a2159 820-01598 power rails rapidly cycling

[kjchatman1]

this board had a shorted u6903. it was pulling to ground both 3v3_g3h_rtc and ppbus. i replaced it but now the rails are quickly cycling.

 

[2informaticos]

Remove R6934/35 and test again.

 

[kjchatman1]

with those resistors removed the rail is 0v but pp3v3_g3h_rtc_reg_r is stable at 3.3v and ppbus is stable at 12.37v

 

[2informaticos]

What is diode mode to ground reading at pin 1 of these resistors?

 

[kjchatman1]

it is 0.454 on both

 

[2informaticos]

Check diode mode to ground on all coils around U7800.
Also check all MLB's big coils.
Compare readings with good board.

 

[kjchatman1]

diode mode to ground on pp3v3_g3h is 0.228. I injected voltage and u3900 gets warm. pp3v3_g3h has 0.0000 ohms resistance to pin 1 of L7810. i've reattached u7800 and nothing changed

 

[2informaticos]

What means "reattached"?
Did you change U7800?

 

[kjchatman1]

once i realized that 3v3_g3h had a direct path to L7810 i reballed it twice to make sure that it wasn't a solder issue

 

[2informaticos]

How can a massage helps to a wooden foot???

CHANGE U7800...

 

[kjchatman1]

i've changed it with a known good one and now pp3v3_gh_pmu_vddmain is a steady 3.3v but none of the coils have a voltage reading that is steady. i'm seeing pulsing on the coils. the resistance to ground on all of them seems reasonable

 

[2informaticos]

Do you mean the big MLB's coils, or U7800's coils?

Test if can enter into DFU...

 

[kjchatman1]

the coils surrounding u7800 have no steady value. they're 0v but they pulse every few seconds

 

[2informaticos]

Both USB-C ports have the same behavior?

Be sure U7800 is properly soldered.

 

[kjchatman1]

yes both ports have the same behavior i've attached u7800 a few times to be sure

 

[2informaticos]

Then T2 is damaged.
Has been overvolted through L7810.

 

[kjchatman1]

i have another one of these boards but someone tore the board when removing the heatshield of the u3900 and u7800 and so now it get no s5 rails. PMU_ONOFF_L is 3.3v. where is 3v3_s5 created on this board?

 

[2informaticos]

The equivalent of 3V3_S5 from old models is 3V3_G3H on this board, page 68.

Actual 3V3_S5 on this model, is a much low power rail.
It is generated by U7800, page 71.