820-2532-A - Intermittent no power issue

[tlough75]

So on the bench I have an 820-2532 with a strange issue. It is usually drawing 40mA and not powering on, so I checked power rails and am missing PP5V_S3. I removed L7220 and injected 5v to mimmic the 5v rail coming from U7201 and then triggered SMC_ONOFF_L and the board started. I removed the wires and put L7220 back on, try it again and the board fired up. Well after the board cooled off, it would no longer start, so I back to square one. I took my heat gun and heated the areas again and lo and behold it started again. I'm beginning to suspect a bad cap.

 

[tlough75]

I believe this board is one of the ones prone to the classic C7771 failures. I'm getting 330ohms to GND but do not have another board for reference.

 

[piernov]

Yes replace C7771.

 

[tlough75]

Thank you piernov. I replaced C7771 and it seems to be working well. I let it cool for an hour and it still turns on. Just out of curiosity, are there any measurements that can be done to confirm this is a fix either way? I understand that the proof is the computer is now running, I was thinking about for the sake of being able to measure something.

 

[2informaticos]

You should check L7770 voltage before replaced it.
Bad C7771 usually causes 0.3V there.
That is 100% confirmation...

 

[tlough75]

Is that before triggering SMC_ONOFF_L? If so, I measured .33V

 

[2informaticos]

Then the board shouldn't have another problem; after changing C7771.