The BF/DF switch was blocked in DF position. Just remove the three bigger screw under and remove the block.
On this one the lubricant dried on the sliding rail for selecting mode, you need to remove it and clean it + add some new lubricant (I use WD-40 everytime but I’m not sure that’s a good idea here). Remove all the screw pointed in red.
After this cleaning everything should be good 🙂 On mine the slide works great and I don’t have switching problem any more.
On the Left you can see that the light goes only on the external ring of the Neo Objectiv serie and come back on the center. The Brighttfield path is a classic prism.
Since I’m thinking to fit a high power laser on the lighting system it gave me some idea, like fitting a laser diode in the darkfield path.