Analog signal receiver changed

Yesterday we found out the analog signal sent through the fiber from the end room had some problem. No matter what signal we sent, it was always -7.5V. At first we thought maybe the problem was from the fiber, so we decided to change the fiber, but the receiver(pic 1) we are using now have a special port, other fibers cannot be used. Other possibility is the receiver or the transmitter had some problem, so we took the transmitter from the west end to the south end. Sent a signal from the west end transmitter, the original fiber and the south end receiver, with this configuration we have a offset about 70mV. So we also changed the receiver to the west end receiver, now everything is fine.