Whodunit? The case of the missing +12valw

[clearchris]

Hey guys, I have been tracing out a failed laptop, but this is a general circuit question.

I have traced +12valw to a resistor. One side of the resistor I get 12v. The other side, nothing. With the power off measured in circuit, the resistor measures 90kohms (schematic says 100k).

Is it as simple as the resistor being bad? I'd have imagined the resistance to change if the resistor failed...

What do you guys think? Is there something on the other side pulling the voltage down? Could a short act this way?

Here's a pic of the relevant piece of the schematic with my annotations.

[fzabkar]

Re: Whodunit? The case of the missing +12valw

MOSFET Q33 is being turned on by the SUSP signal presumably because the laptop is in suspend mode, ie power saving. Measure the gate voltage (pin 2, G) of Q33 to be sure.

[clearchris]

Re: Whodunit? The case of the missing +12valw

So if MOSFET Q33 is on, you would expect zero voltage at my arrow?

If so, that's strange because suspend is actually on all the time in normal states. Here's the relevant portion from the Mobile Embedded Controller:



+5VS powers the PWM Controller that powers the CPU power.

[fzabkar]

Re: Whodunit? The case of the missing +12valw

The PWM controller in your diagram is an ISL6247:

https://cdn.badcaps-static.com/pdfs/...40861c3d30.pdf

The ISL6247 has a "deep sleep" mode which is activated by the DSEN# pin, and a "deeper sleep" mode which is activated by the DRSEN pin. I don't claim to understand your circuit, but clearly the +5VS supply is switched on and off by RUNON, which is in turn controlled by SUSP via Q33.

You need to determine what SUSP is doing and where it is coming from.