You have to remove the top cover to locate the problem. You may wish to refer to the service manual for Spotmatic through this site to see how the top can be removed. However Spotmatic F, which is a younger model, differs slightly from Spotmatic. Be careful. http://www.pentax-manuals.com/manuals/service/servicemanuals.htm
There is one thing I found on slr's and that is that sometimes the seal material gets gummy and sticks up the little finger that senses the door is open and before I disassemble a camera I would check this first and clean the lever.
