Actually a Velleman K4000 kit stereo valve amp from 1994. Not just hum but mains noise, eg dimmer spike noise. It would seem to not have a proper grounding point. Specially thickened ground lines on the ps board, large ground interconnects to the preamp and main amp 0V lines and input sockets insulated isolated from chassis which is bonded to the 3 wire mains input at the IEC socket. The kit construction literature makes no reference to connecting the thickened ps ground lines to the mains bond point and seemingly relying on earthing of whatever is used as a non-integrated preamp or external source. What is the point of reinforced ps ground tracks if they conduct current nowhere , only via signal line braid, so a safety issue as well Surely there should be a heavy strap between the ps ground and the mains ground point ? I only have the amp for attention which is line level input to speaker output so no volume or tone controls.
-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on