I have had this Quad 520F for many years now and it has developed a fault. Buzzing (not hum) particularly in channel 1 (-12.5dbu) and some in channel 2. I have replaced all the elec caps to no avail. If I lift the earth off the transformer there is a 10db improvement. I have tried shorting out R40, still no improvement. Should I send this amp for repair and to whom?