I don't normally post repair tips here any more due to the actions of one particular member, but I'll make an exception. If you contact me by e-mail, I can talk you through the entire machine.
Not all the boards have to be working, but most of them do :-(. And the ones that don't _have_ to work (like the cassette controller, expansion ROM backplane, etc) could fail in such a way that they prevent the machine from working.
The fact that you have 2 RAM boards (in the slots in front of the PSU) doesn't mean you don't have a RAM fault. The 2 boards are assigned fixed addresses (depending on which slot they're in) and
there has to be RAM at some particular addresses for things like
the system stack. So if that board has a problem, the machine won't work. Alas I am working from memory, and can't remember which slot has to contain a working RAM board.
So what I normally do is firstly remove all the 'unnecessary' boards. These are the 4 boards for the tape controller (Blue handles), the tape drive itself, the keyboard connector (in front of the display, but leave the mains cable to the on/off switch connected), the I/O backplane, the expansion ROM module backplane, and any internal expansion ROMS (but leave the black/red handled PCB that contains the last bit of BASIC in place).
Still no display? I then check the display strobe signal (see the scheamtics). It's almost always missing, but you'd feel a right idiot if you spent time debugging the processor section only to find the processor was sending data to the display and the fault was in the latter section.
Then there are several things to try. If you power up the CPU clock PCB (brown/red handles)on its own (either in the machine with all the other boards removed, or using a 5V bench supply), the BitClk and muClk signals will run. 16 bit clocks, then a muclock (microcode clock), and so on. Check this with a 'scope or logic analyser.
Since it's a bit serial machine, the M (memory address) and T (memory data) registers should have all bits changing. With the 'essential' boards back in the machine, check these with a 'scope or logic analyser.
Then it gets harder. I connect a logic analyser to the microcode address lines on the CPU control PCB (brown/orange handles) and grab the microcode listings (ask me nicely by e-mail :-)). Then I see just what the CPU is doing and go from there.