I think you should fear treading into the great unknown world of repairs without adequate preparation MORE than you should fear sending your radio into the great unkown world of specialist repairmen.
That said, if you are determined to repair it yourself, that's great, too.
Just be prepared to LEARN FIRST, by reading enough and practicing enough to acquire sufficient knowledge and skill. If you don't already understand the basics of electricity and electronics, you will need to learn this, as well as the correct and safe techniques for troubleshooting and repairing old gear. Start with books on the subject from the library or bought from Amazon. Get the relevant "Sam's" manual for your unit and/or a service manual before you begin, assuming you can find them.
It is fascinating and fun to learn audio repair, but it also takes time and effort. Be paitent and disciplined. Properly learn the theory, then assemble the right tools and manuals and learn the techniques for using them correctly/intelligently, and you'll end up with a very satisfying hobby, one which can become a profession. If your goal is JUST to get this unit working for your grandparents, and not add this art to your life, it probably isn't worth the investment of time you'll have to put into it.
If you are in a hurry to give this to your grandparents and you don't want to invest the time "doing the homework", then I'd suggest you should let an expert restore it properly. As Sandy G pointed out, there are voltages in that radio that could kill you, and you could just as easily "kill" the radio.
Hope I don't sound too negative; I think it's great when people repair their own gear. I do my own repair work, and I'm still learning to fix these things, having a blast learning as I go along. But I also have a related background/education going back for years, which makes it easier. I remember a few near-fatal mistakes early on (years ago), and just hope to save you similar experiences.