Hi - I too have a Melos SHA Gold with exactly the same problem. In my case, i have tracked it down to a unique PLA chip (near the balance pot), and have thus assumed that it is irreplacable. However....the 2 relays that control the mute are on the pho-teniometer board and are well worth checking, as is the diode and the driver transistor for the relays. Remember, if the tranny is not switching, it is highly probable that the diode is dead too, so replace it at the same time as the transistor.
In my case, I am going to rip out the entire board, replace the volume pot with a decent ALPS one and hard wire whatever I have left. I'll let you have the details when I have done, just in case you end up following my path.