With Win Explorer create a sub folder REPAIRED
Open WinRar, browse to the folder with the damaged Rar file(z). U can find which one is bad by selecting the 1st RAR file and using TEST button.
Select the damaged file, press REPAIR button.
In dialogue box, enter the path to the REPAIR subfolder, or browse to it.
As long as there is a RECOVERY record in that Rar, it will be repaired.
Copy all the NON-DAMAGED rars to your repair folder and rename the REPAIRED file back to it’s original name.
Proceed with extraction in the REPAIR folder as normal.