Hotend temp drops during print

I'm having a brand new problem with my printer. Anywhere from 10-40 minutes into a print, the temperature of my extruder will start randomly dropping. It usually hits the "Cold extrusion prevented" mark, but the print is screwed long before that. After a short time, the temperature starts rising again.

I've got the RAMBo circuit board, running on marlin. My hotend also has a 24v 40W heater cartridge in place of the power resistor, although I doubt that's the culprit, as I've been using it for months without issue.

It doesn't seem to be the thermistor, that seems to be working fine whenever I check it. It doesn't look like the firmware is at fault either: as soon as the temperature starts dropping, it turns on the heat_0 mosfet. I haven't been able to check the voltage across heat_0 during the temp drop, so that's the next test, I suppose.

I've been running this machine for a couple months without any problems (at least like this).

If anyone has experienced this before, I'd love to know what is causing it.

Maybe a broken wire to the heater. If it is ordinary cable then bending it backwards and forwards can break the strands inside but leave the insulation intact. It then forms an intermittent connection, see [hydraraptor.blogspot.co.uk].

I now use ribbon cables backed with polypropylene strips for all my flexible connections, see [hydraraptor.blogspot.co.uk] under the Cables heading.

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[www.hydraraptor.blogspot.com]

I'm experiencing the same thing right now. A j-head/ramps 1.4/marlin. When the temp drops, I see the LED on the ramps board come on solid, so I know the thermistor is calling for heat. Also, the LED is on when there is voltage being output from the MOSFET - it is not simply an indicator for the MOSFET gate, so I know the fuse is good, and most likely the MOSFET. I have alligator clips on my j-head resistor, waiting for the next occurrence so I can measure the voltage there, but I am starting to think like nophead here. I think it's the wiring. I'll let you know if I figure it out.

I solved my problem with the hot end temperature dropping during prints. As theorized, it was a bad connection. The first real clue I had was when I realized that the problem was only happening at the extreme right edge of the print area. Once this was my hypothesis, I heated up the hot end, attached a voltmeter, manually moved to the right edge, and then just moved around. It got to the point where I could reproduce the problem at will. When I disassembled the carriage and its wiring, I found a broken connection where the hot end wire enters the connector. A new connector and some general wiring clean up and I am back in business.