Im currently working on a Time Pilot PCB and tested out Shoestrings diagnostic ROM.
This board is the Atari version which wasnt supported at the time but has now been updated.
I also ran into a problem where nothing appeared on the screen. Working together we eventually found the issue.
The original game code makes a write of 01 to address $c308. Using the Fluke 9010 I found that this write actually initialises the screen output.
MAME doesnt have any support for this so when testing the ROM in MAME it works fine. The test ROM has been updated with this now too and should work fine.
This highlights the need to test things on REAL hardware.
Thanks to Shoestring for making the diagnostic ROM and for putting up with all my questions.