Encountering weird behavior with the thermal sensor. Due to a loose fit of the thermocouple in the heater block, I've been having chronic temperature control problems lately. It started with a frequent problem of the heat cutting off shortly after starting a print. Now, in addition, I see frequent 'flatlining' of the temperature where it sticks at one reading until the actual block temperature has shot far past the set temperature. Usually, I can wiggle the wire for the thermocouple or press the metal pin into the block for a tighter fit to coax it back into reading temperature normally. Unfortunately, in the process this is slowly unraveling the metal braid around that thermocouple wire. Now, this would seem to suggest a short in the thermocouple. But today, while trying to coax the thing to start working, I noticed a truly bizarre phenomenon as I was moving the head module back and forth. With the head near the minimum X/Y position (front/left corner) the sensor would flatline at any particular temperature it was at. I often start out with the head in that corner, otherwise it can't identify the center for "print at center" settings. (which ought to be called "print _from_ center", since it never actually centers the workpiece...) But moved away from that corner, it would read the rising or falling temperature normally. How is this even possible? My only thought is that, since that's the point of maximum reach of the head module cable and tube, it suggests a short along that length. Yet, when the temperature fails mid-printing I can only get it working again by finagling with the thermocouple, trying to get it seated more tightly.
Also been struggling with strange spontaneous mid-printing failures. Had a nearly perfect print run of 6 continuous hours yesterday, then, 10 minutes from finishing a tall workpiece the thing spontaneously pushes the platform up and the print head crashes into the unfinished workpiece knocking it right off! It was as though it had over-run what it thought was the Z axis limit (it was still a good 7-8cm from bottom), jumped-up, and started printing from a new level. Pre-Marlin there was a similar chronic problem at the end of printing, the machine raising instead of lowering the platform and ramming the hot print head into the piece destroying it just as it finished. I couldn't even try printing anything that was higher than a few cm. Now it's back again in this random behavior.
I seem to be having an unusual amount of trouble with my Ultimaker. Its original Atmega 1280 failed shortly after assembly, platform got marred when the screws came loose on the upper z-stop, motor driver current was tuned too low (which took months to figure out), and while most people seemed to be printing just fine out of the box I had to wait 8 months for the combination of ReplicatorG-25 and Marlin 3 to print anything that didn't look like a blob. (until ReplicatorG-25, neither Mac, Windows, or Linux versions would load any version of Marlin) With Marlin it seemed like I had finally gotten my Ultimaker for the first time, producing a couple truly excellent prints, but then this thermocouple nonsense starts and I'm dead in the water again. Sure, you have to expect bugs with beta, and I think I've been patient for what is now just about a whole year. But, really, are other users going through this? It seems like one thing is failing after another. I feel like I'm being left behind by what is increasingly looking like a lemon. If this latest problem is a failed thermocouple, I'm probably looking at yet another month sitting on my hands waiting on mail order for a replacement part--assuming I can get one. It's really getting me down.