Well, maybe not really a DIY project, but this synthesizer was available as a kit to build it yourself. The Maplin 5600/ETI 4600 was a synthesizer with deep editing capabilities and usage of digital technology.
Do you own such a synthesizer, have you built it in the past? Do you want to share experiences? Please let us know through a comment below, or via our contact page. The Maplin manual is available in 4 pdf files, which can be found below. And the ETI 4600 version is one pdf (lower).