This is an old post, preserved for reference.
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As I have mentioned before, the Achilles heel of many vintage computers are custom chips. These PLA (in Commodore terminology) or ULA (in Sinclair terminology) chips were a way to reduce cost and size by combining the functionality of a number of logic chips into a single package. Sometimes doing simple address decoding (as in many Commodore machines) and also generating sound and video (as in the case of the ZX81 and ZX Spectrum, Acorn Electron etc.).
here (in German) The pinouts aren't exactly the same, so a few pins need to be redirected. I built a simple adapter for this.
I've has good results with EPROMs and a capacitor to alter the timing slightly, also a nicer PCB