marcos

hi there i dont know if anyone can help me....i have a sirius 500 second hand desk bought off ebay.....the desk works fine with most my fixtures but when i set up some fixtures like martin 518 pro it works when you first set up .but after the lamp wont come on i tried a differant fixture (varytech) and the same thing happens all the controls work fine like pan tilt etc...but i cant get the light to come on. am i missing something very easy here. please help me.