The disassembly of Nikon's Nikkormat camera is moderately difficult, not something I would recommend as your first camera repair job. To take off the top is easy enough, but the viewfinder is a little bit tricky due to the shutter speed readout, as we will see below. So, why would you bother? After all the cleanliness of the viewfinder has absolute no effect on the quality of the photographic image. All I can say is that dirt in the viewfinder simply annoys me and I know I am not alone with this. A couple of specs of dust is tolerable, but in old cameras there can be fungus, too, and bits of sticky foam from the disintegrating mirror damper. Furthermore, if you have a camera in which the shutter speed readout in the viewfinder does not agree with the setting on the shutter speed dial, then fixing it will definitely require the disassembly of the viewfinder.
So, let 's begin with taking off the camera's top.
I usually start with the areas I am not familiar with because if I can't figure out how to undo something then I might have to abandon the whole exercise. If this happens (and it has on occasion), then at least I haven't wasted time with removing and putting back things. In this particular case the unscrewing of the cover disk on top of the film advance lever is a good place to start. Since there are no slots or holes to fit a tool into, the only way to do this is by using friction. A piece of rubber should to the job. There is another disk underneath with two holes in it and to unscrew it you need a spanner wrench tool. Once that's done, take out the washer and lift off the film advance lever.
Next, remove a small screw from the side of the camera. The film rewind button screws out anticlockwise, as usual. Note, that with many Nikon cameras there isn't a separate screw that holds the fold-out arm in place, so when you take off the knob, the whole assembly falls apart. (This is not an issue with the Nikomat, but I was caught out when dismantling a Nikon FE2; the rewind shaft fell into the film compartment and the back door was locked. It took a while until I managed to open it. The Nikomat's back door, however, opens with a different mechanism, which I much prefer, by the way.) There is another retaining ring to unscrew here for which you also need to use your spanner wrench tool (or equivalent).
One last thing remains to be done; the removal of the name plate. It is held by two screws. Don't loose the copper spacers underneath. The above picture shows the camera with the aforementioned parts already taken off and the top is now about to be lifted. Do it carefully - there are a couple of wires to watch out for as seen on the next picture. These are a source of some trouble. Leave them in place and the attached top will constantly get in your way when performing the remaining tasks. Cut them, and you'll have a soldering task on your hand when you re-assemble the camera. (But this is only an issue if you are not comfortable with the soldering iron.) Note that the elaborate mechanism under the name plate is not part of the top cover, so it will stay in place.
So far this was easy sailing. Now comes the interesting bit. The small circuit board is fixed to the viewfinder eyepiece with a single screw. The viewfinder eyepiece can be detached after undoing a screw on each side. The two light meter sensors are also here. Leave the whole works dangling on wires and carefully scrape out the old foam seal you find under the viewfinder eyepiece. By now the foam would have lost its elasticity and would easily crumble making a mess, so it is best to replace it.
To take out the prism, first remove the two small plates on the two sides near the front. Then unhook the two springs on the top and lift out the prism. Appreciate the quality of Nikon workmanship while you perform these tasks. Isn't this camera nicely built? I think it's marvellous. If there is a camera that deserves the title "built like a tank", than the Nikomat FTN surely does. A lot of metal went into it - that you can also tell by the weight - and it is evident that Nikon used high quality materials throughout.
You will notice that to dig deeper in the viewfinder the light meter unit has to be moved out of the way. It is a self contained unit held by only two screws (the shiny ones), but the rewind shaft's column is in the way, so that has to be taken out, too. Grab you spanner wrench tool and unscrew the tube, in which the rewind shaft is located, by the two notches in its collar. Then carefully guide the light meter's indicator needle out from the gap at the side of the viewfinder.
Now comes the removal of the delicate parts. First out comes a metal frame that holds down the condenser lens, then the condenser lens itself. Below this there is a thin metal frame which has a little window in the middle of its rear side through which you can see the shutter speed currently set on the camera. (When I got this far I was wondering whether the shutter speed scale is sandwiched between the condenser lens and the Fresnel lens, or am I only seeing a projected image. Unfortunately, the first version was more likely, and this indeed turned out to be the case.) Carefully lift out the thin metal frame and observe the shutter speed scale - a delicate plastic strip on a black string. Guide the focusing screen out from under this string. You will see how unusually thick it is and that it has a channel along one side where the plastic strip of the shutter speed scale slides. This will make it obvious which way to put the focusing screen back when you re-assemble the viewfinder.
With all pieces of the viewfinder removed we can start the next task; cleaning.
This entails some or all of the following:
Exercise extra care when handling the optical parts. As to how to go about cleaning them and the cleaning fluids to use, please refer to this text here. Don't expect a perfect result the first time you do this job. But, as they say, practice makes perfect. A clean environment is also important and that you don't reuse the same piece of cleaning cloth across jobs. Practically invisible dirt particles entrapped in the fabric can scratch the mirror.
If all goes well, you will have a beautifully clean and bright viewfinder. If not, well, at least you learned something. :-)