Hi, I have an amp Nad T-743 model and I have a problem about this device. The optic input does not work. It worked before but it has not worked recently. In the past, when I closed and the re-opened the amfi, it started to work again. Now it is completely our of order. There are both optic and Coaxiel inputs in the card of amp. Coaxiel works but optic input does not work. I reset the device but the problem goes on. What can it be the source of the problem?