This is unlikely to be dirt on the mirror or focusing screen. Based on the symptoms and known issues with older Minolta bodies, the dark bubbly band is most likely damage to the viewfinder prism caused by degraded adhesive/foam affecting the prism’s reflective coating. Heat can accelerate that kind of deterioration.

Unfortunately, this usually is not something you can clean away from the outside. A standard CLA may be worthwhile for overall maintenance on an old camera, especially after hot storage, but it will not necessarily fix prism/coating damage because the problem is in an integral prism part.

If you are new to camera repair, this is probably not a good first DIY project. Practical options are:

• have a qualified repair shop inspect it
• replace the prism/camera using a donor body
• replace the camera if repair cost exceeds its value

If the camera otherwise works, note that this affects viewing, not necessarily the image recorded on film.