I purchased a second hand DN-X1600 mixer which works perfectly except having problems with channel faders 3 and 4. The channel 4 fader has no effect whatsoever, and the channel 3 fader affects both channels 3 and 4. The problem exists in both midi and non-midi modes. In midi mode, I observed this behaviour in Midi-Ox:

Channel 4: no messages received at all.
Channel 3: a combination of a 'control change' message and a 'CC: 21 (E-MU)' message is received. Furthermore, the 'CC: 21 (E-MU)' messages continue to be received in a slowing down fashion after letting go of the fader, as if it is slowing to a gentle stop.

I've uploaded videos to YouTube so that you can observe these behaviours:
(when using Midi-Ox)
(when using Traktor)

Is it possible that there's a setting which is controlling this weird behaviour?
Or is there anything that can fix the problem?
Or is it just broken?