My C128 is dead :(
It was having problems being turned on and off, which turned out to be the power switch having horribly carbonized contacts. I desoldered the switch, cleaned it and soldered it back on; Which made the computer reliably turn on, but now the screen only shows grey "jail bars" and won't do anything else. It was doing this before I fixed the switch occasionally too.
I checked the 9VAC and 5VDC on the motherboard side, and both seem to be getting past the switch and to at least past the bridge rectifier, so I don't know what's wrong.
Maybe some of the parts are so old that them getting proper voltages causes them to fail.