kanecvr wrote:Hmmm - do you know exactly what traces need to be cut? I've done some googling and came up with nothing.. some pics or a schematic would help greatly.
There's a schematic at http://www.retroisle.com/amstrad/pcs/Original … _Agujereado.pdf . Page 31 shows the VDU chip, IC126 labelled 40041.
kanecvr wrote:I don't suppose cutting power to the chip would be enough?
I wouldn't recommend doing it that way - for one thing, an unpowered chip might give spurious outputs and/or cause excessive load on inputs. For another, it looks like the 40041 also decodes the addresses for the RTC/NVR, so that would also get broken doing it that way.
I think the best way would be to disconnect the BA9 and BA19 address lines (pins 4 and 75 respectively) on this chip. These pins should instead be grounded (connected to pin 64, 29 or 8, which should all be connected together already). Don't ground the other side of the cut - the one that goes off into the rest of the machine, or nothing will work and magic smoke might even escape.
With zeroes on BA9 and BA19, the VDU should not recognize accesses to ports 0x3dX and memory 0xbXXXX, so your VGA card won't conflict. It will have the side effect of mapping the RTC at ports 0x70 and 0x71 also to ports 0x270 and 0x271. However, that only thing that Ralf Brown's Interrupt List mentions for those ports is the Gravis Ultrasound, and that can be configured to use different ports.
