If no working unit is available, consider to double-check for yourself if the battery used to buffer the clock chip and PRAM is in proper condition. I have not ever seen any broken LC power supply or mainboard, but even Apple computer sellers swapping the whole 475 mainboards instead of the battery. The 475 sometimes is misdiagnosed as dead because of a discharged lithium cell. This original 3 Volt (3,6 Volt) lithium battery is expensive, but a pair of two cheap 1,5 Volt alkaline cells in series will do, as well (batteries of different sizes can be used using an also cheap clip holder connected by wire to the original holder; just mind the polarity).