I have recently been increasing my collection of Miranda vintage cameras and along with a few other models I've acquired two Miranda Sensomat RE models and both of them had the same problem - at slow shutter speeds (i.e. between 1/8 and 1 sec) the mirror flips up, but the operation didn't complete and the mirror is left up and the shutter open.
What I found is that the shutter mechanism was basically dried up and sticking, and this is a brief post to explain what I did to fix it.
First, you need to remove the bottom plate of the camera. This means first opening the battery compartment and then removing the 4 screws which hold the bottom plate in place.
This reveals an electrical board on one side of the camera and a mechanical arrangement on the other side. The mechanics are the escapement mechanism for the slow shutter speeds. I found that when the mirror was locked up, I just needed to touch one of the arms and the operation would complete and the shutter fire.
So I applied a very small amount of thin oil to the cog wheels in that area and fired the shutter a few dozen times to work it in. Once that was complete, the shutter seems to reliably fire on all speeds.
Just a word of caution - it is very easy to go overboard and apply too much oil. I have a small Draper oil pen which applies a drop when you depress the knob on the top, and I used that to drop a tiny amount on one cog and then used a tiny screwdriver to distribute it to other parts.
