Only 17 minutes elapsed diagnostic time, well done!
You are exactly right.
Now the harder work begins - to open it up and put it back together, without injuring the balance spring (hopefully).
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OK, diagnosis over, resuscitation has begun
I've decided to get right to work, as a patient who is handled quickly and carefully has a better chance for survival, don't you think? I removed the back cover. Notice the blued screw near the word SWISS is mostly undone. The adjusting arm has fallen off...
Here's the final result
With a few annoying obstacles along the way, I pulled the balance cock, replaced the stud holder and adjuster, and reinserted the balance into the movement. I thought I would try a new iPad-based timing meter, so downloaded the software and ran some tests...
Here's the ladies tiny Jubilee watch bracelet and the gunk that came out of it! ...
It helps to know what the data represents
But if you just want to see how your watch is performing, it just works. You do need a quiet room and you need to have the mic directly over the escapement- so it‘s nice to have a transparent caseback. Cazalea