I was at the HF yesterday morning along with a few others, and we did some simple testing.

The first time we turned it on and set the target temp to 255, we noticed that the resistor on the left got very hot, while the resistor on the right did not.

The heating resistors are 5-ohm 10w resisters that are wired in parallel. Mr Ohm tells us that in this configuration we should measure 2.5-ohms across the wires that go to the relay board (yes we removed the wires from the relay board, and the resistors were cold). But... we measured 9.5 - 10 ohms. Odd. We then double checked the wiring and that seems correct. We even de-soldered the resistors and measured them separately and got 5 ohms. Something is not right. My gut tells me that one of the resistors are bad, the one on the right I suspect. If I make it by ABC today I'll look and see if they have them.

-John

I did manage to pickup two 5 ohm 10w 1% resistors today. I'll bring them by the HF tomorrow (Saturday) and set them on top of it...

I didn't post on the makerbot operators group yet, as I'm still searching existing posts for possible solutions. If I have time I'll run new wires to the resistors tonight.

edit: found a thread where the user was having the exact same problem as us, ended up being 1 dead resistor

so they've shipped at least 2 bad resistors out? others were complaining about failure to heat with undiagnosed cause no?

Are they shipping bad resistors or are they dying because they're being abused.
I don't think those resistors are designed for use as heating elements.

Interesting.
I just looked up the datasheet for that resistor.
http://www.ohmite.com/catalog/pdf/89_series.pdf

I observed this interesting tidbit therein:
"Derating: Linearly from 100% @ +25°C to 0% @ +275°C."

If we run at 225 Deg C, then we have to derate the resistor to 20% or
0.2 * 10W = 2W

We're running 12V through a 5 ohm resistor.
12V / 5ohm = 2.4A
2.4A * 12V = 28.8 W

We're dissipating 28.8 W from what is effectively a 2W resistor.

I'm not surprised that the resistors are dying.

They're 10W resistors.

abuse they've been done aside, that's not what they're testing at when we were fiddling yesterday.

Only at room temperature. As they get hotter, their effective rating decreases.
That's what the bit about derating that I quoted from the datasheet means.

Even at a 10W rating, they're dissipating nearly 3 times that.
Generally not good for the longevity of any component.

Darus, good points. I was actually wondering about the effects of the heat being generated derating the resistors. So this sounds just bad design (and maybe a bad resistor or two).