Will the MIDIJACK work for my Korg 800 DV (I think it has no gate / CV or any input “holes” at all)?
Yes, you can use the MIDIJACK II: Hertz so good! The lack of CV/gate jacks is no problem. The MIDIJACK II works with a lot of synths that don’t have an analog interface, such as the Moog Prodigy documented on this site under Analog User Modifications. The Korg 800 DV is not listed on the list of known compatible synths for a technical reason: It is not a monosynth, strictly speaking. The DV stands for dual voice, which in those days probably meant something. Now, you would be more likely to stack the oscillators into a super-fat unison mode and you could use the MIDIJACK II to run it as a MIDI analog monosynth. If you really wanted two voices, you could use two MIDIJACK IIs and set them to separate channels and use it as a two-voice multitimbral analog synth.