"Many Omni 1 and Omni 2 keyboards you see for sale are toasted in some way.Apparently this is due to a badly designed power supply which generates transient voltages when you turn the synth on and off. Over time, these power spikes fry the synth's chips and capacitors. ARP later figured this out and sent service bulletins to all their technicians instructing them to perform a mod on any Omnis they repaired."
Click here for copies of the mods and more info.