I got my hands on a Replicator 2 and the MightyBoard Rev H with a missing botstep and 3 blown BOTSTEP17Es ($50), so I went the Pololu 2128 S4988 Stepper Motor Driver, Black Edition ($6.95) with the 68mΩ sense resistors, which is the version made since Jan 2017.
I took voltage measurements of my working Rep 2s (Mightyboard Rev G & H) and carefully studied the schematic and datasheets of the A4982 (Botstep17E) & A4988 (Pololu 2128) to determine the following conclusions:
1. MS1 & MS2 are given +5V on all BOTSTEPs, which means the Mightyboard is doing 1/16 steps. MS3 on the A4988 should also be tied to +5V.
2. The Z motor Botstep's Vref into the A4982 is being fed 0.70V (Mightyboard Rev H). This equates to an Imax of 0.33A. Mightyboard Rev G measured 0.73V => 0.34A.
3. To get Imax on the Pololu 2128 (Jan 2017 or later version) to be 0.33A, adjust the potentiometer to 0.18V.
4. The X, Y and Extruder motor Botstep's Vrefs into the A4982 are being fed 1.68V (Mightyboard Rev H). This equates to an Imax of 0.78A. Mightyboard Rev G measured 1.70V => 0.79A.
5. To get Imax on the Pololu 2128 (Jan 2017 or later version) to be 0.78A, adjust the potentiometer to 0.42V.
6. The A4982 and A4988 are basically the same chips, performance wise (most values are identical, including RDSon and have the same package thermal characteristics). The key differences are as follows: - The A4982 has a sleep mode - The A4988 has a 1/8 step mode and a 3rd mode select pin (MS3) - Both chips are available in difference IC packages
Tips: a. Getting to the Vref trim pot is impossible when put into breadboard or Mightyboard. Must use trial by error to get the voltages correct.
b. Remove the header pin for MS3 and jump MS3 to MS2 with a small wire or solder blob.
c. Label the Z axis module as Z Axis Only with a silver sharpie
d. Put heat sinks on the X, Y & Extruder modules.
WARNING: The reason the Mightyboard is feeding Vref a voltage to the BotSteps is to allow it to adjust the hold current on the stepper motors. This lets them save power during heating up the extruder. So while the extruder is heating up, the steppers can be drawing less power. This can be seen in the start.gcode with a G130 command.
I might not recommend these replacements drivers if you have a heated build plate and the stock 150W power supply. The power draw for the build plate, extruder and steppers may exceed what the power supply can deliver.I would advise using a stiff 24V 300W or higher power supply.
These have been working fine in a Replicator 2, no heated bed with stock power supply.