iMac G5 20" Black screen; good logic board

iMac G5 20" 1.8GHz, OSX 10.5.8 Had bad caps; logic board professionally repaired and tested all good. Fresh power supply. Interior led test lights all on and steady. Fan & hard drive running. Keyboard seems to work; I hear expected noises when hitting volume buttons, etc. Just nothing on screen. Totally black. It was working fine before power supply failure and bad caps issue. I double-checked internal cable connections and see no apparent damage to plugs, receptacles or cables.

For purposes of this question, let's assume (because it was tested by a pro) that logic board is fine; also because all other indications are that computer is working correctly except for the dark screen. Which, to reiterate, was working fine before computer went down due to a combination of power supply and logic board issues. BTW, all caps were replaced, not just bulging ones.

1) is it possible that somehow the brightness adjustment got pushed all the way down? If this is possible, is there some way to adjust it without access to the on-screen menu in system prefs? Internal or external?

2) If that is not possible, then how do I troubleshoot this? I understand this much about the screen: there is the led screen, a backlight, and an inverter. I don't understand how the inverter works or what it does; could it be bad? I also don't know if the backlight is part of the screen assembly or separate, or whether there is a way to test if that is the problem. I tried looking at the screen from all angles in a darkened room and cannot see any light whatsoever.

Any help with this will be greatly appreciated!

Judy