820-4924-A Cant turn on

[phist1987]

i have a unit of 820-4924-A Logic board,condition is water damage,repaired somewhere else before,some chip burnt (seeing by physical) which i replaced,U5000 has been touched by someone before (not sure if it still working),i did reball it already,also U7100 cause the charger fuse to spoil,which after i replace,it doesnt happen anymore.

Now when i plug in the charger and check,magsafe indicator show,sometimes green and sometimes orange,when its green,it will blink
U7100 pin 2 - jumping from 16.62v to 16.69v
U7100 pin 14 - jumping from 3.28v to 3.35v
F7140 both side - 0.00v
L7095 pin 2 - jumping from 3.401 to 3.402

i dont have dc power supply for me to inject into the PPBUS_G3H so a bit hard for me to check,but im wondering if there is something wrong with the SMC IC U5000

appreciate with any help,thanks

 

[dukefawks]

Resistance to GND on F7140?

 

[phist1987]

Resistance to GND on F7140?

Thanks duke for reply.
With charger plugged in,on resistance mode from GND to F7140 showing 0.4 to 0.5ohm.
Does it indicate something short to the ground somewhere?

Thanks
Nafies
Beginner Logic Board Repairer.

 

[dukefawks]

You do not measure resistance with power applied

 

[phist1987]

You do not measure resistance with power applied

oh yeah i got it....it shows 0ohm
ground to ground suppose to be 0ohm...so that mean that PPBUS_G3H is short to ground
how for me to check which part of it that are causing short to ground?

 

[dukefawks]

Really absolutely 0Ohm or 0.something. Also measure resistance form PPBUS to L7310.

 

[phist1987]

Really absolutely 0Ohm or 0.something. Also measure resistance form PPBUS to L7310.

sorry,its actually jumping slowly between 0.0 to 0.2 but because when connect both multimeter probe also its show something like that,jumping from 0.0ohm to 0.1ohm and max 0.2ohm. im using Sanwa CD800a.
when i check from PPBUS_G3H at R7005 pin 1 to L7310 both pin,its shows the same,jumping between 0.0ohm to 0.3ohm max then goes down again to 0.0ohm

 

[dukefawks]

Hmm, I'm afraid the CPU is fried. You can inject voltage on PPBUS and see what heats up. Start with 2V and go up but not higher than 13V of course. Should draw a lot of Amps so make sure your power supply can deliver 10A+. First suspect to heat up will be the CPU and if it does this board is trash.

 

[phist1987]

hehe...seems like i have to keep this board for quite some time...as im still looking for a good power supply as i don't own one....just started doing simple board repair without enough equipment (company doesn't want to give budget for logic board repair research,have to come out from my own pocket). by the way can u recommend any good power supply that will not cost me so much? probably below 200USD? also does the sanwa is good enough for me to do logic board repair? by the way i have a BGA reballing machine,is it possible to change the CPU if its really the CPU heat up when i inject the power?
by the way thanks so much for your effort of replying me,will definitely get the power supply and continue on this.

 

[dukefawks]

There are heaps of variable power supplies on Ebay for <$100. Multimeter will do, although I'm no fan of auto ranging ones. Multimeters are slow to start with, auto ranging makes it 10x worse.
Check prices on new CPUs, with the risk involved it is not worth it to me.