Well I have had some success :-D
I re-partitioned the hard drive with SFS and re-copied the OS4 files, but unfortunately it still wouldn't display anything on the VGA monitor. I started thinking that the BVision must be dead, so I removed it. When I booted again, sure enough there was the OS4 installer on an AGA screenmode!
Then I noticed some bare wires coming from the A1200 motherboard, right underneath where the BVision would have been. They were connected to the floppy connector, and my heart sank when I thought of 5v going somewhere they shouldn't. I removed the cable and re-connected the BVision, booted up, and up popped the OS4 installer on my VGA monitor!
:banana: :banana: :banana: :banana: :banana:
:pint: :pint: :pint: :pint: :pint:
Can I assume at this point that the PPC definitely works?
I clicked through the installer (actually just Locale Prefs...) and it seemed very slow, and after the Locale Prefs had closed I was left with a blank Workbench Screen. This might be a weirdness due to the installation files not being located on the CD, so if someone can confirm that the presence of Workbench at all means that the PPC is working then I'll invest some money in this machine and get a 44->40->40 way IDE cable.
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moto