Just done a total respin of this, based around newer 40-pin SOJ ISSI DRAM chips - IS41C16257-35's and using SMD 1206 capacitors
This will give a total of 1MB (8Mbit) of CHIP RAM and shouldn't impinge on anything in the trapdoor slot - literally a 1:1 replacement of all the 256k x 4 DRAM chips. Gerbers are all updated as well.
Again - GPL v2 on all of my projects, to fork them, play with them, whatever you'd like. Remember to grab A500 Parts symbols, which have been updated, too.
https://github.com/kr239/A500-DRAM-Board
