I decided to “upgrade” my generation 3 electronics to use a real stepper motor driver instead of the PWM hack non-sense.
For those who don’t already know, the Makerbot electronics are not really designed for Mendel, and specifically the extruder controller is designed to run a gearmotor instead of Mendel’s stepper motor. The Mendel firmware was “hacked” to make the gearmotor controller push 12v through a 2.3v stepper motor but, by making the gearmotor controllers simulate PWM output (which reduces the output power) it doesn’t blow the stepper. From the perspective of re-purposing existing electronics, the idea is a “really neat hack”, and in theory it should just work. Needless to say that in practice, in my experience, the extruder controller makes the stepper run hot, and with a high pitched squeak coming out of it at all times.
Re-wiring the motherboard to use a separate extruder is quite a simple task, just connect the SCL output of the motherboard to the Dir pin of the stepper driver, and the SDA output to the Step pin. However when I came to do this myself I didn’t have and couldn’t find this vital bit of information. So using a multimeter connected to the powered up motherboard I tried to determine which output should be connected to which. Well in the process of probing around I managed to short +5v to GND. I heard a fssssszt noise and saw a flash come from the motherboard….oh no!
Well I think I managed a lucky escape, one of the tracks on the Motherboard had acted as a fuse and blew, so with the bit of repair shown above circled in green, the motherboard was working again.