820-3209 partial short on PP3v3_S5 PPBus_G3H not boosting to 8.6V

[grmacs]

Couple of issues but focusing on PP3v3_S5 missing. I pulled L7260 and at pin 2, I get .255V drop in diode mode red to ground which I know is low compared to working board. After inspecting board there was some corrosion near C6201 and Q7800. Looks like there was liquid around J6950.

I tried injecting 3V at pin 2 of L7260 but given not full short nothing getting warm. I tried pulling J2550.

Any suggestions on tracking this down? :-/

 

[grmacs]

Tried pulling some things out to narrow it down... U7201, Q7800, U7930. Took a longshot being not a dead short and tried removing C7290/92/72 also with no change in voltage drop at pin 2 of L7260.

 

[2informaticos]

255 is not too low in diode mode to prevent starting 3V3_S5 power supply.
I wonder why did you waste time in such manner, before checking basic voltages/signals for U7201; as pins 23, 12, 13, 22, 29, 21.

 

[grmacs]

I thought I was headed down the right trail. Ok, I was missing enable voltages at U7201 and tracked down to the SMC. Replacing that from known good 3209 (bad ram) board brought the board back to life and now starts but then stops. PPBUS_G3H is at 8.4V.

SMC bypass keeps it running so I went through the VSense and ISense measurements. None of the VSense measure over 1V. Not really getting anything to register anything notable on the iSense lines. PLT RESET L is 3.3V during smc bypass, it comes on momentarily with normal power on.

 

[2informaticos]

"None of the VSense measure over 1V"
Do you think is normal???
SMC_DCIN_VSENSE should be aprox 2.4-3V, depending on charger type.
Also SMC_PBUS_VSENSE is aprox 2V normally...

Did you check AVREF_SMC voltage?

 

[grmacs]

I was able to get DCIN VSENSE to over 2V by replacing Q5310. Oddly that didn't keep the board running.
PBUS VSENSE is at .9V which matches working board.
AVREF SMC voltage is 3.3V. I referenced other VSENSE and ISENSE readings to working board and not seeing anything unusual.

The board still starts temporarily and stay on with SMC Bypass or will turn on momentarily triggering SMC ONOFF.

 

[2informaticos]

"PBUS VSENSE is at .9V"
You must check it when the board is on.
Should be 2.2V aprox...

 

[grmacs]

The board that isn't working reads .9V during SMC bypass at pin 2 of R5303. At pin 1 of R5303 is 8.2V which from what I can tell is simply PPBUS_G3H not boosted due to bypass? I get the same reading on a board that starts normally both appear that Q5300 is working properly. There is 3.3V at EN pin.
I've checked that R5303/4/1/2 all reading correctly. Diode check at pin 2 of R5303 red to ground is .7V.

On this board are you sure PBUS VSENSE is supposed to be 2.2V?

 

[grmacs]

Reheating the SMC and a tap seems to have gotten the board to boot. Fans are running high but hopefully on the way now to having a working board.
Went back to check and I do now have 1.5V at pin 2 of R5303 now. I can only assume that the board with what used to be intermittent RAM issue that I was using for reference has another issue. What are the chances of both having same bad reading...

Anyway thanks for the help. I'll report back if I can't finish this one up.