I did already say that we are using a converter box. This coverts HD-SDI to HDMI.
I am using a similar SDI to HDMI converter without any problem. It's best for you to troubleshoot the fault this way:-
Try bring your SDI camera to a shop that sells SDI DVR, see if they willing to let you try, if that works that means you might have gotten yourself a spoilt converter.
Also, you can bring along the converter and test with their SDI Camera. BTW, where did you buy the SDI PTZ?