there were problems with interference generated by the pulses caused by the speed control drive frequency if i remember correctly, was worse with certain speed controls, especially the ones of the era that sported 'regenerative' braking. even modern 4 wire fet servo's still require a choke and they are quite cheap and easy to come by, nick adams at demon products will help you out if you have any trouble getting them. i have 2 ps87 still in cars and both going strong, not quite as fast as the modern ones but they still work after all these years, will the modern ones!
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