Is it likely that the power resistor in a J-Head might just burn up and die?
I had a print fail last night but it just looked like a typical “lack-of-adhesion” issue; here’s a time-lapse of the failure:
http://www.youtube.com/watch?v=1jpJj945ZkM&feature=youtu.be
I noticed last night when I was trying to print something else later that when I turned on the heat, it would turn off a few seconds later and the temp reading wouldn’t move much if at all (small enough to be normal ambient variation).
I got a few minutes to look closer at it tonight and then noticed that there is a blob of filament coming out above the head. I’m starting to suspect that maybe the power resistor failed late in my last print causing the filament to freeze in the nozzle and back-up over the top?
I don’t have time tonight to pull it apart but I plan to check the resistance of the resistor to see if it’s functioning as expected (and of course I have to figure out how to get the backed-up filament out of the rest of the head as well), but I’m wondering if it’s typical for resistors like this to fail, or if there is an expected “duty life” in hours that I should be keeping track of and replacing the resistor before something like this happens again?
http://www.youtube.com/watch?v=1jpJj945ZkM
The power resistors we use were never intended for anything like the what we use them for. It shouldn’t be surprising when they fail. That said. It could also be a wiring issue keeping power from getting to the resistor, or a burned FET, or a couple of other things. Even a loose thermistor would cause Marlin to notice that it’s not getting temperature feedback and shut down. The specs list the resistor hole at 5.944mm, so it might fit one of those cheap chinese cartridge heaters that have been going around (they are nominally 6mm, so a little reaming will probably be necessary, depending on the tolerances of each). I think my J-head is at the shop, so I can do a test fitting in the morning.
Yeah when I first heard of a resistor being used this way I raised an eyebrow but it seemed to be commonly-used enough that I went along with it (and up until last night seemed OK 
I have a couple of QU-BD extruders that use those cartridge heaters; I’m not sure if I could extract them from the QU-BD hot-ends w/o damaging them tho (they seem stuck in there with something).
Either way, I’m more worried about how much of a mess the backed-up filament may have made inside the thing, and I’m not entirely sure how I’m going to get it out…
The QU-BD cartridge heaters are held in with set-screws, but they are 5mm diameter instead of 6mm. They should work, but you’d need to make sure they are pressed firmly against the side of the hole. I’ve done this by wedging in bent spade connectors before, or by hammering a nail into the side of the block to collapse the resistor hole enough to clamp a smaller cartridge heater against the side.
I have heated the head with a small gas torch so I could take it a part and clean. You have to be very careful as they are very hot and you risk melting the PEEK and even the aluminium. I have mostly broken the legs of the resistors until I got a couple of cheap cartridge heaters. Had to widen the hole of one J-head a tiny bit, the other J-Head I got included the heater.
Since I have a demo scheduled next weekend (and want to do dual-extrusion eventually anyway) I think I’m just going to order another entire hot-end and then see what I can do with this one while I wait for the replacement to arrive.
I also printed some improvements for the QU-BD so maybe I’ll take that for another spin while I’m waiting.
I wonder if this is related to that black stuff I saw on my prints that I posted about a few days ago?
Yup, they do die. Make yourself a replacement from nichrome wire and fire cement/exhaust putty/fire rope cement etc.
@Vik_Olliver If he doesn’t already have those lying around (as most don’t), a new identical resistor is probably easier to source.
Yeah, and then you do the same thing a month later. If you’re hard up for it Jason, hit me for a free length of nichrome.
I had a nichrome heater in my MG hot-end and I didn’t have much luck with that. I think the j-head design is sound, but I think something like those cartridge heaters might be a better way to go than the resistor.
That said I ordered a second j-head and a replacement resistor just to make sure I had all the bases covered, and in the meantime I’m going to attempt to get one of my QU-BD extruders working using the @Whosa_whatsis 's simplified extruder design
The nichrome in resistors is way too thin to stand up to the abuse. The nichrome from Jaycar isn’t up to the job either - most people use too thin stuff so they don’t have to wrap much wire. Not a good approach. We get ours for industrial meat packing equipment, and it’s correspondingly tougher stuff.