"Symptom Charts/Video" - slide/pg. 8: (http://tim.id.au/laptops/apple/legacy/quadra.lc_630.performa_640.pdf)

Symptom: "Screen is completely dark, fan is not running and LED is not lit"

Resolution:

"1) Check all external power connections.

[the relevance of 2 in this case is: is the keyboard for sure good?]

3) Reseat logic board.

4) Unplug 4.5 battery, wait 20 seconds, plug in battery, and restart computer.

5) Verify that monitor has power.

6) Remove expansion card, if present.

7) Replace power supply.

8 ) Replace logic board.  Retain customer's SIMMs."