Low amplitude finally solved!

thor447 · January 13, 2023

I've been working on a Seiko 6138-0040 Bullhead that belongs to an acquaintance of mine. He bought it new in the 70's, wore it until 1985 when it quit working, and it has sat in a drawer since that time. The issue of the watch not working was simple enough to fix. The hour recording wheel would not reset, and I found that the hour fly-back lever had snapped. Once I got the replacement part in I serviced the movement. After the initial inspection everything appeared to be in good order aside from some play in the upper barrel arbor port. That steel bushing was replaced with a jewel from Vintage Time Australia. Everything went back together nicely, but I was getting amplitudes in the 180-195 range. The Seiko 6138 generally runs at a lower amplitude, but this was too low. I took the watch apart again, thoroughly cleaned everything (again), and set myself to the task of rechecking everything, include end shakes and side shakes. Although the original mainspring looked to be in really good shape, I sourced another. Even with the brand new mainspring, the amplitude still wouldn't get above 200 degrees.

At my wits end I took the watch down for the thrid time. This time I removed the barlow lens from my microscope, lessening my available viewing area and work space, but allowing me to zoom in much closer. I finally found that the inside lip of the lower barrel arbor jewel (located in the calendar plate of the 6138 movement, not in the main plate) had a very small chip in it. It was very difficult to see due to the way the light was reflecting from the jewel, but I finally managed to get a picture of it. The chip is at the 11 o'clock position of the inside jewel rim.

After finding this chipped jewel I doublechecked the barrel arbor and did not see any scratches or other evidence of damage. Unfortunately this jewel is not the same lower arbor jewel that comes in the standard jewel upgrade kits (for the 61 & 63 series movements) because on the 6138 dual register chronograph movement the barrel arbor goes through the main plate and sits in a jewel on the front calendar plate. I sourced a cheap 6138 calendar plate but the barrel arbor jewel was in good shape. After swapping the jewels out and adjusting end shake, everything got cleaned (one last time), reassembled, and I'm happy to report that the amplitude issue has been solved.

This one tested my ability, and most of all my patience. I wish I had a deerstalker hat so I could have called upon my inner Sherlock Holmes and figured this out sooner, but at the end of the day(s) a successful repair has been completed.

Edited January 13, 2023 by thor447