So next thing was to check the RAM which is often the problem on old machines.
This machine has four 2114 1024x4 SRAM chips giving it 2K of memory, but I had no way of testing these chips.
So I thought that it must be possible to create a simple tester with an Arduino, and so I did.
The source code has comments to explain what the different parts do, and how to connect the wires.