You might want to get a multimeter and check your power adapter. All four center pins provide power, with the outside ring as ground. If you search around on here you'll probably come across a post listing which pins provide what voltage. Or if you have an Apple service manual, you could use that.

When my 540c stopped powering up, it turned out that it was a bad fuse (next to the AC plug) in the power adapter. I don't recall any power line anomalies around the time it died, but then the thing is well over ten years of age, so maybe it just got tired and gave up. Anyway, I had to solder a new fuse in there, but the holder it uses is very tight; I broke the old fuse getting the end caps off, and they wouldn't fit on the new fuse, so I just soldered two bits of wire to the ends of the new fuse and soldered those to the PCB. It works great now.

Unfortunately the power adapter wouldn't come apart without disfiguring the casing, so mine is now coated in duct tape with an Apple logo drawn on with a Sharpie.