I always remove either the front or rear lens group from rangefinders in order to get to the aperture area. Then I clean what has to be cleaned with a tiny amount of ammonia on a soft, very clean tissue. I cannot right now speak for the Minolta you have but chances are that there is a spanner wrench nut on the rear which can be loosened and removed to accomplish this. I do not have a spanner wrench; I use a very pointed set of sewing shears to do this. The front is probably just as easy to do.

Of course, set the shutter at Bulb and use a locked cable release to keep it there. - David Lyga