I recently blew out both extruder motor stepper drives on my Makerbot Replicator Dual Stepstruder. I won't go into the reasons why but wanted to show my repair.

I don't think Makerbot sells parts for this unit (at least doesn't show them on their web site. I didn't call and ask) but would expect them to be quite expensive if they did.

I knew the stepper drive was blown when the stepper motor would just jitter back a forth a single step and the circuit board was extremely hot.

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 appear to be exactly the same board with one exception. The Pololu #1182 PCB uses a trim-pot to set the motor current where the Makerbot board uses a digitally (software) controlled voltage to set the motor current.

I don't think this is a big issue as it only needs to work, no acceleration is used on the extruder drives so this was not an issue.

Because of the difference in how they set the motor current, the pin out on the two boards is slightly different. Notice the "VR" pin on the Makerbot board is labeled "MS3" 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 that pin when I soldered in the pin header (included with the Pololu #1182 PCB) and placed a jumper wire between the "MS3" solder pad to GND solder pad. MS1-MS3 is used to configure the step/rotation.

To set the correct motor current, I guessed that it should be about 1-amp or less. These boards are able to supply 2-amps with a appropriate heat sink and 1-amp without a heat sink. Since there is no heat sink on either board I decided to set the motor current to a conservative 800-mA.
According to the Pololu instructions, you do this by adjusting the trim-pot so that the voltage on the test point is 1/2 of the desired current (don't get lost in the weeds between mixing volts/current here). For 800-mA (0.8-Amps), I set the trim-pot so that the measured voltage was 0.400 Volts.

While this repair worked for me, I do not suggest anyone else do it as you would be exposed to dangerous voltages and temperatures and possibly further damaging your Replicator.

ALL RISK IS YOURS IF YOU CHOOSE TO SERVICE YOUR OWN REPLICATOR.

Refer all service to qualified service personnel.

Steps: (Read and understand all steps prior to doing any work on your Replicator).

1. Solder the header pins to the Pololu #1182 PCB, don't install a pin at the MS3 position.
2. Install a jumper between 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. Note the orientation of the Pololu PCB, make sure the GND and Dir pins are at the same side as the original Makerbot PCBs.
5. Place 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 the Replicator off immediately if the voltage is above 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 4 through 9 until the voltage is approximately 0.400-VDC (800-mA motor drive current).
10. This completes the repair, test using the load/unload filament utilities.