If your bed/nozzle is not heating, here are the things to check:
With the power off:
1. Is everything plugged in?
1a. … the power supply to the wall?
1b. … the printer to the power supply?
1c. …the power input to the switch, and then to the RAMBo via the 6 pin connector?
If you can move the motors, chances are all of these things are ok.
2. Are all the screws on the 6 pin connector tightened, and are there any stray wire strands from one lead that might be causing a short to the other pins on the connector?
3. Are all the screws on the 2 pin connectors for the bed and hotend tightened, and are there any stray wire strands from one lead that may be causing a short to the other pin in the connectors?
4. Does the board output voltage to the bed and/or nozzle heating element? There are 2 LEDs near the output pins that should light up when voltage is applied. You can also set your multimeter to DC voltage and measure the output.
If the board does not output voltage, you may have a blown fuse. The fuse is there as a fail-safe, so if it has blown and it’s not a faulty fuse, chances are there is some other issue that needs addressing or it will just blow the fuse again.
5. Are the bed/hotend circuit closed or open? They should be neither.
The hotend heating element circuit wires are red and should read ~4.8 ohms,
the bed heating element circuit wires are purple and should read ~1.6 ohms,
the hotend thermistor circuit wires are orange, and should read ~100,000 ohms at room temp
the bed thermistor circuit wires are yellow and should read ~100,000 ohms at room temp.
You can check these by disconnecting the two-pin connector at the board and reading the resistance across the two pins.
If the hotend is not heating and the heating element circuit is reading open, then there is a chance that the heating element is blown. In this case, the 4.7 ohm heat resistor needs replacement. Contact support@lulzbot.com or source your own if you prefer. To prevent this from happening, you can also remove the heating element and wrap it in foil and replace it, or reflash your firmware with a lower voltage being sent to the element (all future 24v TAZ will have this change made in the firmware at the factory as far as I know).
If the hotend is not heating and the heating element is reading a short, it is possible that the insulation around the element leads is not seated correctly and the current is flowing through the heater block and not the resistor.
6. Is the problem in the heating element/thermistor itself, or the leads to the heating element/thermistor? Check this by disconnecting the connector near the bed/hotend (two two-pins for the bed, one four-pin for the budaschnozzle) and measuring the resistance there, instead of at the place where it connects to the board.
7. Is there continuity in the leads from the board to the heating element connector? Set your multimeter to ohms and put one terminal on the connector on the board, and the other terminal on the connector near the heating element in question. It should read a small resistance, about the same resistance that is read when the two terminals of the multimeter are touched together – usually about .1 ohms.
On older iterations of lulzbots, there was no strain relief for the bed or hotend leads, which led to failures due to flexing of the crimps which would loosen over time and open up the circuit. With the newer versions, there is a strain relief, so this failure mode is much less common, but still worth checking if there is not continuity or the correct resistance. You can remove the pins from the connectors by using a pokey thing to bend the retaining tab in and then pulling out the wire and pin for inspection. If everything seems ok, then bend the retaining tab back out a bit and push it back into the connector.