Mitsubishi- power switching chip / 733 Digital Audio G4

[FdB]

This thing will not power up. Light comes on then quickly goes off. I tried everything else short of testing the PSU and thought I'd pull the board and have a look. This chip located on the backside of the M'board (near the battery), looks pretty bad and I'm wondering if this may be the culprit. It's evident that this machine has been very neglected but thought I'd share what such neglect looks like and see if anyone knew the actual role this chip might play in the grand scheme of things. I did clean it up a bit and may tuner-clean it tomorrow and re-install to test, before questing a replacement motherboard.

And, what's the slime-ooze dielectric paste needed between the processor and heat sink?

Hope pics make it...

[MacOS Plus]

  Wow, what a mess!  Was that corrosion?  The photos are slightly out of focus so I can't tell properly.  What I can see though is it looks like there's actually a pin completely missing now near the bottom left of the chip in the second photo.  That certainly would be an indicator of heavy corrosion, and likely a death-blow if it wasn't already dead beforehand.

[FdB]

Pardon the bad late-night, hand-held, via iPhone close-up photographs. And yes, it was indeed corrosion.

A friend's machine that he simply set aside over 10 years ago when he thought his HD had failed. The circuit path from that "missing pin" travels up to what appears to be (only), a probe test point. I hope to get back to this project today and I've advised him of suitable replacement boards (Apple 820-1173-A), available on eBay.

And yes, the machine looked like it was stored outside with the case open, possibly in a barn. I should have photographed a series of "before images" to provide under the new topic of "How Not to Treat Your Mac" to possibly send many here shrieking from their own machines in definite horror. (Feeling a bit like Vincent Price there, shivering with delight, standing next to a working bug zapper.)