A4000 does nothing on power-up; keyboard light, power light, and hard drive spin-up show that power supply is working, but screen is gray or black, and disk drive doesn't click.
Possible cause: the processor board is not firmly connected to the mother board, or has worked its way loose (the power LED may flash quickly in this situation). Reseat the connector strip that is parallel to the floppy drive. Since this connector is quite long, it is possible for the ends to be firmly attached while the center is not making adequate contact. Be careful not to flex either board too much while reconnecting them; using a large rubber eraser or other flat object will protect your fingers while pressing the boards together gently. In extreme cases, it may be necessary to use a contact cleaner to clean accumulated gunk (like silicon heatsink compound) from the connectors.
Possible cause: expansion cards not inserted fully or properly. Remove and reinstall.
Possible cause: the motherboard is not supported very well near the internal IDE connector. Solution: gently lift the motherboard near this connector. Installing an insulating support underneath the motherboard near here will be a more permanent solution.
Possible cause: badly-trimmed pins of the soldered power supply connector have poked through the insulating plastic under the motherboard and shorted to the case. Solution: trim the pins and install more insulating plastic.
