Replacement Stepper Drives for Replicator Dual Extruder

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I recently suffered a catastrophic failure of both extruder motor stepper drives on my Makerbot Replicator Dual Stepstruder. I won't delve into the specifics, but I wanted to document my repair process. I suspect that Makerbot no longer sells replacement parts for this unit, at least they don't display them on their website, and I didn't bother calling to inquire. If they did sell them, I expect they would be quite pricey. When the stepper motor started jittering back and forth a single step, and the circuit board became extremely hot, I knew the stepper drive was blown. I replaced both stepper drivers with Pololu A4988 Stepper Motor Driver Carrier Boards, Item #1182 @ $5.95 each (https://www.pololu.com/product/1182/resources). These boards appear to be identical except for one key difference: the Pololu #1182 PCB uses a trim-pot to set the motor current, whereas the Makerbot board controls it digitally through software. I don't think this discrepancy is a major issue since acceleration is not used on the extruder drives. Therefore, this difference didn't pose a problem in my case. Due to the differing pinouts between the two boards, I noticed that the "VR" pin on the Makerbot board corresponds to the "MS3" pin on the Pololu #1182 PCB. I assume the "VR" is the variable motor current adjustment pin on the Makerbot board and the "MS3" is used to set the steps/revolution on the Pololu #1182 PCB. To make the Pololu #1182 PCB compatible, I simply removed the pin when soldering in the pin header (included with the Pololu #1182 PCB) and connected a jumper wire between the "MS3" solder pad and the GND solder pad. The MS1-MS3 configuration is used to set the step/rotation. To set the correct motor current, I estimated it should be around 1-amp or less. These boards can supply up to 2-amps with an appropriate heat sink, but without one, I decided to set the motor current to a conservative 800-mA. According to the Pololu instructions, you adjust the trim-pot so that the voltage on the test point is half of the desired current. For 800-mA (0.8-Amps), I adjusted the trim-pot so that the measured voltage was 0.400 Volts. While this repair worked for me, I do not recommend anyone else attempt it, as they would be exposed to hazardous voltages and temperatures, potentially causing further damage to their Replicator. ALL RISK IS YOURS IF YOU CHOOSE TO SERVICE YOUR OWN REPLICATOR. Refer all service to qualified personnel. Steps: Read and understand all steps before proceeding with any work on your Replicator. 1. Solder the header pins to the Pololu #1182 PCB, omitting the pin at the MS3 position. 2. Install a jumper between the MS3 and GND solder pads. 3. Set the trim-pot to the full-CCW position to set the motor current to 0-mA. 4. Place the Pololu PCB into the Replicator, ensuring that the GND and Dir pins are at the same side as the original Makerbot PCBs. 5. Connect a digital volt meter between the GND and motor current test point. 6. Turn on the Replicator and measure the voltage; it should be roughly 0-Vdc, indicating 0-mA of motor drive current. Shut off the Replicator immediately if the voltage exceeds 0.4-VDC. 7. Turn off the Replicator and remove the Pololu #1182 PCB. 8. Turn the trim-pot slightly CW to increase the voltage and consequently the motor drive current. 9. Repeat steps 5 through 8 until the voltage reaches approximately 0.400-VDC (800-mA motor drive current). 10. This completes the repair; test using the load/unload filament utilities. Print Settings: Printer Brand: MakerBot Printer: MakerBot Replicator Rafts: Doesn't Matter Supports: Doesn't Matter