I think I sang victory too soon, because it seems I still have some small problems to solve.
After switching CPUs, my STacy boots up about 2 or 3 times out of 10 attempts. When it does boot it works fine, it loads the floppy, I can use games and utilities, I can even reset the system - it always restarts. The only problem is when I turn it off, in the sense that I don't know when it will want to start: in fact when it doesn't start it turns on and shows (like last time) a white screen (on a color monitor) or black with a monochrome monitor . I tried to use the diagnostic cartridge and it passes all tests, so the problem is just when you press the ON key immediately sending the CPU into HALT (0V oscilloscope signal, active low), while addresses and data are all high (exactly like the last time). I have two CPUs (I have a spare, just in case), and with both the situation is the same, so I think the CPU is good. Sometimes, with the diagnostic cartridge in, it won't even read it and act like it's not there, other times it will read it perfectly and run all the tests. Another thing: if I attach its original lcd display, it just doesn't start, not even the floppy starts reading (but perhaps this has nothing to do with the previous problem).
