Musical Fidelity M3si Phono stage hum

Hi Guys wondering if there is some advice on this amplifier and the phono stage.

A friend asked me to look at an amplifier that seem to hum on the phono stage. Now all I have is DJ decks and I plugged them in and indeed it hummed.

I swapped round the cables and the hum went from left to right channel. So I presumed its my decks and I swapped it for the other deck and still the same. I was now puzzled as it seems to indicate I have faulty leads/decks so I then accidentally removed the the separate ground cable and the hum completely disappeared.

My DJ days are done but I remember it only used to hum when the ground cable was NOT plugged in and not the other way round!!

Is this normal?? Any advice from someone with experience on this would be greatly appreciated!
 

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Hum can be weird like that. One integrated I have hums if you connect phono ground, another hums if you don't and it goes away if you connect it. Anyway it doesn't indicate a fault as such, just leave it in whichever configuration works best.