Never seen one, but circuits are circuits, they all work the same.
120Hz hum is power supply ripple. It MAY be from lack of filtration, but many many times it is signal grounds sharing power supply grounds. the return current for the supply gets into the signal circuits.
Before you wholesale replace a lot of caps, just tack a largish one in parallel with the existing filter caps. If that makes an improvement, then your caps are either worn out or were just not large enough to begin with. (very possible)
How is your signal ground connected to the chassis ground? It seems they are not bonded together listening to your scope experience.
How about a schematic of what you built? How does the negative side - assuming that is the grounded side - of your powr supply get to ground?