aramis,

You are interested about information on internal design of arrangers (and workstations, which are different kind of keyboards), and all manufacturers consider these informations confidential. So there is no way of acquiring these type of information unless You apply for a job in R&D department of some keyboard manufacturer.

Of course, You can see some simplified schematics in the manuals, but nothing more, although if You have experience with electronics it's quite easy to understand the operating principles (any keyboard operate quite similar to any other computer, have CPU, ROM, RAM, DSP or other custom chips, etc).