pin2=0v
pin12 en=3.4 v

replaced u7501, but same result. no direction now, what should i do?

thanks

 

[aprendiz]

Around??? please be more accurate on the info you give... SMC_PM_G2_EN is present??

 

[roy.ruan]

when i said around,it's pretty much like around 1v meaning 1.01-1.05,sth like that. pretty solid on the numbers
SMC_PM_G2_EN is there 3.3v-3.4v

 

[aprendiz]

SMC_PM_G2_EN = P5VS5_EN = Pin 2 on U7501.....

 

[roy.ruan]

...
pin2=v5sw=0v

pin12=smc_pm_g2_en=p5v_s5_en=3.3v....

pin12 is the en pin as I see here,it's 820-4924

 

[aprendiz]

sorry you are absolutely right, check L7520 and L7560 for short....

 

[roy.ruan]

i already checked. one is 300 sth, one is 100 sth, (diode mode ,resistance to ground) i have another same model machine here and compared. they all seem to be right.

 

[dukefawks]

VREF2, VREG3 and VREG5 present?

 

[roy.ruan]

Vref2=0v
vreg3=0.71v
vreg5=0.87v

 

[smiba]

Check if Pin 23 of U7501 (PPVIN_S5_HS_OTHER3V3_ISNS) is there.

If it is, replace U7501. Dead chip

 

[roy.ruan]

that was what i did the first place..

 

[smiba]

Are you sure the chip is well? Because if we have VIN(23) and EN(12) we have everything that is needed to get VREG2,VREG3 and VREG5 going. Unlikely that all of them are shorted

 

[roy.ruan]

my thought would be something in the output side pulling it down. common seen when pp3v3 5v having a shorted sub circuit. usually can be cap in the usb 5 circuit or something else.

that's why apredez told me to check l7520 and l7560, but both of them look fine.no shorts.

 

[smiba]

VREG3 only goes to C7503 and U7501 as a possible way to ground. There is no way this is being pulled down (Or not created) by anything other then U7501 itself as the cap never goes bad. Especially not in a way that all VREG's would be low

You just have a bad chip, honestly replace it.

 

[dukefawks]

Either U7501 is fucked or it is not receiving VIN at pin 23 or EN pin 12 is not present. Nothing else is needed to get the 3 LDOs to work. If the chip is getting hot there may be a short on one of the LDOs.

 

[roy.ruan]

LOL, problem found. not the chip. you guys are right .something to do with the LDO
VREG3 is shorted to ground.it's C703 that is bad. removed and replaced,fire up right away

 

[roy.ruan]

thanks guys, appreciate it. can be marked as solved.