Time to pull out the tools. Remove the devices and find the part number(s). Those aren't always obvious to laymen, (such as saying my Nortel system is a M7310).Your profile states you hobby as working on all systems so here is a new challenge for you.
The mechanics are identical with everything electrical that you have worked on previously, so troubleshooting is the same.
You cannot install an additional device without being onsite and find out first hand what you are working with. And then research Nutone Website
for the proper devices and possible replacements for legacy equipment.