I sprayed cleaner in the volume control, hum still present. Something
input else... the hum is audible no matter what input is selected from
the switch, FM, AM, Phono, tape/aux. But there is a Monitor switch
and when switched from Source to Monitor, the hum ceases.

This suggests that there's nothing wrong with the power supply, as the tape
monitor is part of the overall input-selection circuitry.