Section 6.5 of the repair manual referred to in Huss’ link:
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6.5 The mirror frame doesn't work.
6.5.1. Unscrew the lens 3, take away the mirror frame, see if spring 161 jumped off the lever 160. Remove the shutter, then turn off the trigger button counterclockwise, put the spring on the lever of the mirror mechanism 28 with tweezers.
If the spring is deformed or the frame of mirror 177 does not reach housing 167, disassemble the device in paragraph 6.1.1.
Unscrew the screws 13, remove the shield 12. Unscrew the screws 26, remove assembly 28. Replace spring 161.
6.5.2. Inspect the mirror lifting mechanism - spring 182 may have been torn off.
Unscrew screws 5, remove cover 4, unscrew screws 10 and remove the mirror lift mechanism 11. In bridge 182, unscrew the screw 180, nut 181 with wrench 7812-4070, remove the axis-screw 188.
Carefully open the screw groove, unfold the second end of the spring tucked into the gear hole 186 and remove the spring. Put the new spring 189 on the screw axis 188. One end, U-shaped, is tucked into the groove of the screw axis and compressed with pliers.
The second end is filled into the gear hole and fix it.
Insert the knot (axis-screw) into the hole of bridge 182, wrap the nut 181 and fix it with a screw 180. Install the mechanism for lifting mirror 11 in the housing and fix it with screws 10.
6.5.3. Install assembly 28 in the body of the device, check the pin engagement (lever 160) with the cam 185. The link must be equal to the pin size. Unscrew the 187 screw in assembly 11 (from the mirror mechanism) and by rehooking the gears to place the cam 185 so that when checking the stroke of the frame of the mirror "by hand" in the lowered and cocked position of the shutter, the lever pin 160 does not touch the end of the cam 185. Weaken the 163 screws in assembly 28, by rotating the 164 screw, adjust the gap of 0.5--1 mm between the mirror frame and the assembly body 28.
Note: when adjusting the gap, set the trigger button counterclockwise after the shutter platoon.
The mirror mechanism should be removed and installed in the body of the device. Fix the mirror mechanism on the body with 26 screws.
On the side of the bottom cover, unscrew the stopper 180, loosen the nut 181.
Rotation of the 188 propeller (clockwise) to start the spring of the mirror lifting mechanism by 1.5--2 revolutions with the key 7812-4380.
Check the operation of locking device 28 with shutter curtains.
Release the shutter, press the release button on shutter speed "B". The first curtain and mirror frame should open the frame window.
Release the trigger button, the second curtain and frame should return to their original position. Check the operation of the curtains at 1/500 (the slot should be opened at the beginning of the frame).
Wrap the bottom cover 4 with screws 5. Assemble the apparatus under paragraph 6.1.3.
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The parts described above are illustrated in the second reference at the bottom of the document Huss linked.