I'm using the full game power board to boot it, but no playfield load etc.

The TOP video (for BH) and Clay's guide show them being booted on a bench (similar setup to mine) and all documentation states that the IRQ (even with only CPU powered) should stay high for 5 sec, then drop once game boots (around 3.5v I think).

Link here:

http://www.pinrepair.com/sys80/bhcpu.gif