This post shows how to repair a Pentax Spotmatic stuck mirror, which is a common problem with this iconic camera from the 1960s. Although it's not certain that this will be the cause on all occasions, it worked on the two Pentax Spotmatic models I had, which showed the problem of the mirror sticking in the up position during shutter release.
The pictures above are of the Spotmatic ES because it has a PCB under the bottom cover and is therefore slightly more complex than the original spotmatic models, but the mechanics are basically the same in all models that I've seen. The problem is that the mirror, instead of returning to the position which allows the viewfinder to be used to compose an image after the shutter has been released, sticks in the up position leading to a black viewfinder.
I did a bit of Internet research and found the culprit is a lack of lubrication in the cog which acts as the mirror return system, and that it is a relatively easy fix to perform. I did however find that I had to do several searches to find the actual cog, so I've added this post as an aide memoir to myself, and hopefully as a help to other people.
The pictures above show the process for removing the bottom cover and the PCB. If the model is an earlier version than the Spotmatic ES, there will be no PCB and the bottom cover is all that needs to be removed.
The most important pictures are the last two which show the cog which needs the oil, and I've copied the last picture here as well. However, firstly, when the mirror is locked it's necessary to release it with a small screwdriver. If you look at the last picture in the series above, you can see there is a slot above the cog, and the lever which is shown down in the picture will be at the top of the slot. A slight push sideways on the lever which is holding it in place will allow the mirror to return and the lever to move to the position shown in the picture.
Once the lever is released, the next thing is to apply a small amount of oil onto the cog shown in the picture. It's important to try to get the oil into the gap between the steel inner circle and the black outer cog and once a small drop of oil is applied, fire the shutter about 10 times in order to work the oil in and check the mirror returns every time.
I found this worked on both my Spotmatics and returned the mirror to perfect operation.
