Geared motor for MG400 J4


I have an application where I am moving a rack of test-tubes, but an attached coiled cable is imposing a very small amount of torque to J4 of my MG400 and it is causing it to misalign.

An old post on this forum (MG 400 Joint 4 Buzzing) says that there is a version with a reduction gear.

Does anyone know if this is available for retro-fitting and if so, where do I get one from?