[Cobaltblue]

Has it been modified for use with CPM?

I built and repaired dozens of these back in the very late 1970's still have a couple.

If so its possible that the boot ROM has been disabled since CPM starts at address 0000 same as the standard Nascom operating system.

When working one of the tasks is to zero the video memory.

I did part time work for a local Nascom supplier and the local colledge students would build them and sometimes (often) they didnt work.

The first thing I did was to check that all the data lines and address lines could change state.

Worth checking all the address lines since the Z80 has a refresh register and is expecting to address dynamic memory so they should be changing state even when the processor is idling.

Incidentaly you can read the refresh register as its asynchronous to other processing functions can be used to create random numbers.

Cheers

Mike T