Our Mendels currently use Gen 3 electronics from Techzone. So far we've had the heater output on two of the (ver 2.2) Extruder boards fail, probably due to miswired tip heaters.
Testing to see if the MOSFETs are at fault is easy.
* Disconnect the board from everything (power, tip, etc) * Use a multimeter to measure the resistance between the heater terminals (the source and drain of the MOSFET). * You should measure a very large resistance, probably several megohms. * If you measure a a low resistance, (a few hundred ohms or less), the MOSFET has burnt out and should be replaced.
The heater drivers on the techzone boards are NTD4856N MOSFETs from ON Semiconductor. These are simple power MOSFETS with no short-circuit protection, so they burn out quickly when the output is shorted.
The standard Extruder board uses a different heater driver, the NIF5002N or NIF5003N, also from ON Semiconductor. This is a power MOSFET with short-circuit and temperature protection, which means that it shouldn't burn out even if the heater is badly miswired.
To repair the extuder boards, we purchased some NIF5003Ns (part# NIF5003NT1GOSCT-ND from digikey.com, $1.44 each). The pin spacing is similar to the NTD485Ns we were replacing, but the new parts were shorter. Fortunately, the top (drain) pad on the circuit boards is very large, and the new parts fit with no issues.
Replacing the parts is fairly straightforward:
Note that they are pairs of terminals that slide together, so you'll need to start from the end furthest from the power input and remove them one piece at a time.
1. Unsolder and remove any screw terminals which are too close to the drain pins of the MOSFETS. 2. Unsolder the MOSFETS. Bending the gate and source pins up and then pull on the part gently while heating the drain pin. 3. Put some solder on the pads, put the new part down, and solder it in place. 4. Solder the screw terminals back on 5. Test.