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Tried the new bios file. No change. It acts like the other new bios files. It has the LED flash on the keyboard (original one didn't) and has no chime (original one did chime once). It doesn't respond to PRAM reset. Neither bios has ever allowed a PRAM reset.
Are you 100% positively sure this board did not have a spill? Inspect CPU power supplies and regulator/controller IC very well, and all surrounding caps. Even if just a single drop or sign of residue, then that's enough. I am still not convinced this isn't a BIOS issue (one of your original reads could have been corrupt/bad, original ME damaged and none of the successive ones good/bootable with board, etc), but it could be a damaged CPU.
Tried multiple keyboards. Won't respond to PRAM reset. Tried USB flash drive. The light comes on and stays on. No activity, just a constant light. I've never seen that one before.
Can never be 100% sure, but there is no sign of a spill that I can find. I've combed over it exhaustively. Can't find anything that looks damaged in any way.
• Original bios: chimes once only. no usb light flash at all
• New bios chip: no chime. has usb light, but no actual activity
• Repaired bios with original ME and serial: no chime. has usb light, but no actual activity (I tried two different chips just to be sure.)