@chiark
The keyboard Caps LED flash does not prove that the 68000 is running only that the Keyboard MPU has initialised.
3 suggestions
Swap X1 or use frequency analyser to ensure the 68000 is receiving it's clock, I also presume that you have tried swapping the 68000 (does it get hot?).
What rev is your Motherboard does it contain a Buster IC, If so check BBoAH as certain revisions req a Buster Tower to correct a bug in the Buster Logic which is not required for later revisions.
Contact issues on AGNUS can also cause a Black Screen
The Tick signal is also setup correctly.
Good luck my friend & let us know.
TC
