[Heathkit] GR-78 modulation hum

[SX-25]

Hello all,

Has anyone any experience dealing with modulation hum on the mid-70s vintage Heathkit GR-78 portable shortwave receiver? As with most modulation hum it is prevalent on stronger stations and the greater the signal strength, the greater the hum. I have checked out the usual things, bonding of the tuning capacitor to ground...voltage being supplied to the front end and removal of AC charging and allowing it to run just from batteries. In my tube type radios modulation hum is often cured by arrangements of filament to grid bypassing, but obviously such is not the case here. Of course, my wire bundle may be the culprit, although it is tightly dressed and routed precisely as Heath described in their assembly manual.

Anyone have any other ideas?

Thanks and hope all on this reflector have a safe, healthy and happy Christmas in peace.

WPE9GHF