Replacing the decoders on the Williams boards can be a little trick if not frustrating. The original part was a 7641 PROM. Don't bother looking for it. If by chance you do find it you probably won't be able to burn the image on it because only a few programmers can access the 7641. There is a sub, 74s474 which is a little easier to find and a few programmers can access the chip.
For Defender boards is pretty easy but for later game boards it's another story. I will explain the Defender way and then the later CPU boards for games like Robotron, Joust, Sinistar, Bubble......This page will show you a quick and easy way to resolve the Williams Video Decoder ROM replacement problem.
Defender CPU boards don't seem to mind what chip you use. You can use either 2716 or 2732 as either Decoder 2 or 3. Here are the steps to follow:
1) Burn the appropriate image file on to the chip.
2) Cut pins 18 and 19 short but keep enough to solder a jumper wire to them in the next step.
3) Run a jumper wire from pin 12 to 18 and 19. (This will ground pins 18 and 19)
4) Plug it into the CPU board and you're done.
For later games that use the Rev. B, C, or D CPU boards it's another story. I have not had to much luck making any EPROM work 100%. It seems that at first the chips works but after the game has been running for about 10 minutes it will sometimes reboot and give you bogies RAM error like 3-1-3. It will then work again until a few minutes later and reboot again. I found that when I use the 2732 Hitachi or Intel chips I have the best luck but still not 100%. The procedure is the same as the Defender decoder replacement but make sure you only use 2732 chips and if possible only Hitachi or Intel chips, the faster the better (just as a reminder,100us is faster then 400us (us stands for micro seconds)).
The best solution for the Rev. B, C and D boards are the original 7641 BPROM, these PROMs are almost extinct and if you do find them, they will cost you a hefty sum. An easier part to find is the 74s474. It is a sub for the 7641 and will work without any modifications.
That's it. If you need any help with the above procedure or have any questions please email me. If you need to purchase the parts they are $12 per chip.