You press the shutter button on your Pentax MX to capture a once-in-a-lifetime photo opportunity when the viewfinder goes completely dark --and stays that way. This is a common problem on many old single lens reflex (SLR) cameras, and it is often caused by the eroded foam pad that the mirror touches when being flipped up and out of the way of the shutter. If the mirror damper pad on your Pentax MX is gummed up and sticky, use the following guide to replace it and avoid missing another precious moment.
Depress the small tab near the mount thread and turn the lens counterclockwise to remove it from your camera. Set it aside in a safe place.
Orient the camera so you can see the inside of it clearly. If the mirror is still stuck in the up position, gently touch the edge of it and pull it down until it stays in place.
Locate the foam damper pad. It will be at the edge of the focusing screen -- a small square of milky-white glass on the top inside of the camera. Grasp one end of the pad with tweezers and pull it up slowly, gradually cutting it away from the camera with a hobby knife. If it begins to crumble, ensure that no particles fall onto the focusing screen.
Use a toothpick to gently clean away excess bits of foam or sticky adhesive from the edge of the focus screen.
Cut a small strip of 3-mm thick foam to size. Although dimensions do not have to be perfectly exact, it should be long enough to line the edge of the focusing screen. Use the size of the old damper pad as a reference.
Peel off the backing of the foam and lick the adhesive backing to make placement easier. Use tweezers to set the damper pad at the edge of the focusing screen and gently hold it in place for several minutes.
Moisten the end of a cotton swab with glass cleaner and gently clean away any adhesive residue that may be stuck to the edge of the mirror. Allow the cleaner to fully dry before test-firing the shutter.
Replace the lens by screwing it on to the mount in a clockwise direction.
Replacing a mirror damper pad is simple work but it requires a well-lit area and much patience. Ensure that you have an abundant light source so you can see your work area clearly. To help keep the damper pad in place as the adhesive sets, set the camera's shutter speed to bulb mode ("B" on the shutter speed dial) and depress the release button. This will hold the mirror in the up position and keep the pad in place.
Avoid touching or allowing any tools to touch the MX's focusing screen. It is made from a highly sensitive type of ground glass and can be ruined very easily.