Troubleshooting 101 like johnp says. Unplug everything. Go to 66 block. DT or NDT?? DT--plug one device in--NDT --go to each jack and take wires off of the jacks. Keep going---step by step. Slow.

I've had customers kids put crap in an unused jack--shorted out all of the lines. I've had a fork coming out of a jack one time. Goofy.

Troubleshooting 101.