In my last post I was suspecting the clocks.
Now I’m not so sure there is a problem with the clocks. Having traced them through everything looks fine. I’m not sure what I was doing wrong but the symptoms haven’t changed but I’m seeing bus activity now and also the _CS_ on the main 2708 EPROM is wiggling around as I suspect it should.
I’ll see if I can read the EPROM and see if it has bit-rot.
EDIT…. It turns out my programmer can’t read 2708s. Doh.