The extruder motor gear won't turn - even after removing the hotend (with the clogged tube) and cutting the filament. Commands sent from the Control Panel do nothing - either retract or extrude.
What it did on the last print was run rather fast, which I thought was a bit strange since I hadn't changed any speed settings, and then it just stopped extruding about half an hour into a 2 hour print. (I probably should have stuck around and stopped it at the beginning when I noticed it running fast, but this is still fairly new and hindsight is 20/20. I remember thinking that the higher speeds might have been in the code for the print that I'd downloaded.) Still don't know why it was running so fast, because the codes are all the same as I'd been using.
When I looked into the hotend assembly, I could see that the filament was just chewed up and that was why the extruder wasn't moving it. But it must have been trying to move it against the extremely clogged barrel, so i thought that was why it might have burned out.
This is what the filament looked like:
You can see the chunk cut/worn out of it by the spinning gear. There are teeth marks farther down on the filament, so the tension was probably okay - it just hit a point where it couldn't be forced into the brass tube any more.
Didn't know that at the time of course - I did disassemble the hotend, and the diameter had been restricted to about 1/2 of the diameter necessary to let the filament pass through it. So the extruder had obviously been
forcing it through for a while (obviously it took some time for that much buildup in the tube).
I've printed the better part of a roll of the MakerGear black PLA, then i switched to a Brown PLA from eSun. And at that point I did start to notice some issues. I had put it down to a different manufacturer, but now I'm thinking that's where the buildup started. I was having to use a much higher extrusion adjustment, and that was making the extruder motor force through the desired volume. Probably compounded the problem.
Finally I switched from the Brown eSun back to MakerGear, but the White PLA, which has a different texture (less smooth) than the black. And that was what finally jammed it. I'd just started with that one, and I did have a harder time getting that to extrude. (Which was why I asked earlier about how to recognize partial clogging.) The nozzle flow was fine - I didn't realize the tube had buildup.
And this buildup is not like a wad - it's a smooth, rock hard coating all around the diameter of the tube - it was just restricting the diameter.
Once I drill out that restriction, I'll be able to see which of the filaments contributed to that coating inside the tube - so far, everything I've managed to ream out has been the white, but that was the last one used so I expect it in the center area. I'm expecting to find a rather large layer of the brown closer to the walls of the tube.
(I do have a spare hotend, so I'm just cleaning this one out to see what caused the problem.)
Would it be easier to melt that out? One thing we've got at this house is a heck of a lot of torches. (Husband sells them - very handy

.)