I have 3 extended Mackie MIDI Controller faders that I use with my d8b (plus the main controller) equaling 32 ch for controllering my DAW ( I use the d8b more as a line mixer). These Controllers are about 17 years old, so no surprise the P&G motor faders have become problematic. This month, I order a 3 of them to test how they worked from 40hz-20khz. 2 of 3 are working, but looks like the 3rd one needs to be returned. 40hz-20khz agree to send me another one for free 🙂 Will keep you posted on how things progress, but I suspect in time, more will stop working. The joy of maintaining vintage gear !
failed diagnostic 
label ch bad 
remove screws 
note which ch is problematic 
fold metal tabs to open 
looks like no a mechanical issue