last Thursday I plucked up the courage to take the camera apart and took a look. I found that the part marked in the picture for triggering the shutter was loose, and it was not possible to push the upper shutter blades and shutter delay assembly to open when pressing the shutter trigger.
The two GIF files demonstrate:
belair-shutter.gif : Demonstrates the action of the shutter when it is opened normally (I did not push the shutter lever all the way at this time, the metal piece below did not pop up, and there was no click sound)
belair-failure.gif : If the shutter lever is pushed to the bottom, the shutter fails to be triggered due to a malfunction (although a clicking sound is made, the part marked in the picture above fails to push the upper paddle open, and the shutter fails to open completely)
I removed this part, reinstalled it and tightened the screws, and the function is now completely normal. I hope it can help people with similar problems to troubleshoot the problem.
This reply is machine translated, there may be some inappropriate expressions, please bear with me.