It's been a long time since I posted anything here, first of all because I'm no expert so what help can I provide… and second because I have had no issues whatsoever in a long time either.
When the MK5 and MK6 came out (I have a ToM, initially with MK5, later upgraded to MK6) people provided all kinds of evidence that the setup with power resistors for the heater was a recipe for disaster.
I have lowered the power just a notch (by adding around 0.5 ohms in the circuit) and have been printing quite a lot for over 18 months now. I have bought a spare set of resistors to be able to keep going when the first set would blow but they are still sitting in the box.
My ToM has seen its fair share of abuse, mainly because of some "less than optimal" filament I once got, but it's actually built very strongly. My machine is not a speed demon but it just keeps on going and going.
This is just a reassuring message for people who wonder how durable these machines are.
Thanks for the update and report. I too still have a working MK5 head, so I know it can be done. I think the single biggest trick is the wire attachement and the directions are at fault. Anything you do to the resistor ends puts stress that over time, so the dumb crimp method in the instructions caused a lot of premature failures. From day 1, I used female spade ends on the wire to slide over the terminals of the resistor. By not bending or stressing the resistor leads, you don't crack the material around the resistor, it stays tight in the aluminum block which disipates the heat correctly and then never have the failures. Also, while on the subject, ensure the thermocouple (applies only to gen4 electronics) was insulated with Kapton tape and I also slid and taped it into a terminal lug to hold it and conduct the temp. I actually changed the instructions to add that step back in the day. In the event a resistor shorts (somebody used solder that melts, or just the wire coming loose), it will blow a gen4 extruder controller sky high as the 12 volts attempt to ground through the thermocouple. Luckily, thermistors used with gen3 are glass beads and a little harder to short, but still possible. Anyway, just posting those tips since there are a lot of folks who posted lately running older hardware.
I did do the crimping but making sure not to hurt the resistors in any way. The female spade ends are certainly a better solution (if they make good enough contact to not have any resistance). Also important is to ground the heater block so that if a resistor blows you short out the power supply (which should have protection) and do not blow the extruder controller.
How well things continue to work depends a lot on the abs you get. I had good experience with makerbot's abs (white and black) but pretty bad experience with white abs from reprapsource.com. It required more heat and even then the extruder would miss steps and produce clicking sounds. I now have (natural) white abs from rerapworld.com and it's absolutely the very best. Very smooth extrusion and the output really looks and feels like Lego material (which is also abs).