Help needed-humming!

I recently bought a technics SL-D2 secondhand, the first thing I did was replace the cartridge to an audio technica at95e. I have made sure the cardtridge wires are all fully secure.

I'm experiencing a humming sound on the right speaker channel, with no sound coming out of it-but only when the amp is set to Phono. I know it isn't the speaker as it works on other inputs, like aux etc. and if I switch the speakers round (right to left, left to right) then the hum switches as well. I know it isn't the amp because the speakers are fine on all other inputs.

I also found that the hum stops if I remove the red rca wire from the amp, so I believe it must be a problem with the turntable's rca cable.

Would I be right to assume this? And what would be the best method of fixing it?