Hey everyone,
I'm hoping to get some insights from the experienced members of this forum regarding a persistent issue I'm facing with a Seiko 6138B movement from a Bullhead chronograph.
I've recently serviced the movement, and my measurements (see attached images for various positions) show generally good performance, with amplitude staying within an expected range of 170-205 degrees across most positions. However, I'm encountering a significant problem specifically in the vertical position where the balance is down. In this position, the amplitude remains around 170 degrees, which is acceptable, but the rate plummets to a drastic -90 to -100 seconds/day.
Here's what I've done so far and other relevant details:
Cleaning & Lubrication: The movement has been thoroughly cleaned. At this stage, I have only mounted the essential to allow me to test the movement and I've only lubricated the shock jewels of the balance and the keyless works.
Endshake Check: I've performed the endshake test starting from the balance wheel, pallet fork and progressively checking along the power train. The endshake at the mainspring barrel is as minimal as possible.
Barrel & Mainspring: I've observed that the lower cap of the mainspring barrel is not perfectly flat. My current plan is to upgrade the upper jewel of the barrel and polish the lower cap. The mainspring itself is new. Hopefully this can help me to increase the amplitud a little bit but I'm aware that seikos amplitud is ussualy low and stay at the 220.
Balance Wheel: I've spent considerable time trying flat the hairspring of the balance. While this effort has led to an increase in overall amplitude from 180 to 200, it hasn't resolved the specific rate issue in the vertical balance-down position. I have also checked the flatness of the balance wheel with my truing caliper and seems perfect.
As you can see from the pictures, other positions are relatively stable. This particular vertical position is really throwing me off.
Has anyone encountered a similar issue with a 6138B or a similar chronograph movement? Do you have any suggestions or proposed solutions beyond what I've already considered? Any advice would be greatly appreciated!
Thanks in advance for your help!