The 2230 is one of my least favorite Tektronix scopes to work on. You have to remove the top row of front panel pushbuttons, then remove the screws holding the top board, swing the board away, remove some more brackets and hardware, remove the power supply cover, and gently maneuver it out. Be careful with the screws on the back panel securing the power supply cover - they sometimes are frozen after years of use and are easily stripped (I had to drill some of mine out).
Might I suggest two things to aid in your effort. First, use your phone/camera and take lots of pictures as you go. Second, get a small tray with compartments and put the screws in them in a logical, labeled manner. Several of the screws in the same disassembly step have different lengths.
As others have indicated, it's most likely the two Rifa Y-suppression capacitors. A two minute job to replace after you've spent an hour getting to them.