I have a NuVista preamp and at one time the selector switch was causing a similar problem. It was resolve when I opened up the preamp and gave the switch a shot of cleaner. (this issue also happened with a marantz preamp I had years ago).
The only other "simple" check is to verify the issue isn't with your cartridge. If you have another table you can plug in this would be a quick check. Or plug your table IC's into another input and see if you have a signal from both channelsvfrom it. It will sound bad and faint but you should may be able to hear if you have a signal. If the table is the source, the solution may be as simple as remove-clean-replace your cartridge headshell leads.
If none of these work you may either have a bad cartridge, or something more significant going on with your NuVista. My money would be on the selector switch being your problem.
The only other "simple" check is to verify the issue isn't with your cartridge. If you have another table you can plug in this would be a quick check. Or plug your table IC's into another input and see if you have a signal from both channelsvfrom it. It will sound bad and faint but you should may be able to hear if you have a signal. If the table is the source, the solution may be as simple as remove-clean-replace your cartridge headshell leads.
If none of these work you may either have a bad cartridge, or something more significant going on with your NuVista. My money would be on the selector switch being your problem.