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  I have a 3513.50, based on the 7750, and I noticed a strange behaviour.
  
  When I stop the Chrono and don't reset it, after a few hours I notice that the Chrono hour hand in the dial at 6 o'clock keeps moving, although at a very slow pace.
  
  For example, if I stop the Chrono after 1 hour, 3 hours later the hour hand advanced close to the 2 hour mark. The rest of the hands stay where they should.
  
  It's not a big problem, but I want to be sure it's a problem and not regular behaviour of this movement before sending it for repair.
  
  Thanks

 

I believe that's what is known as "chrono creep," and would occur even when the chronograph function hasn't been used----the hour chrono hand will move very slowly past 12 . A competent watchmaker can fix it.

 

I had it on a 145.022-74 and it was fixed by the watchmaker in 30 minutes.
However, some of the teeth of the chrono hour subdial can be damaged and cause the creep, so it could have to be replaced. I don't think that it could be expensive though, especially on a 7750 based movement

 

It's not normal, so it is a problem that needs to be repaired at some point. It's more common on the manual wind Speedmasters, but not unheard of on a 7750 based watch.

How long ago was this watch last serviced?

Cheers, Al