If you are using Screwmount Leica cameras, and want to use focal lengths other than 50mm, sooner or later an external viewfinder will become part of your toolcase.
You have the choice between fixed focal length viewfinders and those that allow to select multiple focal lengths. Out of those, the Leitz models VIDOM and VIHOO are probably the most common ones. The VIDOM as the earliest incarnation has the disadvantage of a vertically inversed picture which can puzzle your mind (except if you are used to waist level finders…)
A huge improvement is the VIOOH, which, usining a prisma, shows the image correctly.
Unfortunately, not only your camera, but also these finders are many decades old and have been thoroughly used. So, they are in need of some cleaning. Also, a common fault is the chipping of the prism, which manifests itself like this:
If you are lucky, it is like here just on the outer edge and not entering the image frame. It is is just a nusiance and not really a problem for normal use. Still you can improve the symptom, more on that later.
The first step is opening the shell of the viewfinder. This is easily done by removing the four black screws on the back. You can now take off the the backplate with the ocular lens.
Once open, remove the two screws on the right hand side to take out the prism block.
DO NOT remove the two small screws on the prism assembly, as they are used for adjusting the prisms, and you will have a very hard time getting them readjusted (speaking of personal pain here…)
Take care when handling the prism, as it is somewhat fragile!
Now, the interior should be empty and you should have a good view on the inner lens of the optical system of the viewfinder. Usually, there is no haze inside the viewfinder optics, just dirt on the front lens and the inner lens. Use a rocket blower to get rid of any loose dirt that has accumulated in the past half-century. After that, give the lens a thourough cleaning with isopropyl alcohol and a cotton swab.
Now clean all surfaces of the prism block. Any dirt on any glass surface of the prism will show and deteriorate your viewfinder image. I would highly recommend to wear cotton gloves when handling the prism, as it is small in surface and every fingerprint will show up later. Also note, that the prism has NO silvered surfaces, so you can also use an ultrasonic bath to clean it.
Don’t be shocked if some surfaces have turned dark and light transmission is limited:
This is not because of de-silvering, but because of the reflective change when liquid is in contact with a glass surface. Simply give it enough time to dry thorougly!
Dont worry, it will dry up and just be fine.
If you have a chipped prism, you can cure some of the symptoms by painting the broken surface with black paint. Easiest is using an edding or sharpie for that. Just paint the chipped surface black and take pecial care of all edges and borders. This is to suppress additional reflections.
If you got too much ink on the prism, it is easily removed with a cotton swab and some alcohol. Don’t forget to polish the surface clean again.
Carefully put the prism block back into the housing. When you have cleaned the ocular lens, reassemble everything.
Looking through your viewfinder, the image should be bright and clean now!
As a bonus, here a scan of an old brochure (in German only, sorry!)