Update: Repaired ~ Yep mirror sticking ...
Using a JIS Japanese Screw driver set, that we just received in the mail, we opened the RB67, and first inspected the mirror springs. They looked fine, and unbroken. The mirror was evenly spaced from right to left, at the hinge also.
Investigating further, by physically moving the mirror from the open left side of the camera body, we found out that the mirror was hanging up as it came closed, near the top of its arc. Gazing inside the camera body, towards the front left of the camera body, we saw a tell tell mark. The mirror was being caught by the camera body.
Reached in with a finger and gently pushed it out.
Problem solved, and
... SIGH, ... we really didn't have to disassemble the camera to see this, but the question remained, what caused it?
Investigating further, we cranked out the bellows and found a misalignment of the bellows, its lower on the side, making it thicker when it closes up on that side, which puts put more pressure on that side of the camera, pushing the front in, thus hanging up the mirror.
When we were first checking out this problem, we had several times moved the bellows out, and cocked the camera, but the mirror would not come down. Perhaps some time in the camera bag, the front of the camera was pressed on, and caused the dent. Who knows!
It way happen again, and wondered if is there any way to fix the bellows alignment?
But glad its up and running again, and it was really an easy fix, that did not require the camera to be taken apart, but wanted to post this so in order to help someone else with a similar problem. ;-)
Also .. BTW ~
one should not use US standard Phillips head screw drivers on Japanese equipment, it will not work correctly and most likely mess up the head of the screw.
