Flektogon 50mm f4 aperture opens but doesn't close

[Yobo57]

Recently got a Flektogon 50mm f4.
It seemed to be stuck open on arrival. Opening the back, I can get the blades closed to f22, but upon reopening the aperture they remain fully open.
Is this a mechanical problem or do the blades just need a cleaning?

Here is a video

If it’s just a cleaning, what is the best solution to use, 99% alcohol?
Also, what is the best way to access the blades, removing rear element? Any repair manuals for this lens?
Thank you

 

[blee1996]

This is a common problem with Pentacon Six lenses, that lubricant migrated to the aperture blades which made them stick together. Out of 5 Pentacon lenses I bought, 3 need service. Including one Flektogon 50/4.

Initially I just removed both front and back lens groups (quite easy), and wipe the blades as clean as possible using lighter fluid. This is the quick and dirty solution. It worked for a couple of days or weeks, and soon the blades are struck again.

Eventually I have to do a full disassembly, taking out all the aperture blades and associated mechanism, and thoroughly clean everything. And they are still working properly today. However, you need lots of patience and time to do the full service yourself. It is not super difficult if you had some experience and the right tools. The lens construction is not complicated if you take lots of photos along the way, but putting the aperture blades back together can be quite frustrating.

 

[Alexander6x6]

Once you have fixed it, it is essential to always keep the lens with the closed (f22) aperture.