Certainly replace U975/U976 if there has been any visible corrosion around these shift registers. I've seen faults where the computer fails to boot because the MDAT line from U975 to Lisa/U450 is floating which causes U450 not to generate certain clocks and the system gets stuck in this weird half-reset state at power on.
If the RTC U178 has been corroded it can cause hangups on the system data bus and prevent booting. A fairly sure sign is if there's any heat at all detected with U178, which indicates it's defective. If in doubt, remove both U177 and U178, the computer will boot fine without them. U975 and U976 are required else the computer can fail to boot, or at best, the system software will fail to read the 'AGA capable' hardware flags as read from U976, so you'll only have ECS screenmodes available.
I've written various repair guides such as the ones
here and
here which might help further. Details on the booting process and "caps lock test"
here which is written for the A3000, but it's virtually identical in the A4000.
Failing that, I'll be resuming Amiga hardware repairs in November when I'm back in the real world. Won't be a problem to have a look then if you get stuck.
