Hi everyone:
Just thought this might be of some help to you Asahi Pentax S1a and related cameras.
There is a common problem that these cameras get stuck due to age. the lubrification is no longer there or is semi dry hence the problem of having the shutter screen not fully closing and mirror remains in the up position.
I was able to fix my newly acquired camera by removing the bottom plate, adding a drop of oil in the moving parts at the bottom of the camera.
I am including some photo links so that hopefully some people might benefit from this trick.
Asahi Pentax S1a shutter curtain mirror fix | Flickr
Note: use only camera oil or something of high quality for small mechanisms. Just add one drop and clean excess off with Qtips.
After the oiling is done, crank the camera at different speeds. eventually the oil will do its job and hopefully you should have a decent working device.
Have fun!