Have a problem with a home security system. Any suggestions?

I have an older security system. Exxis is the brand with telephone connectors and wire not coax. I get audio but no picture from any of the cameras, three set up. I'm guessing it's a problem with the monitor and would like to know if it is worth repairing or just replacing. Any suggestions on how to find the problem myself? Three cameras with motion detectors run through recorder. Have contacted Exxis and the monitor is obsolete and they, of course, suggest replacing with one of their current models.