The Nikon SA-21 film strip feeder has a limit of 6 frames, where the SA-30 handles up to 40 frames. There is really almost no difference between these two devices, except that the SA-21 is free and the SA-30 costs extra money. After doing this conversion, you'll be able to scan film rolls past the limit of 6 frames, but still use the film strip feeder for the shorter strips of 2 to 6 frames (I'm using mine in both modes.)
Here's how to convert the SA-21 to handle strips of any size up to 40 frames:
Remove the inner assembly from the outer skin of the SA-21. Unscrew about 9 screws from the top of the assembly, including 2 that are tucked under the steel wing spring tab (photo, above). You'll pull out the plastic frame, as shown, to reveal the final 2 screws.
Lift the top of the assembly and remove the flex cable from the circuit board. The flex connector has a pressure clip that pops up to release the flex cable. (Later, when re-attaching the flex cable, you'll push the pressure clip back down to lock the cable in place.)
Unscrew the circuit board, lift it up to reveal the solder side, as shown above. Locate the two solder bumps adjacent to the "N" in "Nikon" (above, far right). Use a precise solder tool such as Metcal soldering station, join the two solder bumps together. This will indicate that you have an SA-31 instead of SA-20.
I have found virtually no difference in using the modified feeder, other than the limit of 40 frames instead of 6. There is no cause for alarm!