[Reece]

Let's track down this buzz.
1. Do other radios working off the same wall receptacle buzz? If so, could be something in the house: light dimmer, phone charger, shaver charger, etc. etc. Could even be at the neighbor's.
2. The AC switch is on the volume control, which is hum sensitive. How are the wires to the control routed? They should be tightly twisted to reduce hum and routed away from audio grids.
3. Did you replace the cap from volume control to the grid of the first audio with a new cylindrical cap? Routing of this cap may be critical. Some radios shield this cap. Try this: wrap some foil around the cap careful not to short to the end wires, fasten with tape, and connect foil via test clip to chassis ground: hum may be reduced. If so, devise a permanent connection for same. Sometimes where space permits, placing this cap tight against the chassis has a similar effect, the chassis acting as a shield.

If you could post an underchassis view, maybe two, up close of both sides of chassis, we might spot something.