Chrono sub dial not resetting

[qhartman]

I own a Hamilton Khaki autochrono like this: https://www.jomashop.com/hamilton-khaki-field-watch-h71566733.html

 

The upper most subdial stopped resetting today. Still works just fine when running the chronograph though. I've never worked on a chronograph movement before, and I'm reluctant to tackle this one as a first crack. The movement is a modified Valjoux 7750. Any suggestions on where to start?

 

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[nickelsilver]

Did it stop zeroing altogether or it doesn't quite make it to zero? The hammer (which zeros the hands on their heart cams) should be readily visible through the display back. If it's hitting the minute counter cam properly then most likely the cam is loose (or hand).

[qhartman]

It's not zeroing at all.

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