I would suspect the fuse on the incoming line pin maybe blown or the filter capacitors are going bad. The Synchrolinc looks like it may use a switching power supply and have the same issues as the 2413 PLM, 2450 I/O Linc, hardware V2 2443 Access Points.
I hope you can find some for your needs.
If not have you seen some of the threads on using a current sensor and an I/O Linc? Not as compact and neat but may work.