I've been using the Pentacon 50mm f1.8 with pleasure on my Canon 550D with the help of a M42 to EOS adapter.
Some weeks ago however the lens housing got damaged in my luggage such that turning the focus ring became difficult. After I applied some torque on the focussing ring (by hand, no tools used) the lens unscrewed. The lens now consists of two parts as shown below.
Screwing it back together seemed easy, but it proves to be quite hard. I have to align some notches and the aperture control lever before it can be screwed together. I've tried it now for a while, but either the notch doesn't align or the lever doesn't fit.
Is there some kind of trick in doing this? The lens cost me €10, so going to a repair shop is not worth it.
Provided you have more than average skills with fine mechanics, it can be done.
The lens focusing mechanism has a couple of guide rails, which are probably loose and must be reattached in order to have it working normally again. When the second of them came loose, focusing became hard, as the focusing rail probably would twist and not be aligned when turning the focusing ring.
To get things back into the envelope, you first need to disassemble it further. Then you can again attach the guide rails.
The stop for focusing movement must be removed to put the lens back in, when the guide bars are in place. Then the focusing helicoid is turned a bit past the close focusing mark, and the lens pushed in from front. Turning back the focusing helicoid will set the lens in its position. You might need more than one try, as you might put it into the wrong helicoids - it has several "threads".
When putting it in, you must make sure that the stop-down mechanism for the diaphragm also engages.
When you have got it into its correct position, the screw for stopping focus movement in the close end shall be put in place again, and the lens will then work normally if you haven't damaged anything or lost parts.