Hey guys, I've replaced the PROM with the new programmed one but all I get is flashing green light when I turn it back on. Same thing happens with the old PROM now. Any ideas what might be happening? The socket hasn't been fitter properly maybe?
Almost certainly you've got an open or shorted track that's happened during the socket installation. As you've found, it's not easy. You can trace most of the lines, but not all of them.
First step don't panic you haven't wrecked it yet as mentioned check for obvious shorts or even a pin from the socket not soldered to the pad (the corner ones are harder to see) add a little more no clean flux if you have it - it will help any solder bridges to separate and make for nice shiny solder connections onto the pads.
If there is quite a bit of solder on the pads you might have to remove excess with copper solder wick - a very clean tip regularly re-cleaned as it starts to oxidize, a very lightly tinned tip (less is more) use the tinning from the solder tip against the solder pad don't apply solder directly make sure there is a healthy amount of flux - afterwards clean with isopropyl alcohol though you can test if its fixed without cleaning it. If it still has a flashing light afterwards you might have a solder bridge under the socket some additional flux with heat might make it naturally separate and you could possibly get at it with a few strands from the solder wick but worst case you will have to remove the socket again.
If you have to remove the socket I recommend sacrificing it and cutting it into four parts to make it easier to desolder less likely to cause damage to the board this way. If you do cut the socket apart watch the force as it splits apart that it doesn't rip anything off I would hold the socket with one hand either side of where your cutting.