Question: Q: Power Mac 5500/ 225 will not start-up

It's likely the pram battery if it's been sitting around. At least it's a cheap place to start.

The slide-in motherboard in a 5500, like some other Performas and Power Macs of that era, engages a long slot inside the computer's chassis. Over time, the upper and lower gold-plated pins on those motherboards can oxidize. To improve the connection, I usually remove the motherboard and gently scrub those pins with a soft, white eraser. You need to be sure and blow off any residual rubber shavings from the pins, before reinstalling the motherboard. While you have the motherboard out of the chassis, you'll need to remove its 4.5-volt alkaline battery. It is constantly drained after the computer is unplugged from electric power. After it is fully depleted of any charge, those batteries often leak alkaline solution onto the surrounding motherboard components. Hopefully, that's not the case with your 5500. The battery is a Rayovac 840 — a small black cube with black and red wires that connect to pins on the motherboard. It has a fuzzy velcro pad that grips a plastic-toothed base that's stuck to the motherboard. I use a non-conductive popsicle stick to apply downward pressure on the toothed base, to avoid pulling it off when removing the battery. If the plastic base pulls off the motherboard, a new battery usually comes with a replacement.