I made this for some people on other forums, but I'll share it here too... A lot of people have issues using the SST 27SF512 EPROM chips with Willem burners, especially when it comes to erasing them. There may be a simpler way to do it, but here is what works for me and the particular Willem burner that I have...

http://www.turbols.net/TechInfo/27SF512.html

The 27SF256 chip and other 27SFxxx chips should be about the same, just probably with different DIP switch settings.

 

http://www.pgmfi.org/twiki/bin/view/Library/Willem
Links not working but heres another one also