My IBM 5154 had some blown capacitors in the power supply (the AC line filters). It worked, though I decided I should replace the caps in the power supply just to be safe. I went ahead and replaced all of the electrolytic capacitors as well (they were out of spec).
Put it back together and it works, sort of. I can clearly see the picture that is supposed to be there but the blacks are not black in EGA graphics mode and there is an undulating wave. In non-graphics (text) mode, the blacks are black but the undulating wave is still there.
Here's a video. How can I further troubleshoot this?