With the lens off, try fiddling slightly up and down on the aperture-actuator - left from the mirror when viewed from the front - while also playing on the advance-lever. That way you can make the camera advance without the shutter firing, with a bit of patience on a working sample. Also try moving the mirror up with the tip of a pen. Have you tried the double-exposure lever? The connected mirror-assembly is probably the weakest part of the construction. You can also try on the winder-coupling,
Good luck, never had one of mine do that, thogh.