Yep, definitely the Power supply by the lookooks of things. It's a bit more involved and daunting than the logic board but if you don't rush things, be organised with your workspace and get familiar with things first, you should be right. The hardest part is realistically seperating the top and front case plastics from the inner bezel without breaking anything to access the PAV board. WHilst I say this, beware that there is the potential for there to be residual high, possibly deadly voltages present in some components of the PAV board so you must have an idea of where these may be and be dilligent about where you put your fingers or tools.

This link to an entry on my blog will give you a pretty foolproof step by step as far goes taking the case apart and reassembling it later. It doesnt go into details about the PAV board as for the project that blog entry covered, this wasnt removed. My advice would be to google a teardown of the slotload iMac and also do some reading up on the procedures you should observe to safely work around exposed CRT's, CRT analogue circuitry and other high voltage areas such as Power supplies.