Sometimes, re-plugging in the monitor wall power cord to the wall quickly turns on the picture. Out 1 second back in. Like flicking a wall breaker. May take a few tries. Look for the screen flicker.
When you get frustrated unplug the Apple computer wall power cord (not the monitor) and walk away.
Even with the refurbished boards it just seemed like the capacitors needed some time to energize and learn.
I do have a spare power supply board #661-4821 if someone is ambitious enough to troubleshoot it.