
It could be one of two things. The cable connecting TV to amplifier is too long (unlikely) or you are connecting the input of the amplifier to a switched out output or input of the TV (more likely).
If the phono leads are over 13m long or are long and very very tightly coiled, the resistance or the inductance (respectively) of the cable will get in the way of the sound. Move the amps closer, or uncoil the cable.
It's more likely to be the TV end. Check the back of the set to see if the phono sockets have a plastic logo with an arrow pointing in or out of the little circle. If it's out, it should be fine; if not, not. Assuming this is okay, check the manual or the manual to see if there is an audio output option on the menu.