Anycubic Chiron - Extruder Cable Stabilizer

After several years of use, my Anycubic Chiron started getting weird temperature fluctuations.  Sometimes it was minor, deviating at most 3-5 degrees off the setpoint, and sometimes ... not.

When I first started seeing the problem, I tried PID tuning the printer a few times, but that didn't help.  Then I suspected the thermistor, so I replaced the entire hotend, also to no effect.

I spent quite awhile trying to diagnose/fix the issue, but it was difficult to test because it was intermittent and unpredictable: a print could behave normally for a long time before the issue suddenly appeared without warning.  For example, this started over four hours into an otherwise stable print...

The magnitude of the temperature drops got larger (often dipping 50-150C and then recovering over a second or two, as shown above) which ruled out a real temperature change, because the printer simply can't heat or cool that rapidly under its own power.  Well, more precisely, a real temperature change could happen ... but only in response to the printer seeing these fluctuations and trying to adjust, rather than being the source of the spikes.  It had to be an electrical issue in the thermistor path.  Combined with a separate issue where the bearing under the extruder gear was failing (that I've since fixed), this often resulted in sudden under-extrusion and ruined prints.

Another hypothesis was a broken or dodgy wire in the cable to the extruder on the Z-axis, but replacing the cable once again did little to help.  The problem kept getting worse, and eventually started triggering thermal runaway protection on the printer, aborting my prints.

Last year, it occurred to me that the issue might actually be caused by slight movements of the cable relative to the connector on top of the hotend that it plugs into.  The cable is somewhat bulky and stiff, so the X-axis movement might be causing it to shift slightly in the connector, creating a bad connection on the thermistor lines, thereby changing the apparent temperature. I'd never really noticed any movement with naked eye (maybe I just was never looking at the right time or the X-axis movement masked them), but it was the only reasonable thing left that I could think of.

So out of desperation, I hot glued the connector in place to attempt to stabilize it (it's far enough from the heated portion that the glue won't melt during operation) and zip tied the cable to the end of the drag chain where the cable emerges to help cut down on its movement even more.  Lo and behold, the issue practically disappeared ... and stayed away for almost exactly one year after these changes.

It did come back, but I quickly came up with this (maybe) more permanent solution and just needed the printer to work long enough to implement it.  So I added more hot glue and printed this part.  Once it was installed, the issue went away completely, and hopefully for good.

The white printed part replaces the little black Z-shaped piece (the piece being held in the photo below) that's attached with a screw above the X-axis limit switch on the hotend carrier.  That part normally just serves as a guide or rest for the drag chain folding back on itself as the extruder moves to the right.  The replacement part allows the cable to be secured to the extruder with a zip tie, with the result that the cable can no longer move relative to the connector, it can only flex above the zip tie, thereby keeping the cable from wiggling around and causing loose connections).

Installation is easy: just undo the screw above the limit switch that holds the Z-shaped guide on the extruder, remove the part, and use the screw to secure the new part in its place.  Then tighten a zip tie around the Z-axis cable and the recessed portion of the post.  Hot gluing the connector in place is optional, as is zip tying the cable at the point where it emerges from the drag chain (as you can see just to the right of the blue Bowden tube). Neither really hurts, but they may offer no additional benefit once this is installed.