I remember the 70s.The ground thing is critical. The electrical fields in the rat’s nest of wiring behind a receiver can make that hum tough to crack. It’s a good idea to begin with the turntable as the first item you connect to the receiver. If the “hum” is there through your headphones when the turntable is the sole component, you can play with the cables to attempt to reduce the effect. With the addition of each component, don the headphones and check for the hum. Check with the new component switched “on”. Save the speakers for last. Sometimes the fault lies with the sensitivity of the pickup in the phono cartridge itself, and those things designed to track at a gram or less, well no 2 of them are the same. As stated above, you want to attempt to keep signal cables ( as much as feasible) away from power cables. In addition, you should make liberal use of contact cleaner on those connecting screws and RCA plugs and jacks on circa 70s equipment before installation. The other thing that’s critical about those low tracking turntables is that you MUST isolate them from vibration. Mounting your turntable on a rack or platform that in any way touches your speakers is just asking for “rumble”.