I have a post on psx-place for this but thought I'd try here as well in case anyone has experience or the schematics or any kind of help. Here's the original post from psx-place:
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Hi, so I bought a YLOD Fat PS3 as I wanted a repair project and thought it would be a good way to get experience. I was able to solder to the RX, TX and Diag pads on the COK-002 motherboard and was able to get external access and retrieved the codes. I haven't powered it on beforehand so these should be the last ones. I did try to get internal access immediately after but kept getting scopen response...