MakerBot Industries

Well, hopefully this is basic enough. I assume you are still using the MK5 heater with resistors correct?

If you are using MK5, if the resistors are exposed I would check those connections first. Then with the heater wires disconnected at the extruder end-measure the resistance of the extruder heater circuit using the lowest resistance range on the meter. It should be aproximately 2.5-3.5 Ohms worst case. If it reads 5 ohms or open-you either have a bad connection at the resistor or one has failed. The only way to tell if the resistor is failed to to disconnect all the wiring (because they are in parallel) and you might as well just replace both resistors at the same time.

Look-the bad news is you are going to learn basic electricity-but the good news is that it's a great skill to have in your life.

If the resistors are bad-honestly, save yourself future headache and get a MK6+ heater upgrade. If you don't have skills and don't want to do this more than once-the upgrade is worth the price.

Ok first, the kit you probably should use is http://store.makerbot.com/mk6-plus-heater-upgrade-kit.html

You have a problem with your heater-thats the new style heater-but they also sell the resistors for your current heater should you decide to not upgrade and just replace them. Always replace the resistors in pairs as they likely fail at the same time or nearly the same time.

1. We're talking about measuring stuff without the TOM being powered up, right? This is because I'm trying to check the integrity of the heater circuit elements without power, right?

2. The first measurement should be made by touching the meter leads to the ends of the heater wire that were connected to the extruder controller, after disconnecting them from the controller, right?

Yes, you are correct on both of those questions. YOU MUST ALWAYS measure RESISTANCE (AKA Ohms) with the circuit powered off and preferably disconnected in most cases. The meter itself supplies the voltage to measure the circuit. If the circuit is powered or connected-the voltage from the meter may take a different path-and then you are not measuring the resistor-which is what we care about.

3. To go beyond that, I'll have to unwrap the thermal insulation from the extruder heater core, right?

Yes again. Infact-this is most likely your problem area-neatly tucked underneath that insulation. Much more likely than a problem at the extruder controller-but anything is possible.

I'd actually check it in these steps:
1. Unwrap the extruder (don't open up the bottom of the bot and disconnect from the extruder controller yet)
2. Check for DC voltage across the resistors-so in the bot-one meter lead on the backside terminal and one on the frontside terminal (since the resistors run along the sides front to back). Turn on the bot, connect to the computer, turn on the heater in the control panel.
Now you should get a voltage higher than 10volts. If not-you have a wiring problem. If you do, now you have two choices. Remove the bottom of the bot and disconnect at the extruder controller and then measure resistance and see if you get the required resistance somewhere around 2.5 to 3 Ohms. Or, skip that, we know we had the voltage and it didn't heat-so it can only be a bad resistor or wiring connection at the resistor. The only way to tell if the individual resistor is bad is to remove it from the circuit by disconnecting the wires. By the time you are there-you might as well have just replaced it.

Yes, sounds like a bad terminal connection-knowing that that is the weak spot in MK5, and the instructions and method were not optimal in the wiki.

This is where some basic wiring experience would really have helped you out, but your learning now, and that's a good thing.

Honestly, I can't recommend it enough to just upgrade to MK6+ heaters and get away from the nightmare of wiring and resistors MK5 is.

This isn't to say Mk5 is horrible. My first one from my cupcake is still on it's original resistors and is still printing. Just I used spade crimps on the terminals, allowing for easy replacement or even checking of a resistor if one failed. My terminals are exposed so I could test it at the first sign of an issue.
http://www.thingiverse.com/derivative:6203

Notice the kapton wrapped thermocouple in the middle that can be easily removed or checked, the bent 90 degree female spade terminals crimped to the wires and connected over the ends of the resistors.