So I've swapped out the 30 year old Z80 for a brand new 10Mhz CMOS one. Which should mean I can drop the clock speed down as low as I like without problems. And I've put a UM232H on the board so that I can power it from the board instead of having the keep switching power supplies when I do any JTAG programming. (There's also and SD card, a real-time clock and two backup batteries on the board, but I'm not using them yet).

And it still doesn't quite work. Oh well, more playing tomorrow.