You have isolated the CPU board by removing all the ribbon cables and the blanking led stays stuck on. At this point, there is no point checking anything else. The issue is within that board and that board alone. Perhaps there was some damage when you desoldered and replaced your old components.

If it were me, I would probably just buy a new CPU board.
Mad has one nos: http://mad-amusements.com/product.php?id_product=1821 but you need your CPU and security chip. Pinled makes a nice replacement.

Sorry of this wasn't helpful. But if you're already offering $50 may as well pay a bit more for a new board.