on those 300s, I be worrying about the power supply filters. They have a known history of drying out due to the heat they are exposed to.
I would replace those big filter caps (check mouser or digikey for comparables) then reflow all the octal socket tube connections. Should be good after that.
I would replace those big filter caps (check mouser or digikey for comparables) then reflow all the octal socket tube connections. Should be good after that.