The device is shown as:
1a86:752d QinHeng Electronics CH345 MIDI adapter
The infamous cheap MIDI interface! [MP4]
Cheap bakelite PCB. Nothing to see here. Apart of the unconnected loose cables shields braids menacing to make shorts with the circuit!
CJS1039B-MD01C ... No crystal for USB connection? and... Wait! There is also no opto on the input?
Hahaha! What they even tried to do?
For reference, this is the standard MIDI wiring diagram (found on Wikipedia)
This was before opening the enclosure. It should at least read itself when 4 is connected to 4 and 5 connected to 5, but impossible to make it work even lines crossed, unless at some point I tried to invert the signal too...
Allez hop, tout le monde dégage !
Optocoupler added on top, secured with some epoxy
Added a 220Ω resistor, and I keept R1 (4.7kΩ) as the 6N137 is having a open collector output.