spiny wrote:I know they haven't even been delivered yet, but how would the testkit work on a broken ST that doesn't show anything on screen ?
Depends. I suggest that you grab the appropriate schematics and service manual for your "patient" here http://dev-docs.atariforge.org/
You may then use a Nullmodem cable and attach it to another computer (PC/Atari, whatever with a RS-232 and a terminal program). If at least RAM and CPU/MFP are working then the test cart will send the menu over the serial line to the other computer and you can remote control the dead unit. If the Atari is dead then see the chapter "Troubleshooting a dead unit" in the service manual. But I assume that you need a scope then. I've been able to resurrect STs with that chapter and a scope (Multimeter is often too slow).
I have an ST here that either just shows a white screen, or graphics corruption - will the cart help in cases like these ?
in fact, and this may be a daft question, does the ST boot off the actual cart when it's connected, as in, does it bypass the STs hardware ?
From what you write I assume that the ST isn't completely dead so you may try the Nullmodem solution I described before.
I just recently had a look into Atari ST Intern 1. Here is a commented disassembly of (probably Disk based TOS 1.0) the Boot code. As far as I see it the TOS ROMs, DRAM and CPU need to work to start the cart's own code. CPU runs the code in TOS ROMs and inits IRQ vectors and system variables below 1K then checks for a diag cart and jumps there.