Thread breakage or shredding in the Brother PE770 Embroidery Machine can be caused by several factors. Here are some steps you can take to try and fix it:

1. Check the threading: Make sure the machine is threaded correctly and the thread is properly seated in the tension disks.
2. Change the needle: A dull or damaged needle can cause thread breakage. Make sure you are using the correct needle type and size for the fabric and thread being used.
3. Adjust the tension: Adjust the thread tension to ensure that the upper and lower threads are properly balanced.
4. Clean the machine: Dust and lint buildup can cause thread breakage. Make sure the machine is clean and properly lubricated.
5. Check the bobbin: Make sure the bobbin is properly wound and inserted correctly.
6. Slow down the machine: Running the machine at high speeds can cause thread breakage. Slow down the machine speed and observe the stitching to see if the problem is resolved.

If these steps do not resolve the issue, it may be necessary to take the machine to a technician for further inspection and repair.