Sometimes the hum is a little farther down the path.You might try removing the headshell and buffing the spring loaded(arm) side connections with a pencil eraser.Also try pinching the RCA connectors to ensure that you are getting good ground.I have found this to be a problem with several Thorens tables I have owned.Good Luck.
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I would first try redressing the cables. My 'table will do the same exact thing, sometimes.
Then, try running an additional ground wire from a non-moving metal part of the tonearm to your ground at the preamp. That's worked for me, before, too.
I think your leads may be acting like an antenna. But, I don't really know how this stuff works, to be honest.
Some great ideas here. I should have mentioned the hum is present even with the power supply disconnected. I looked at the inside of the RCA plugs, and since they looked tarnished I rolled up some sandpaper and sanded the interior contact surfaces, and then reconnected them. I took care to bend them inwards a little so they fit tightly. Better, but still a bit of hum. I then switched various power cords, no change. But when I removed the ground wire, dead silence. Interesting! But when I connect the power adapter (which is an 120V AC to 16V AC) the hum went through the roof. The ground wire then reduces the hum to what is was before.
So I am stumped. I am going to look for a new AC adapter. Fortunately 16V is a standard size for home alarm systems.
It's the [email protected]™ phono board!
After hours of research, countless power cord and interconnect changes, replacing the AC transformer power supply, etc, I tracked down the source of the hum.
It's the board, or at least the circuit the board is connected to. There is hum only when the RCA cords are connected directly from turntable to these jacks. When I coonect my turntable through another input jack using my Rotel RQ-970BX pre, the system is dead silent, as are all other input sources.
Is the Arcam phono board a POS, or is it installed wrong?