I once put a voltage regulator (built on a mini breadboard) in my mac. I powered it from a 9V PP3. It was terrible with bits of prestick and sticky tape amid wires running from end to end across the top of the hard drive :O ! But before that, it was worse. I had a small phone charger-style transformer and I used that to power the regulator and had to leave the case of the mac off 8-o !