Servicing Mission 3A, flown by Discovery for the STS-103 mission, was a split-off from Servicing Mission 3 after three of the six onboard gyroscopes had failed. A fourth failed a few weeks before the mission, rendering the telescope incapable of performing scientific observations. The mission replaced all six gyroscopes, replaced a Fine Guidance Sensor and the computer, installed a Voltage/temperature Improvement Kit (VIK) to prevent battery overcharging, and replaced thermal insulation blankets. The new computer is 20 times faster, with six times more memory, than the DF-224 it replaced. It increases throughput by moving some computing tasks from the ground to the spacecraft, and saves money by allowing the use of modern programming languages.
As a side note, this was Scott Kelly’s first spaceflight. Scott (@StationCDRKelly), now a veteran space flier of 3 missions, is now serving on a one year long duration mission aboard the International Space Station.