If you can access the memory at location using a programming language(stos, gfa etc...) to read the address $ffff8001 (byte length) this will tell you what the computer thinks is installed and in what configuration. By doing this you can then focus your efforts to the area in question instead of going in blind.
My first suggestion is to extract the MMU and re insert it. Making sure that all the pins are in contact.
The next part will be the resistors which can be tested using an omh meter with the machine switched OFF.
Let me know what version of ST you have (the revision letter printed on the motherboard) and I'll try and guide you through it step by step. I have access to quite a few revisions.
I also need to know if the computer has had any upgrades installed or any removed that you know of.
