Wow, that's some serious work you have done with this motherboard repair!
I am no expert or anything close to it but just out of curiosity, is it anyhow possible to system hang on bootup if Buster chip went busted? I mean some of its pins are connected to CPU slot afterall, which theretically can mess it up? It's funny that's only chip on your motherboard that still remains soldered without socket.. Which is just opposite of mine Rev.B which had Buster rev.9 in the socket, and I just pop it out and rev11 went in.. All other chips aren't socketed.
Offtopic, I'm struggling to get my RTC back to life..
https://forum.amiga.org/index.php?topic=74685.0I wish you good luck with your A4000 "adventure"!
