The SG3524 pulse width modulator is used on the daughterboard in the "DataPower" variant of the Lisa Power Supply, often referred to as the "1.8A" PSU due to the rating label.
This component does sometimes fail, but is not often a problem.
This part is responsible for regulating the +5V used for the PSU's internal logic (it does not exit the PSU), as well as being the comparison reference used for adjusting the PSU output and detecting over-voltage and over-current conditions.
The datasheet formula for the oscillation frequency predicts 51.5 KHz with the components used.