Thanks, I think the eBay EPROMS are version 1.07. Which I already have installed in my S4 rack. This EPROM 1.07 does not upgrade the S4 to S4 Plus. I also own an S4 Plus (with version 2.00 EPROM) and just swapping EPROM did not work - there must be other chips needed. It would be good to know what was included in the kit. I note my S4 main board is marked S4 and the S4 Plus is marked S8, so maybe the S4 upgrade required a complete swap out of the main board and not just the EPROM and other chips. Maybe circuit board modifications were needed? (trace cuts, jumper wires?).
Questions on the S4 rack (1993), what is required to upgrade to the S4 Plus rack (1995)? 1) EPROM OS update, new CPU, 4 MB piano ROM expansion board? Other chips? 2) Does upgrade require a new mainboard (S4 Plus uses S8 mainboard)? Thanks, Mike