I have Tamiya double gearbox problem (pololu.com/product/114/specs)
I am using one of these gearboxes on my robot project. The gear ratio is 1:114.7. The gearbox is brand new and after ~20mins of operation, one of the motors became much slower. Both motors have 0.9 Ohm resistance, both work, but it became much slower. I tried with another SN754410NE driver, the result was the same. After i changed the motor with a spare one from another double gearbox kit all became normal again. Is that a brushed motor? If it is, is it possible that the brushes were destroyed after 20mins of operation? What could be the problem?