That really depends what is wrong with it....Hi!
My first post here and it's a question, is it possible to repair a chronograph wheel to a 6138 movement ? i've found the parts on a couple of sites who sells watch parts, they sell them for repair or parts.
I have repaired two different failure modes....
A lose/wobbly forth wheel on a 6138b. 'Repaired' using a little metal putty to carefully keep the forth wheel pushed evenly off the clutch plate that you get on the 'b' movements. Not a great repair but the only option to me at the time and it worked perfectly.
And if you find that the clutch spring is no longer effective I find it is possible to put a little bend in the spring to make it function once again.