Is the humidity lower in the evening than during the day (HVAC running more)? If so you may be experiencing static buildup increasing surface noise.
Do you live near large transmitting towers? If so you need a Faraday cage, maybe only around the phono preamp but possibly around the entire turntable and phono preamp.
Not sure, how to isolate if some particular appliance is causing an issue with the main. I don't live near a tower either. I tried @JK47's advice and converted the phono with tinfoil. Then I raised the volume a bit without playing any record.
I could still pick up the stray radio signal. Then i disconnected the TT from phono preamp and the radio signal did go away(almost). That would mean that the TT is somehow acting as an antenna. Or the signal is coming from the mains. I have the TT plugged directly into the wall socket.
Try unplugging from the TT side and see if the radio station is still there to rule out the RCA's (may need better shielding? wrap foil around them) or the TT (maybe picking it up from the tonearm and head shell/cart leads?)
@JK47 I was able to diagnose one thing. If shield the RCA cables from phono to the amp, the radio signals do subside. Do you have any suggestions for decent shielded RCA cables. Tin foiled cables really look ugly.
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