Do you have a curtis handset to check inputs and outputs? It is possible also to have a bad f-n-r switch as these switches do at times break leaf contactors inside. Simple check of the switch can be achieved with handset. If no handset is available then simply remove switch and crack it open and inspect leafs. Also as Edward posted, check for motor shorts, brushes and raised/blown commutator bars and power cables. This series of truck are pretty hard on motor brushes.
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