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Yesterday for the third time the printing just stopped after a few hours, leaving the head in the last position (in the job!), maintaining temperature as usual and leaving the display like running a normal job. The homing key just works as expected, but the job is completely standing still without any movement. This happened a few weeks ago also, on a smaller part I was working on, and I wrote it off to the general USB miserability. Since then I have been printing through the SD card and that seemed just fine after getting used to use the display and knob. But yesterday the same thing happened again, with the USB not even attached to the PC. I was standing at the printer looking at the job and it just stopped moving. I usually don't print unattended, but I do let some jobs finnish during my nights sleep. I'd hate to see this happen while asleep and this is therefore a BIG throwback in my confidentiality on the printer. I can't possibly be continuously around during a 7 hour printjob...

How big a APC would I need to rule out 230V power fluctuations in the grid? (Maybe I could lend one from my work). Or is my rambo failing? Does someone here have some other clues as to what could be the cause, or what to check? I'd like my confidence restored...

There used to be an issue with the old 0.8.x Repetier firmwares that did this, but I assume your running 0.9.x?
Usually with a power fluctuation the RAMBO resets, the only time I've seen this happen is recently when I knocked the power cable on my laptop and discovered the battery wasn't holding a charge.
Repetier itself has one big weakness it supports a binary protocol as well as ascii gcode, if the first bit of the command gets corrupted then it assumes the command is binary, when it fails to decode the command it requests a resend, and it usually doesn't recover. If your seeing this and you turn on logging in the preferences, at the point the print stops you see hundreds of resend requests in the log.
If this is what you are seeing.
Over USB it's pretty obvious how this happens, off SD card it's still possible because it's just an SPI device, I would check the way your screen and SD card are wired to the RAMBO, making sure to route the ribbon cables away from any of the other wires.
It could also be a bad SD Card, or a bad connection to it.

Thanks for the tips. The wiring actually is quite messy behind the sidepanels. That sure is worth some investigation. I am running the standard 0.9 firmware from Seeme indeed. The fact that the Rambo did not reset was of curiosity for me too. If it really is a fluctuating 230V outlet that would most like be happening, a complete reset. I'll check the wiring this weekend.

If it's just fluctuations, I would think a standard surge suppressor would do the trick. A small 230V USP which would handle the printer for ~3 minutes is 92 pounds (Sterling, not weight). You want at least 350W (As that is the maximum draw through the Rambo without busting fuses), and the higher your VA number, the longer it will last. Pick something you could deal with, and which doesn't break your budget. (I have heard that actual power outages lasting longer than a minute or two are rather rare over much of Europe, so if that's the case, one with a small internal battery would serve just fine for pretty much everything)

http://uk.farnell.com/ea-elektro-automa ... dp/1812164 The 91 pound UPS I referred to (The voltage is right, but the plug may be wrong. I know little about European electrical standards)