To repair the gearbox, a certain qualification and special tools are required (pullers, micrometer heads and fixtures, shims to compensate for axial play of shafts and gears). In addition, spare parts for gearboxes are expensive and difficult to obtain. Therefore, in case of abnormal operation of the gearbox, it is recommended to hand it over for repair or purchase a reconditioned unit.
However, if you have some experience and tools, then self-repair with careful and accurate work is quite affordable. When disassembling the gearbox, pay attention to the installation order and the orientation of the parts. Repair is greatly simplified if it was possible to install a faulty node.