I did do a final test of the resistor, and as you can see in the temp log below, it heated briefly, then flatlined.

The cartridge is actually measuring 5mm diameter, and the heater block hole is 7mm, so it needs quite a bit of filling in.  So I first wrapped some Aluminum foil around and inserted the cartridge for the initial test.  within seconds, there was a ton of smoke and you could see the foil burning.  So that wasn't a good solution.

I then found some brass tubing, and with a little bit of work, I got it around the cartridge and it fits in the hole, with just a little bit of space.

I then retested and you can see from the log below that it works quite well... and no smoke either.

I just need to finish the job and wrap everything in Kapton.

Could it be connectors/crimps? Are the electrical connections solid? Or perhaps a short somewhere (damaged insulation, ...)? Any signs of overheating, melting, etc.? Perhaps do continuity check between connector pins (at hotend and at board)? Just a thought...

Edit: My guess would be an intermittent short that's eventually killing your MOSFET.  If you're lucky, you might be able to detect it now.

It looks like its either a loose connection - or its thermally failing... (which will cause it to work again once its cooled sufficently).

Stick a fan right on the mosfets and see if it behaves

Tonight I was doing a fairly lengthy print (~6 hours), and with about 1:30 hours to go, The extruder heater started acting up again.  Here is a printout of the last 3 hours, about midway is where I noticed it flaking out and killed the print.  I then tried to reheat the extruder only to get those huge drop offs. Now it's once again off line permanently.

I haven't checked the MOSFET yet, will do that tomorrow, but I have a feeling that it's dead like before.  Could it be that the resistor is killing my MOSFET ?

Mosfet you mean ? since there isn't a voltage regulator on the heaters?

You're right, I'm used to working on power supplies with voltage regulators (7805, 7812) that look the same. 

Anyway, when my bed heater had gone bad the first time I bought some upgraded MOSFETS from Mouser Electronics; PSMN7R6-60PS.  They're rated at 92A continuous drain.  The original is only 55A.

Update...

I pulled the resistor out the hot end and on whim I hooked it up to a 12V battery.   It got hot instantly.  I then tested the output at the Sanguino board, and it got nothing.  It was the voltage regulator all along, not the heater.

Luckily I happened to have a few lying around since the last time when I replaced the VR for the bed heater with a higher capacity one, so I got the soldering iron out and now my printer is working. 

I'll probably still use the heater cartridge when I get it, but in the meantime, I'm back up and running.

Mosfet you mean ? since there isn't a voltage regulator on the heaters?

I pulled the resistor out the hot end and on whim I hooked it up to a 12V battery.   It got hot instantly.  I then tested the output at the Sanguino board, and it got nothing.  It was the voltage regulator all along, not the heater.

Luckily I happened to have a few lying around since the last time when I replaced the VR for the bed heater with a higher capacity one, so I got the soldering iron out and now my printer is working. 

I'll probably still use the heater cartridge when I get it, but in the meantime, I'm back up and running.

One more question.

I've always hear it told here that the extruder temperature readings are skewed by as much as 30°C because of the location of the thermistor. 

The thermistor seems to be attached to the side of the copper nozzle, which to me it would be the logical place to put it, as it reads the temperature of the area where the plastic is actually being melted.  Isn't that what you want to measure?

It is, but most hot ends measure heater temperature instead.

I've always hear it told here that the extruder temperature readings are skewed by as much as 30°C because of the location of the thermistor. 

The thermistor seems to be attached to the side of the copper nozzle, which to me it would be the logical place to put it, as it reads the temperature of the area where the plastic is actually being melted.  Isn't that what you want to measure?

http://www.soliforum.com/topic/1501/how … g-element/

A cartridge heater will work, just make sure of the diameter and realize that sometimes the cheaper cartridges are oval and you may have to ream the block a bit.

Long story short, my extruder won't heat up any longer.

So I know that Solidoodle uses a 3W resistor, but can I use a heater cartridge instead?  I'm looking at one of those 12V 40W cartridges.  Diameter wise they're the same as the resistor (6mm).

Also what's the best (simplest) way to remove the resistor?