Gradual temp decline until Heater Failure #3 on Replicator 2

I'm troubleshooting an extruder heating problem. The extruder consistently preheats to 230C and starts printing at 225C (with PLA filament), but once the raft and model start to print it looks like the heater has shut down. The reported temperature starts to drop until after 3-5 minutes it gets below 180C and the printing stops with Heating Failure #3.

Is this more indicative of a bad thermocouple or a bad heating block? I guess I have spare parts from another R2, so I can start replacing both of them. Which should I try first?

I don't understand why it heats up nicely before the printer starts to move. Could it be a loose wire (inside the thermocouple itself or at the heating block)?

SOLVED! (I hope)

I started to realize, after trying various physical tricks with the thermocouple wiring (at the motherboard connection and where it comes out of the heating block), that the temperature decline started to happen precisely when the "active cooling fan" (the big centrifugal one on the side of the extruder) came on during the print. So I tried another print with "Use Active Cooling" unchecked in the MakerBot Desktop settings. After killing that fan, the extruder temperature stays within 1 degree C of the target. If necessary to get good PLA prints, I may try to re-enable the fan, but set the maximum power level down below the 50% default.

Any suggestions from expert users why the active cooling would drop the temperature so much? Is it possible that the thermocouple is getting cooled by the fan and not giving accurate readings of the actual extruder temperature?

Quote:

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Originally Posted by pzingg

SOLVED! (I hope)

I started to realize, after trying various physical tricks with the thermocouple wiring (at the motherboard connection and where it comes out of the heating block), that the temperature decline started to happen precisely when the "active cooling fan" (the big centrifugal one on the side of the extruder) came on during the print. So I tried another print with "Use Active Cooling" unchecked in the MakerBot Desktop settings. After killing that fan, the extruder temperature stays within 1 degree C of the target. If necessary to get good PLA prints, I may try to re-enable the fan, but set the maximum power level down below the 50% default.

Any suggestions from expert users why the active cooling would drop the temperature so much? Is it possible that the thermocouple is getting cooled by the fan and not giving accurate readings of the actual extruder temperature?

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I have the MB5 and had the exact same issue. I cleaned the contacts, checked connections, etc with no luck. Turning off the fan or reducing print speed temporarily corrected the issue. I switched extruders and everything is back to 100%. I took the original extruder apart to see if there was anything wrong but everything measured with an ohm meter perfectly fine. No idea what the issue is except the temperature would sag. Since the thermocouple was registering the temperature and the heating elements should either work or not, it probably is something in the electronics in the extruder assembly.