There are several web resources available online about this quite common project. I navigated all of them and used info from several. Basically I used this approach with some additional info on wiring from here. But mostly, I always double-checked every connection with the pin numbers stenciled on the board itself. This being a rare board, I didn't try to tweak the rules, so I got a proper USB-MIDI converter. It's a cheap $10 unit from aliexpress. My only keyboard has USB out, so I will have to control the DB50XG via computer or via my keyboard (but with a computer in between). It has triple power supply, +5, +12 and -12V.  I had to use some min load resistors, so the +/- 12V buck converter kicks in correctly.

I got sound at first try, sequencing The Entertainer midi file. Then I tried something more ambitious like Take Five, and the instrument mappings didn't sound convincing. Now I have to delve into the world of General-Midi and XG-Midi programming.

The big 4 sided IC close to the center heats a lot, maybe it's a good idea to glue a small heatsink on it.