After a request from one of our members, I'm posting this brief "how to" on accessing the diaphragm mechanism of the Industar-50 lens. It's not intended as a complete servicing tutorial, but should serve as a useful starting point for those who need to service or clean the diaphragm.
First, we need to separate the optical and focusing blocks. On the rear of the lens, there's a retaining ring with two holes.
One or both of these holes should have a flat-headed set screw in it (sometimes, they're missing due to previous service attempts). Remove them as necessary, then use a pointed-end lens wrench to unscrew the ring. The optical and focusing blocks can now be separated.
The optical block has two main parts - the tube containing the optical elements and diaphragm, and a front section which includes the diaphragm control ring. The front section is threaded onto the optical tube and held in place by a set screw (this set screw is also one of two that keeps the diaphragm blade "carousel" in place).
Note the size of the gap between the two tube sections for reference during reassembly. Then, remove the set screw. The front and optical tubes can now be separated by unscrewing them.
Now, remove the front optical groups from the optical tube (unscrew the outer ring, and the groups can be removed in one block).
The diaphragm "carousel" is now visible. The red arrow above points to the carousel, which is holding the blades in place. Before it can be removed, another set screw must also be removed.
And now, the diaphragm "carousel" and aperture blades can be removed for cleaning and servicing...
Apologies for the brevity of this short article - it was created in haste specifically to help another member, but I felt it may be useful to others here
Please feel free to PM me if you have any queries!