Here's what it says in the Service Manual:

Symptom:

Fan is running, LED is on, drive is accessed at startup, but no startup chime and screen is

black

1. Check jumper block J28. Make sure setting is correct for type of power supply

installed (see “Logic Board” in Take-Apart chapter for more information).

2. Reseat ROM DIMM.

I'll try this in the morning. Funny thing, it hasn't been messed with, oh well.