Phono3S it's humming again.

[bikerhifinut]

The Phono3S has a rather nasty 50hz hum on the MM input.
As far as I can ascertain, there's no hum with the inputs shorted out on the Mute setting and even open circuit on the MC setting there isnt a hum issue. I tried the arm leads plugged in to the MC input and again no hum issue even with a MM cartridge in the arm.
The issue seems to be as soon as the arm leads are plugged into the MM input an awful hum ensues.
My suspicions are that there is something amiss with the relay board, perhaps a faulty earth return on the Mm sockets or a dodgy relay?
I am sure it isnt a positioning issue, there has never been a problem previously with the layout and the big mains transformer on the power amp (WD 88VA) is well away from the sensitive phono stage.
Likewise the arm earthing is fine. I also tried a separate earth lead from the arm base to the earth post as it's a Rega arm with the earth incorporated in the LH arm lead. Just to make sure I swapped in an arm with a separate earth wire and the issue is still there.
So where do i start with the fault finding?
I was tempted to wire up a pair of Floating phono sockets directly to the input pins on the PCB on the assumption if that eliminated the hum then the problem does lie with the relay board.

Andy.