In the alarm industry, when we refer to a "Control Panel", we're talking about the "brain" of the system, a PCB located in a steel cabinet somewhere, usually in some out-of-the-way place like an unfinished basement, closet, or utility area. What you're talking about is 3 Keypads, the interfaces between User and Control Panel (or just "Panel"). May seem nit-picky, but in troubleshooting, it's really helpful if we're using the same terminology.
If one of your Keypads has the power led off and the other two KPs show it on, it probably means you have one malfunctioning KP. The Power LED shows that your Panel (the 8980 PCB in the steel cabinet) is getting its nominal 16.5 VAC power from your power transformer plugged into an outlet somewhere. Which it probably is, if two of your KPs are showing it is. The malfunction could be as simple as a bad LED, or it could be something more complex. How handy are you at troubleshooting? It's possible that your errant KP can be repaired, but I wouldn't bet on it.
Keypads are not considered field-repairable by dealers and are usually just replaced, but a KP that old is going to be hard to find and I personally would be wary of any replacement you might be able to find on eBay or wherever: Whatever any replacement's source, those KPs went out of production ~25 years ago. Since KPs have moving parts (the keys & their contacts), their lifespan is finite, and your other two KPs are on borrowed time. You might be able to find a replacement on eBay or similar, good enough to keep your system going another years, but it's rolling dice, and new/old keypads are usually expensive enough to make me think of buying a new system.
Congrats on maintaining your system for 28 years, but it was never going to last forever. I'm personally a big fan of keeping old equipment working, but I'm afraid you may be looking at using your system with just two KPs, or replacing it with something newer. I'm happy to offer advice if you go that way.
