I know a couple of electronic engineers who sometimes take on projects like this just for fun. I've seen them trace and reverse-engineer circuits and do board level work, changing small parts and ICs to get the gadget working.

Decades ago that kind of repair was actually the standard. It wasn't just for fun, people made a pretty decent living driving their vans to other folks' houses to fix their big, heavy 23" and 25" console television sets (and before that their big console radios).

In 1970, a Motorola "works in a drawer" Quasar color television cost $600. That's about $3,800 in today's dollars! When you spent that kind of dough on your electronics, it made sense to call in a repair tech and spend $50 or $75 to get it fixed when it quit working (and quit they did, fairly often).

I still see a few old-timers who work on television sets and other similar electronics, but they're nearly extinct in the US. These days electronics are much more reliable and longer-lived than they were 40-50 years ago, but when they do finally fail, most people just junk them and buy new.

(As an aside, though, I was chatting a few months ago with a friend of ours who lives in South Korea. She said she'd taken the day off work to wait for the television repairman! So while they may be obsolete here in the US, I guess they survive in some other places.)

To get back on topic, what you can do yourself to troubleshoot that intercom is going to be limited by the depth of your electronics background. If you understand electrical safety, know how to use test gear, and can read a service manual and schematic, you can probably at least find and test the power supply.

But you still need to be able to get the manual and spare parts. That in turn requires that the manufacturer still exists and still provides service info and parts for their older products. (I can't see the brand name on yours in the picture.)

Much as I hate to say it, probably your most practical solution is to buy a new intercom. Nutone, for one, has been around since the 1930s, and IIRC has been making intercoms since at least the 1950s. I expect there are other brands too.

In the remote connection (second photo), there is one 4-wire cable. In the main unit, I don't see that one, but many more. Have you shown the complete system? I would uninstall both units, keeping track of where the interconnecting wires go, and bench test it. If it doesn't work, then probably best to get a new one. If it does work, then something is wrong with the wiring. Happy tracing!

So does it HAVE a fuse or battery that you can see? The internal pic is a little dark and not big enough to see much detail.

How does it get its power? Is there a cord for 120V power, or an adaptor or transformer?

Actually, looking at the pic closer, the big black square at bottom right says INTERSTATE BATTERY on it. This is likely a sealed lead acid gel battery that may power the system as a backup if the power is off. The system actually may not work without it, and they do wear out just like car batteries and won't hold a charge. It has to have a charging circuit and the unit probably has an internal charger with 120V power from somewhere. Find out where the 120V AC is coming from and verify that it's got power. If so, you could invest a few bucks and replace that battery and see if it springs to life. No guarantee that this is the only problem since I really have no idea how the whole system is designed.

For a cheap test you could disconnect it and connect any charged 12V battery such as one from a riding mower, motorcycle etc. If it works, get a new battery. If not, something else is also wrong.