I'm at a bit of a disadvantage because I haven't seen this brand before, but I deduced from the manufacturer's description and the problem you are having that the track signals do not actually run through the faders, but the fader position is digitally encoded and the number applied to the track target="_blank">HD16 System Version 1.12 and installed the latest key mapping file for your mixing software. I mention this because perhaps there is an unannounced bug in the older version(s) that causes unexpected fader action on certain operator inputs; there was one such announced glitch in track status indication just for Tracks 1 and 4.

Regarding disassembly:
Can you remove the bottom case with the circuit still attached to the top panel (this is the most common arrangement for mixers)? You may have to remove the hardware from the backside connectors to do so. If this doesn't work, can you separate the pieces far enough to look inside along the right? You may find that the encoder and shuttle assembly are on a separate board that remains attached to the top panel at the first step of disassembly, and there would be cabling from that board to the rest of the unit. It is also possible that these knobs are unlatched from the inside if they don't simply pull off, or they may simply go through big holes in the front panel. I find it hard to believe that anyone would be so mean as to make these one-way, one-time snap-on knobs; the warranty service department techs would complain.

Unless you can use an oscilloscope and multimeter, I'm not sure what you could do inside the mixer other than make sure the cable connectors are clean and properly seated on the fader assembly. You could check the power supply for failing capacitors - perhaps the fader is less forgiving of poor voltage regulation than the rest of the circuit. But then, that is best done with an oscilloscope; failing electrolytic capacitors do not always show externally visible signs of their demise.

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Restoring the gadget to factory settings should resolved the issue. Since problem went back, the firmware for this gadget needs to be reconfigured or reinstalled. Sounds like you need to bring the gadget to the zoom repair center.

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You MIGHT try putting a little bit of CRC226 into the faders through the slot and work them a bit. You can get CRC226 at Home Depot in the electrical department. Not there are other CRC products there so use ONLY CRC226. Spray a little with the spout near th center and work each fader right after putting it in.

The probl;em is are the control and sliders need clean and volume controls, remove the knobs and get a electronic cleaner and spray every control and you will see a big improvement, on everything.