In this set of video tutorials Macho and Neway from Floating Piano Factory in Hong Kong show us how to replace hammer return spring cords on a Yamaha U1 upright piano. It is quite common for these cords to wear out and break. In this piano, virtually all of them have become useless!
Some of the background work has been assumed in these videos. By the point we have reached here you should have removed the action from the piano and removed the hammer rest rail.
Step 1. The first step that Neway explains is how to make room for your work by removing the hammers from the flanges. In this model of piano there is a plate screw which can be loosened so that the hammer can detach from the center pin.
Step 2. The second step is to clean the remaining cord (a.k.a. rope or string) material that is within the groove of the hammer butt flange.
Step 3. Add glue to the groove on only one side of the hammer flanges and insert one side of the cord. Here we add glue and insert cords into to the left sides.
Step 4. Add glue to the groove on the other side of the hammer flanges and insert the other side of the cord. Here we add glue and insert the remaining end into the right side of the flanges.