Thanks for the reply!
All the cameras work on channels 3 and 4.
The problem seems to be with the unit itself. I can unplug the video cable from channel 3 and attach it to channel 1, but no picture appears on channel 1.
The video leads connect to the back of the unit with BNC connectors, and then to the cameras with "phono" plugs.
My main need is to find out what make this machine is, and then to get "admin" login. At the moment I only have a basic "operator" sign-on, so I cannot do anything with the set up.
Anyone know who makes this device, or what a default signon is? Or how to do a total "factory reset"?
All hints gratefully received!